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2016

Position and time resolution measurements with a microchannel plate image intensifier: A comparison of monolithic and pixelated CeBr3 scintillators
U. Ackermann, S. Eschbaumer, A. Bergmaier, W. Egger, P. Sperr, C. Greubel, B. Löwe, P. Schotanus and G. Dollinger; Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 823 (2016) 56 - 64.
Abstract: Abstract To perform Four Dimensional Age Momentum Correlation measurements in the near future, where one obtains the positron lifetime in coincidence with the three dimensional momentum of the electron annihilating with the positron, we have investigated the time and position resolution of two CeBr3 scintillators (monolithic and an array of pixels) using a Photek IPD340/Q/BI/RS microchannel plate image intensifier. The microchannel plate image intensifier has an active diameter of 40 mm and a stack of two microchannel plates in chevron configuration. The monolithic CeBr3 scintillator was cylindrically shaped with a diameter of 40 mm and a height of 5 mm. The pixelated scintillator array covered the whole active area of the microchannel plate image intensifier and the shape of each pixel was 2.5·2.5·8 mm3 with a pixel pitch of 3.3 mm. For the monolithic setup the measured mean single time resolution was 330 ps (FWHM) at a gamma energy of 511 keV. No significant dependence on the position was detected. The position resolution at the center of the monolithic scintillator was about 2.5 mm (FWHM) at a gamma energy of 662 keV. The single time resolution of the pixelated crystal setup reached 320 ps (FWHM) in the region of the center of the active area of the microchannel plate image intensifier. The position resolution was limited by the cross-section of the pixels. The gamma energy for the pixel setup measurements was 511 keV.
BibTeX:
	@article{Ackermann2016,
	  author = {Ulrich Ackermann and Stephan Eschbaumer and Andreas Bergmaier and Werner Egger and Peter Sperr and Christoph Greubel and Benjamin Löwe and Paul Schotanus and Günther Dollinger},
	  title = {Position and time resolution measurements with a microchannel plate image intensifier: A comparison of monolithic and pixelated CeBr3 scintillators},
	  journal = {Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
	  year = {2016},
	  volume = {823},
	  pages = {56 -- 64},
	  url = {http://www.sciencedirect.com/science/article/pii/S016890021630153X},
	  doi = {http://doi.org/10.1016/j.nima.2016.03.110}
	}
	
Four-dimensional positron age-momentum correlation
U. Ackermann, B. Löwe, M. Dickmann, J. Mitteneder, P. Sperr, W. Egger, M. Reiner and G. Dollinger; New Journal of Physics 18 (11) (2016) 113030.
Abstract: We have performed first four-dimensional age-momentum correlation (4D-AMOC) measurements at a pulsed high intensity positron micro beam and determined the absolute value of the three-dimensional momentum of the electrons annihilating with the positrons in coincidence with the positron age in the sample material. We operated two position sensitive detectors in coincidence to measure the annihilation radiation: a pixelated HPGe-detector and a microchannel plate image intensifier with a CeBr 3 scintillator pixel array. The transversal momentum resolution of the 4D-AMOC setup was measured to be about 17 × 10e-3 (FWHM) and was circa 3.5 times larger than the longitudinal momentum resolution. The total time resolution was 540 ps (FWHM). We measured two samples: a gold foil and a carbon tape at a positron implantation energy of 2 keV. For each sample discrete electron momentum states and their respective positron lifetimes were extracted.
BibTeX:
	@article{Ackermann2016a,
	  author = {Ulrich Ackermann and Benjamin Löwe and Marcel Dickmann and Johannes Mitteneder and Peter Sperr and Werner Egger and Markus Reiner and Günther Dollinger},
	  title = {Four-dimensional positron age-momentum correlation},
	  journal = {New Journal of Physics},
	  year = {2016},
	  volume = {18},
	  number = {11},
	  pages = {113030},
	  url = {http://stacks.iop.org/1367-2630/18/i=11/a=113030},
	  doi = {http://doi.org/10.1088/1367-2630/18/11/113030}
	}
	
Radio frequency elevator for a pulsed positron beam
M. Dickmann, J. Mitteneder, G. Kögel, W. Egger, P. Sperr, U. Ackermann, C. Piochacz and G. Dollinger; Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 821 (2016) 40 - 43.
Abstract: Abstract An elevator increases the potential energy of a particle beam with respect to ground potential without any alteration of kinetic energy and other beam parameters. This elevator is necessary for the implementation of the Munich Scanning Positron Microscope (SPM) at the intense positron source NEPOMUC at the research reactor FRM II in Munich. The principles of the rf elevator for pure electrostatically guided positrons are described. Measurements of beam quality behind the elevator are reported, which confirm that after the implementation of elevator and SPM at NEPOMUC the SPM can be operated at a considerably improved resolution (  0.3 ?m) and event rate ( 3.7 kHz) compared to the laboratory based ?+-source.
BibTeX:
	@article{Dickmann2016,
	  author = {Marcel Dickmann and Johannes Mitteneder and Gottfried Kögel and Werner Egger and Peter Sperr and Ulrich Ackermann and Christian Piochacz and Günther Dollinger},
	  title = {Radio frequency elevator for a pulsed positron beam},
	  journal = {Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
	  year = {2016},
	  volume = {821},
	  pages = {40 -- 43},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168900216300298},
	  doi = {http://doi.org/10.1016/j.nima.2016.03.011}
	}
	
Positron annihilation lifetime spectroscopy study of Kapton thin foils
G.S. Kanda, L. Ravelli, B. Löwe, W. Egger and D.J. Keeble; Journal of Physics D: Applied Physics 49 (2) (2016) 025305.
Abstract: Variable energy positron annihilation lifetime spectroscopy (VE-PALS) experiments on polyimide material Kapton are reported. Thin Kapton foils are widely used in a variety of mechanical, electronic applications. PALS provides a sensitive probe of vacancy-related defects in a wide range of materials, including open volume in polymers. Varying the positron implantation energy enables direct measurement of thin foils. Thin Kapton foils are also commonly used to enclose the positron source material in conventional PALS measurements performed with unmoderated radionuclide sources. The results of depth-profiled positron lifetime measurements on 7.6 ? m and 25 ? m Kapton foils are reported and determine a dominant 385(1) ps lifetime component. The absence of significant nanosecond lifetime component due to positronium formation is confirmed.
BibTeX:
	@article{Kanda2016,
	  author = {Gurmeet S Kanda and Luca Ravelli and Benjamin Löwe and Werner Egger and David J Keeble},
	  title = {Positron annihilation lifetime spectroscopy study of Kapton thin foils},
	  journal = {Journal of Physics D: Applied Physics},
	  year = {2016},
	  volume = {49},
	  number = {2},
	  pages = {025305},
	  url = {http://stacks.iop.org/0022-3727/49/i=2/a=025305},
	  doi = {http://doi.org/10.1088/0022-3727/49/2/025305}
	}
	

2015

Time- and energy-resolution measurements of BaF2, BC-418, LYSO and CeBr3 scintillators
U. Ackermann, W. Egger, P. Sperr and G. Dollinger; Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 786 (0) (2015) 5-11.
Abstract: We have investigated the time and energy resolution (TTiming, ?E/E) of four different scintillator materials BaF2, BC-418, LYSO and CeBr3 at a gamma energy of 511 keV in views of their possible usage for time, energy and position resolution for positron annihilation experiments. The shape of each scintillator was a cylinder with a diameter of 25 mm and a height of 10 mm readout by a Photonis XP2020/URQ photomultiplier tube. The best single time resolution was determined for each photomultiplier–scintillator setup in a three step optimization process. The optimized single time resolutions (FWHM) for BaF2, BC-418, LYSO and CeBr3 were 119 ps, 117 ps, 269 ps and 127 ps, respectively. We measured the energy resolution of the photomultiplier–scintillator setups which show a photopeak in the energy spectrum. The energy resolutions ?E/E of BaF2, LYSO and CeBr3 were 9.8%, 9.7% and 5.4%, respectively. The overall most promising material for measuring simultaneously time, energy and position resolution for positron annihilation experiments seems to be CeBr3 due to its very good time and energy resolution characteristics.
BibTeX:
	@article{Ackermann2015,
	  author = {Ackermann, Ulrich and Egger, Werner and Sperr, Peter and Dollinger, Günther},
	  title = {Time- and energy-resolution measurements of BaF2, BC-418, LYSO and CeBr3 scintillators},
	  journal = {Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
	  year = {2015},
	  volume = {786},
	  number = {0},
	  pages = {5--11},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168900215003101},
	  doi = {http://doi.org/10.1016/j.nima.2015.03.016}
	}
	
Free Volume and Gas Permeation in Anthracene Maleimide-Based Polymers of Intrinsic Microporosity
M.M. Khan, V. Filiz, T. Emmler, V. Abetz, T. Koschine, K. Rätzke, F. Faupel, W. Egger and L. Ravelli; Membranes 5 (2) (2015) 214.
Abstract: High free-volume copolymers were prepared via polycondensation with 2,3,5,6,-tetrafluoroterephthalonitrile (TFTPN) in which a portion of the 3,3,3',3'-tetramethyl- 1,1'-spirobisindane (TTSBI) of PIM-1 was replaced with dibutyl anthracene maleimide (4bIII). An investigation of free volume using positron annihilation lifetime spectroscopy (PALS), and gas permeation measurements was carried out for the thin film composite copolymer membranes and compared to PIM-1. The average free volume hole size and the gas permeance of the copolymer membranes increased with decreasing TTSBI content in the copolymer.
BibTeX:
	@article{Khan2015,
	  author = {Khan, Muntazim Munir and Filiz, Volkan and Emmler, Thomas and Abetz, Volker and Koschine, Toenjes and Rätzke, Klaus and Faupel, Franz and Egger, Werner and Ravelli, Luca},
	  title = {Free Volume and Gas Permeation in Anthracene Maleimide-Based Polymers of Intrinsic Microporosity},
	  journal = {Membranes},
	  year = {2015},
	  volume = {5},
	  number = {2},
	  pages = {214},
	  url = {http://www.mdpi.com/2077-0375/5/2/214},
	  doi = {http://doi.org/10.3390/membranes5020214}
	}
	
Correlation of gas permeation and free volume in new and used high free volume thin film composite membranes
T. Koschine, K. Rätzke, F. Faupel, M. Khan, T. Emmler, V. Filiz, V. Abetz, L. Ravelli and W. Egger; Journal of Polymer Science, Part B: Polymer Physics 53 (3) (2015) 213-217.
Abstract: Polymeric gas separation membranes frequently undergo the phenomenon of aging, that is, performance parameters like permeability decrease with storage or usage time. Here, we report on a new approach of reducing aging by incorporation of functionalized multiwalled carbon nanotubes into a polymer of intrinsic microporosity. Free volume and permeability measurements clearly show a reduced aging with incorporation of the carbon nantubes.
BibTeX:
	@article{Koschine2015,
	  author = {Koschine, T. and Rätzke, K. and Faupel, F. and Khan, M.M. and Emmler, T. and Filiz, V. and Abetz, V. and Ravelli, L. and Egger, W.},
	  title = {Correlation of gas permeation and free volume in new and used high free volume thin film composite membranes},
	  journal = {Journal of Polymer Science, Part B: Polymer Physics},
	  year = {2015},
	  volume = {53},
	  number = {3},
	  pages = {213--217},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/polb.23616/abstract;jsessionid=794C1386DED7BBED63EA824F50C74C4F.f03t01},
	  doi = {http://doi.org/10.1002/polb.23616}
	}
	
Erste ortsauflösende Positronen Lebensdauerspektroskopie am Scanning-Positron-Microscope Interface
Johannes Mitteneder; Masters-Thesis, Hochschule München, 2015.
Abstract: Das Scanning-Positron-Microscope (SPM) ist ein Instrument zur zerstörungsfreien Materialuntersuchung mittels der Positronen Lebensdauerspektroskopie. Positronen sind sensitiv für Materialdefekte wie Leerstellen, Versetzungen oder Korngrenzen.
An Defekten variiert die lokale Elektronendichte und damit die mittlere Lebensdauer der Positronen im Probenmaterial.
An der Universität der Bundeswehr wurde das SPM an einer Labor-Positronenquelle betrieben. Für einen höheren Positronenfluss wird das SPM zukünftig durch das SPM-Interface an die Reaktor Positronenquelle NEPOMUC gekoppelt. Durch den höheren Fluss des Positronenstrahls sollen kürzere Messzeiten erreicht werden.
Das SPM-Interface verbessert die Strahleigenschaften, indem es die Divergenz und den Strahldurchmesser des Positronenstrahls verkleinert. Zusätzlich formt das SPM-Interface die Positronenpulse aus dem kontinuierlichen Positronenstrahl von NEPOMUC.
Die Pulsung des Strahls wird als Startsignal für die Lebensdauermessungen benötigt. Damit passt das SPM-Interface den Positronenstrahl von NEPOMUC an die Bedürfnisse des SPM an.
Am SPM sollen ortsauflösende Positronen Lebensdauerspektroskopie Messungen mit einer Ortsauflösung von unter 1 µm und einer Pulsbreite von weniger als 150 ps erreicht werden. Dazu sind mehrere Stufen der Pulsformung und Fokussierung nötig.
Um die projektierten Ziele des SPM zu erreichen, muss am Abschluss des SPM-Interfaces ein Strahldurchmesser kleiner 200 µm und eine Pulsbreite von unter 250 ps (FWHM) erreicht werden. Die hohen Ansprüche an die Strahleigenschaften am Abschluss des SPM-Interfaces dienen außerdem dem effizienten Einkoppeln des Positronenstrahls in das SPM.
Um die Strahleigenschaften am Abschluss des SPM-Interfaces zu bestimmen, wurden mehrere Messungen durchgeführt. Die Messungen liefern die Grundlage für die Planung des weiteren Aufbaus des SPM an der Quelle NEPOMUC.
Für die Vermessungen des Strahlprofils des SPM-Interfaces wurde der Strahl mit einer Micro-Channel-Plate (MCP) direkt beobachtet. Der Bereich der höchsten Intensität des Positronenstrahls ist kreisförmig mit einem Durchmesser von 4mm FWHM. Das Intensitätsprofil ist gaußförmig. Durch eine Abbildung des Strahls mit einer elektrostatischen Linse auf die MCP konnte ein Strahldurchmesser von 2mm FWHM bei einer Brennweite von mm erreicht werden. Mit dieser Messung kann die kinetische Energie der transversalen Bewegung der Positronen auf 70meV abgeschätzt werden.
Ein rundes Strahlprofil mit gaußförmiger Intensität ist die Voraussetzung für die weiteren Messungen. In dieser Arbeit wurde eine Probenkammer zur ortsauflösenden Lebensdauerspektroskopie für das SPM-Interface gebaut. Die Probenkammer fokussiert den gepulsten Strahl mit Hilfe einer magnetischen Linse. Der magnetisch fokussierte Strahl kann durch Scannig-Spulen über eine Probe gerastert werden. Mit Hilfe der Probenkammer sind so ortsauflösende Messungen zur Bestimmung des minimal erreichbaren Strahldurchmessers möglich.
Als Grundlage für die ortsaufgelösten Messungen muss die erreichbare Zeitauflösung bekannt sein. Die Zeitauflösung ergibt sich durch die totale Zeitauflösung des Detektors und der Pulsbreite der erzeugten Positronenpulse. Für Pulsbreiten von unter 250 ps muss der kontinuierliche Positronenstrahl der Quelle NEPOMUC in mehreren Schritten zu Pulsen geformt werden. Dazu werden am SPM-Interface ein Sägezahn-Vorbuncher und zwei Sinus-Buncher sowie ein Chopper genutzt. Zur Bestimmung der Pulsbreite sind alle Pulsungkomponenten einzeln eingestellt und optimiert worden. Der Zeitfokus kann durch geeignete Einstellungen der Buncher- Amplitude und der Driftgeschwindigkeiten auf den Probenort in der Kammer gelegt werden. Durch die 50MHz Frequenz der Pulsung erreichen die Positronen alle vielfache von 20 ns die Probenposition t_n =n 20 ns (n e N). Liegt der Zeitfokus auf der Probe, ist die zeitliche Verteilung der ankommenden Positronen um t_n minimal. Die FWHM der gemessenen zeitlichen Verteilung (für viele Positronen aus mehreren Pulsen), der im SPM-Interface gepulsten Positronen entspricht der Pulsbreite.
Die Pulsbreite T_Puls wurde auf gute (257+/-3) ps bestimmt. Aus dem Vergleich der Zählrate mit und ohne Pulsung an der Probenposition ergibt sich ein Wirkungsgrad der Pulsung von 70%.
Am späteren Aufbau des SPM werden die im SPM-Interface erzeugten Pulse noch einmal gebuncht. Damit können aus den im SPM-Interface vorgeformten Pulsen schärfere Pulse im Bereich von 150 ps erzeugt werden.
Mit der erreichten Pulsbreite sind die Positronen Lebensdauermessungen mit der für das SPM-Interface gebauten Probenkammer möglich. Die Probenkammer erlaubt durch die im Laufe der Arbeit gebaute Hardware und in LabView programmierte Software vollautomatisierte ortsaufgelöste Positronen Lebensdauermessungen. Um mögliche Einstellungen der elektrischen Potenziale in der Kammer sowie für den Strom der fokussierenden magnetischen Linse für die Messungen zu finden, ist der Strahlverlauf in der Probenkammer mit COMSOL Multiphysics® simuliert worden. Vor den Messungen am SPM-Interface ist die Probenkammer auf Vakuumdichtigkeit und Hochspannungsfestigkeit bis 10 kV getestet worden. Die Tests verliefen problemlos, nach einem längeren Abpumpen wurde ein Druck von 1 10^(-6) mbar erreicht.
Aus den Messungen mit der Probenkammer des SPM-Interfaces soll der minimal erreichbare Strahldurchmesser bestimmt werden. Dazu wurden ortsaufgelöste Messungen durchgeführt. Die Probenkammer des SPM-Interfaces erlaubt einen Scan- Bereich von 1,5 x 1,5 mm. Der Scan-Bereich ist dabei weitestgehend frei von Verzeichnungen.
Durch die Geometrie der Probenkammer und die Abschirmung des Detektors sind Messungen mit geringem Untergrund möglich. Als Untergrund zählen alle 511 keV-Quanten, die nicht von einer Annihilation im Probenmaterial stammen. Das Peak zu Untergrund Verhältnis beträgt 1000. Damit ist die Aufnahme guter Einzelspektren ausgewählter Orte der Probe möglich. Mit Hilfe von Line-Scans ist die Ortsauflösung in der Probenkammer zu (180+-10) µm bestimmt. Um diesen Strahldurchmesser zu erreichen, wurde der Positronenstrahl in der Probenkammer mit der magnetischen Linse des zweiten Remoderators des SPM fokussiert. Durch die Verwendung dieser Linse ist am weiteren Aufbau des SPM ein vergleichbar kleiner Strahldurchmesser auf dem zweiten Remoderator möglich. Der Remoderator wirkt als gepulste Positronenquelle für die Positronen Lebensdauermessungen in der Probenkammer des SPM. Mit einem Quellendurchmesser von ca. 200 µm und der schmalen Energieverteilung des remoderierten Strahls sind Strahldurchmesser um 1 µm beim finalen Aufbau des SPM möglich.
Der Aufbau der Probenkammer des SPM-Interfaces sowie die durchgeführten Messungen waren sehr erfolgreich. Die in dieser Arbeit erreichten Ergebnisse erlauben den finalen Aufbau des SPM. Durch die Kenntnis des Strahldurchmessers kann die Positronen-Strahloptik zum Ankoppeln des SPM an das SPM-Interface effektiv ausgelegt werden. Schon mit dem Aufbau der Probenkammer des SPM-Interfaces sind Positronen Lebensdauerspektroskopie Messungen mit guter Orts- und sehr guter Zeitauflösung möglich.
Positronen Lebensdauermessungen mit einer Pulsbreite von 260 ps, einem Strahldurchmesser von 180 µm und mit einr hoher Intensität von 2000 Counts pro Sekunde sind weltweit einzigartig. Sie sind nur am Scanning-Positron-Microscope Interface mit der in dieser Arbeit gebauten Probenkammer des SPM-Interafces der Universität der Bundeswehr an der Positronenquelle NEPOMUC möglich.
BibTeX:
	@mastersthesis{Mitteneder2015ma,
	  author = {Mitteneder, Johannes},
	  title = {Erste ortsauflösende Positronen Lebensdauerspektroskopie am Scanning-Positron-Microscope Interface},
	  school = {Hochschule München},
	  year = {2015}
	}
	

2014

Interphase of a polymer at a solid interface
H.J. Butt, H. Duran, W. Egger, F. Faupel, V. Harmandaris, S. Harms, K. Johnston, K. Kremer, F.Y. Lin, L. Lue, C. Ohrt, K. Rätzke, L. Ravelli, W. Steffen and S.D.B. Vianna; Macromolecules 47 (23) (2014) 8459-8465.
Abstract: Atomistic molecular dynamics simulations of a chemically realistic model of atactic short-chain polystyrene between gold surfaces (111) and positron annihilation lifetime spectroscopy experiments on similar polystyrene thin films on gold were performed. Results from both approaches show that the free volume voids in the film have a slightly smaller average size than in bulk polystyrene. In agreement to that the existence of an interphase of higher density at the polymer-solid substrate interface is shown both by the simulation as well as in the experiment. The average shape of the voids is similar in the bulk and the film.
BibTeX:
	@article{Butt2014,
	  author = {Butt, H. J. and Duran, H. and Egger, W. and Faupel, F. and Harmandaris, V. and Harms, S. and Johnston, K. and Kremer, K. and Lin, F. Y. and Lue, L. and Ohrt, C. and Rätzke, K. and Ravelli, L. and Steffen, W. and Vianna, S. D. B.},
	  title = {Interphase of a polymer at a solid interface},
	  journal = {Macromolecules},
	  year = {2014},
	  volume = {47},
	  number = {23},
	  pages = {8459-8465},
	  url = {http://pubs.acs.org/doi/abs/10.1021/ma501747j},
	  doi = {http://doi.org/10.1021/ma501747j}
	}
	
Interphases in polymer solid-contacts and nanocomposites probed by positron annihilation lifetime spectroscopy
C. Ohrt, T. Koschine, S. Harms, F. Faupel, K. Rätzke, W. Egger, L. Ravelli, L. Willner and G. Schneider; Soft Materials 12 (2014) S135-S141.
Abstract: At polymer-solid contacts with thermoplastic polymers, the chain characteristics can deviate from the bulk on a length-scale of several nanometer in the so-called interphase. Such an interphase depends on the interaction between respective macromolecule and substrate, and affects the free volume in the polymer. Here, we review our experiments on the characterization of the free volume by positron lifetime spectroscopy at planar and curved interfaces. For Teflon AF on silicon, we identify a layer of increased density, corresponding to an interphase width of some 10 nm. PEP based nanocomposites with functionalized silica show no interphase, whereas for functionalized POSS an interphase is detected.
BibTeX:
	@article{Ohrt2014,
	  author = {Ohrt, C. and Koschine, T. and Harms, S. and Faupel, F. and Rätzke, K. and Egger, W. and Ravelli, L. and Willner, L. and Schneider, G.J.},
	  title = {Interphases in polymer solid-contacts and nanocomposites probed by positron annihilation lifetime spectroscopy},
	  journal = {Soft Materials},
	  year = {2014},
	  volume = {12},
	  pages = {S135-S141},
	  url = {http://www.tandfonline.com/doi/abs/10.1080/1539445X.2014.957835},
	  doi = {http://doi.org/10.1080/1539445X.2014.957835}
	}
	
Improvement of the Pulsed Low Energy Positron System (PLEPS) for complex problems in materials science
Luca Ravelli; Dissertation, Universität der Bundeswehr München, Fakultät für Luft- und Raumfahrttechnik, 2014.
Abstract: This thesis concerns the application and the improvement of the Pulsed Low Energy Positron System (PLEPS) at the high intensity positron source NEPOMUC at the Munich research reactor FRM-II. This system is used for the defect study in complex materials. Positrons are the ideal probe for non-destructive investigations of vacancy-like defects in matter. The combination of positron lifetime spectroscopy with a pulsed, monochromatic positron beam of variable energy conveys information on the type and the concentration of defects down to the sub-ppm range and their depth-profile with nm resolution. Defect structures in two materials were investigated with PLEPS for this thesis. First, we studied strontium titanate (STO), which is a material of great relevance in modern oxide electronics. The cation vacancies (strontium and titanium vacancies, VSr and VTi, respectively) were identified in STO films deposited by Pulsed Laser Deposition (PLD). It was also shown, that in commercially available STO substrates only titanium vacancies with a concentration of (1.26±0.16) ppm could be detected and that upon annealing in the same conditions as for the PLD procedure a 400 nm thick layer of titanium-oxygen divacancies VTi-O was introduced. The second investigation was performed in permanently densified silica glasses. In combination with XRD measurements the structure evolution upon densification was analyzed. In particular, it was demonstrated that the average inter-tetrahedral void radius measured with PLEPS permits to predict the shift of the first sharp diffraction peak of the static structure factor as a function of the density. In the second part of this work, from the experience gained with PLEPS in the course of this thesis, the limits of the apparatus were analyzed, measures to improve the quality of the positron lifetime spectra measured with PLEPS were identified and tested. Comprehensive simulations were performed to understand the structures in the background of the measured lifetime spectra and possible countermeasures were found. Modifications of the pulsing system allowed to a) improve the time resolution of PLEPS to about 250 ps, b) measure precisely positron lifetime longer than 3 ns, which enhances the capabibility of PLEPS for the determination of free volumes in polymer samples and membranes and c) get rid of disturbing structures in the background of the positron lifetime spectra. Thus, PLEPS in combination with the high intensity positron source NEPOMUC can be considered as the most productive pulsed positron beam for defect depth-profiling in materials currently available world-wide.
BibTeX:
	@phdthesis{Ravelli2014diss,
	  author = {Ravelli, Luca},
	  title = {Improvement of the Pulsed Low Energy Positron System (PLEPS) for complex problems in materials science},
	  school = {Universität der Bundeswehr München, Fakultät für Luft- und Raumfahrttechnik},
	  year = {2014},
	  url = {http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:706-3513}
	}
	
Erprobung von Szintillator-Detektoren bei tiefen Temperaturen
Johannes Schütze; Masters-Thesis, Universität der Bundeswehr München, 2014.
BibTeX:
	@mastersthesis{Schuetze2014ma,
	  author = {Schütze, Johannes},
	  title = {Erprobung von Szintillator-Detektoren bei tiefen Temperaturen},
	  school = {Universität der Bundeswehr München},
	  year = {2014}
	}
	
PLEPS study of ions implanted RAFM steels
S. Sojak, V. Sluge?, W. Egger, L. Ravelli, M. Petriska, J. Veterníková, M. Stacho and V. Sabelová; Journal of Physics: Conference Series 505 (1) (2014) 012016.
Abstract: Current nuclear power plants (NPP) require radiation, heat and mechanical resistance of their structural materials with the ability to stay operational during NPP planned lifetime. Radiation damage much higher, than in the current NPP, is expected in new generations of nuclear power plants, such as Generation IV and fusion reactors. Investigation of perspective structural materials for new generations of nuclear power plants is among others focused on study of reduced activation ferritic/martensitic (RAFM) steels. These steels have good characteristics as reduced activation, good resistance to volume swelling, good radiation, and heat resistance. Our experiments were focused on the study of microstructural changes of binary Fe-Cr alloys with different chromium content after irradiation, experimentally simulated by ion implantations. Fe-Cr alloys were examined, by Pulsed Low Energy Positron System (PLEPS) at FRM II reactor in Garching (Munich), after helium ion implantations at the dose of 0.1 C/cm(2). The investigation was focused on the chromium effect and the radiation defects resistivity. In particular, the vacancy type defects (monovacancies, vacancy clusters) have been studied. Based on our previous results achieved by conventional lifetime technique, the decrease of the defects size with increasing content of chromium is expected also for PLEPS measurements.
BibTeX:
	@article{Sojak2014,
	  author = {Sojak, S. and Sluge?, V. and Egger, W. and Ravelli, L. and Petriska, M. and Veterníková, J. and Stacho, M. and Sabelová, V.},
	  title = {PLEPS study of ions implanted RAFM steels},
	  journal = {Journal of Physics: Conference Series},
	  year = {2014},
	  volume = {505},
	  number = {1},
	  pages = {012016},
	  editor = {Hugenschmidt, C and Piochacz, C},
	  note = {13th International Workshop on Slow Positron Beam Techniques and Applications (SLOPOS), Tech Univ Munchen, Munich, GERMANY, SEP 15-20, 2013},
	  url = {http://iopscience.iop.org/1742-6596/505/1/012016/},
	  doi = {http://doi.org/10.1088/1742-6596/505/1/012016}
	}
	
Structural evolution and medium range order in permanently densified vitreous SiO2
M. Zanatta, G. Baldi, R.S. Brusa, W. Egger, A. Fontana, E. Gilioli, S. Mariazzi, G. Monaco, L. Ravelli and F. Sacchetti; Physical Review Letters 112 (4) (2014) 045501.
Abstract: Positron annihilation lifetime spectroscopy is employed to measure the size of the interstitial void spaces characterizing the structure of a set of permanently densified SiO2 glasses. The average volume of the voids is markedly affected by the densification process and linearly shrinks by almost an order of magnitude after a relative density variation of 22%. In addition, x-ray diffraction shows that this change of density does not modify appreciably the short range order, which remains organized in SiO4 tetrahedra. These results strongly suggest a porous medium description for v-SiO2 glasses where the compressibility and the medium range order are dominated by the density variation of the voids volume up to densities close to that of ?-quartz. © 2014 American Physical Society.
BibTeX:
	@article{Zanatta2014,
	  author = {Zanatta, M. and Baldi, G. and Brusa, R. S. and Egger, W. and Fontana, A. and Gilioli, E. and Mariazzi, S. and Monaco, G. and Ravelli, L. and Sacchetti, F.},
	  title = {Structural evolution and medium range order in permanently densified vitreous SiO2},
	  journal = {Physical Review Letters},
	  year = {2014},
	  volume = {112},
	  number = {4},
	  pages = {045501},
	  url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.045501},
	  doi = {http://doi.org/10.1103/PhysRevLett.112.045501}
	}
	

2013

Development of a new time and position resolving detector for the pulsed low energy positron system PLEPS
U. Ackermann, W. Egger, P. Sperr, B. Löwe, L. Ravelli, G. Kögel, G. Dollinger and O. Jagutzki; Journal of Physics: Conference Series 443 (1) (2013) 012095.
Abstract: The pulsed low energy positron system PLEPS at the Munich research reactor FRM-II is a user facility for depth resolved positron lifetime measurements. Besides positron lifetime measurements 2D-AMOC (Two Dimensional Age Momentum Correlation) experiments are also possible. 2D-AMOC provides in coincidence the lifetime of the positron and the longitudinal momentum distribution of the annihilated electron. It would be of great scientific concern to measure simultaneously the entire 3D-momentum distribution of the electron annihilating with the positron and the corresponding lifetime of the positron (4D-AMOC). To perform 4D-AMOC measurements a time and position resolving detector is required in coincidence with a pixelated Germanium detector. Therefore a time and spatially resolving detector is currently developed at our institute with envisaged time resolution of 100 ps (FWHM) and a spatial resolution of about 2.6 mm (FWHM) over an area of 12 cm2. First test measurements have been carried out with a 25 mm diameter MCP (Micro Channel Plate) image intensifier and with special delay-line anode readout for the spatial information. Up to now 178 ps (FWHM) time resolution and on average 3.4 mm (FWHM) position resolution have been achieved with BaF2 as scintillator material and a 60Co source.
BibTeX:
	@article{Ackermann2013,
	  author = {Ackermann, U. and Egger, W. and Sperr, P. and Löwe, B. and Ravelli, L. and Kögel, G. and Dollinger, G. and Jagutzki, O.},
	  title = {Development of a new time and position resolving detector for the pulsed low energy positron system PLEPS},
	  journal = {Journal of Physics: Conference Series},
	  year = {2013},
	  volume = {443},
	  number = {1},
	  pages = {012095},
	  url = {http://iopscience.iop.org/1742-6596/443/1/012095/},
	  doi = {http://doi.org/10.1088/1742-6596/443/1/012095}
	}
	
Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy
L. Chai, W. Al-Sawai, Y. Gao, A. Houtepen, P. Mijnarends, B. Barbiellini, H. Schut, L. Van Schaarenburg, M. Van Huis, L. Ravelli, W. Egger, S. Kaprzyk, A. Bansil and S. Eijt; APL Materials 1 (2) (2013) 022111.
Abstract: Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.
BibTeX:
	@article{Chai2013,
	  author = {Chai, L. and Al-Sawai, W. and Gao, Y. and Houtepen, A.J. and Mijnarends, P.E. and Barbiellini, B. and Schut, H. and Van Schaarenburg, L.C. and Van Huis, M.A. and Ravelli, L. and Egger, W. and Kaprzyk, S. and Bansil, A. and Eijt, S.W.H.},
	  title = {Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy},
	  journal = {APL Materials},
	  year = {2013},
	  volume = {1},
	  number = {2},
	  pages = {022111},
	  url = {http://aip.scitation.org/doi/10.1063/1.4818001},
	  doi = {http://doi.org/10.1063/1.4818001}
	}
	
Nonstoichiometry accommodation in SrTiO3 thin films studied by positron annihilation and electron microscopy
D. Keeble, S. Wicklein, L. Jin, C. Jia, W. Egger and R. Dittmann; Physical Review B - Condensed Matter and Materials Physics 87 (19) (2013) 195409.
Abstract: Accommodation of nonstoichiometry in SrTiO3 pulsed laser deposited (PLD) films was investigated using positron annihilation lifetime spectroscopy and (scanning) transmission electron microscopy. Increasing PLD laser fluence changed the stoichiometry from Ti to Sr deficient. Cation vacancy defects were detected, and the concentration ratio of Sr to Ti vacancies, [VSr]/[VTi], was observed to increase systematically in the Sr-deficient region, although no change in the electron microscopy lattice images was detected. Increasing Ti deficiency resulted in the accommodation of SrO layers in planar defects, and in the formation of vacancy cluster defects. A change from VTi to VSr defect positron trapping was also detected.
BibTeX:
	@article{Keeble2013,
	  author = {Keeble, D.J. and Wicklein, S. and Jin, L. and Jia, C.L. and Egger, W. and Dittmann, R.},
	  title = {Nonstoichiometry accommodation in SrTiO3 thin films studied by positron annihilation and electron microscopy},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2013},
	  volume = {87},
	  number = {19},
	  pages = {195409},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.87.195409},
	  doi = {http://doi.org/10.1103/PhysRevB.87.195409}
	}
	
A position sensitive germanium detector for the measurement of angular deviation of annihilation radiation
B. Löwe, M. Reiner, W. Egger, C. Hugenschmidt and G. Dollinger; Journal of Physics: Conference Series 443 (1) (2013) 012098.
Abstract: To improve electron momentum sensitivity in Coincidence Doppler Broadening Spectroscopy (CDBS) measurements it is envisaged to measure the angular correlation of annihilation radiation along with the energy of both annihilation photons. For this purpose two position sensitive 36-fold pixelated, planar germanium detectors will be utilized. The position sensitivity of one of those detectors has been tested with a collimated gamma source. A data acquisition system consisting of 37 sampling analogue-to-digital converters with PC based online/off-line processing has been installed. A position sensitivity of 1.6 mm has been achieved.
BibTeX:
	@article{Loewe2013,
	  author = {Löwe, B. and Reiner, M. and Egger, W. and Hugenschmidt, C. and Dollinger, G.},
	  title = {A position sensitive germanium detector for the measurement of angular deviation of annihilation radiation},
	  journal = {Journal of Physics: Conference Series},
	  year = {2013},
	  volume = {443},
	  number = {1},
	  pages = {012098},
	  url = {http://iopscience.iop.org/1742-6596/443/1/012098/},
	  doi = {http://doi.org/10.1088/1742-6596/443/1/012098}
	}
	
Study of Positronium formation in nano-channelled silicon as a function of sample temperature
S. Mariazzi, L. Dinoto, L. Ravelli, W. Egger and R. Brusa; Journal of Physics: Conference Series 443 (1) (2013) 012061.
Abstract: Oxidized nanochannel in silicon have been demonstrated to be suitable for positronium (Ps) formation and cooling also at low sample temperature. To investigate the Ps yield and to clarify the Ps formation mechanism we studied, by Positron Annihilation Spectroscopy (PAS), nanochanneled Si p-type samples in the 150-430 K temperature range. Ps yield was found to be constant in the 150-300 K temperature range, then it increases up to ?50% of its value from 350-400 K. This effect is associated to a decrease of the fraction of positrons annihilating in Si and in the SiO2 layer on the nanochannels surface. This finding is compatible with the thermal decrease of the positive charge distribution at the Si/SiO2 interface limiting e+ reaching the SiO2 layer and to a charge rearrangement at the SiO2 surfaces.
BibTeX:
	@article{Mariazzi2013,
	  author = {Mariazzi, S. and Dinoto, L. and Ravelli, L. and Egger, W. and Brusa, R.S.},
	  title = {Study of Positronium formation in nano-channelled silicon as a function of sample temperature},
	  journal = {Journal of Physics: Conference Series},
	  year = {2013},
	  volume = {443},
	  number = {1},
	  pages = {012061},
	  url = {http://iopscience.iop.org/article/10.1088/1742-6596/443/1/012061/meta;jsessionid=D1DE141D0665C855F0EED9CB9570D8E4.c1.iopscience.cld.iop.org},
	  doi = {http://doi.org/10.1088/1742-6596/443/1/012061}
	}
	
Geant4 simulation of the effect of backscattered positrons on the lifetime spectra of PLEPS
L. Ravelli, B. Löwe, W. Egger, G. Kögel, P. Sperr and G. Dollinger; Journal of Physics: Conference Series 443 (1) (2013) 012096.
Abstract: The Pulsed Low Energy Positron System (PLEPS) allows the measurement of positron lifetime spectra of very high quality with peak-to-background ratios up to 3 · 104. At those peak-to-background ratios small structures appear in the lifetime spectra due to backscattered positrons. Despite their small overall contribution-less than 2 % of the total events in the spectrum even with backscattering coefficient as high as 40 %-those satellite structures can render the data analysis difficult. To understand the origin of those satellite structures and to further improve the performance of the system, comprehensive simulations of the target chamber of PLEPS have been undertaken. The results reproduce fairly well the background of the lifetime spectrum. It is now possible to identify the origin of the background structures and also plan some additional countermeasures.
BibTeX:
	@article{Ravelli2013,
	  author = {Ravelli, L. and Löwe, B. and Egger, W. and Kögel, G. and Sperr, P. and Dollinger, G.},
	  title = {Geant4 simulation of the effect of backscattered positrons on the lifetime spectra of PLEPS},
	  journal = {Journal of Physics: Conference Series},
	  year = {2013},
	  volume = {443},
	  number = {1},
	  pages = {012096},
	  url = {http://iopscience.iop.org/1742-6596/443/1/012096/},
	  doi = {http://doi.org/10.1088/1742-6596/443/1/012096}
	}
	
Untersuchung des zeitlichen Verhaltens von Siliziumphotomultipliern
Johannes Schütze; Bachelors-Thesis, Universität der Bundeswehr München, 2013.
Abstract: Diese Bachelorarbeit beschäftigt sich mit der Untersuchung des zeitlichen Verhaltens von Siliziumphotomultipliern (SiPMs). Dazu werden die Zeitauflösungen verschiedener SiPMs der Firma Ketek ermittelt. Der Schwerpunkt liegt dabei auf der Untersuchung des Zeitverhaltens bei unterschiedlichen Versorgungsspannungen. Im Folgenden wird zunächst die Funktionsweise von SiPMs, sowie die Funktionsweise der verwendeten Messelektronik erklärt. Anschließend wird der Ablauf der einzelnen Messmethoden erläutert. Zum Abschluss werden die Ergebnisse dargestellt und es wird auf Besonderheiten, die während der Messungen aufgetreten sind, eingegangen.
BibTeX:
	@mastersthesis{Schuetze2013ba,
	  author = {Schütze, Johannes},
	  title = {Untersuchung des zeitlichen Verhaltens von Siliziumphotomultipliern},
	  school = {Universität der Bundeswehr München},
	  year = {2013}
	}
	
Microstructural examination of reactor pressure vessel steels by positron annihilation point of view
V. Sluge?, H. Hein, S. Sojak, W. Egger, M. Pavúk, J. Veterníková, M. Petriska, V. Sabelová and R. Hinca; Progress in Nuclear Energy 62 (2013) 1-7.
Abstract: This paper presents a comparison of commercially used German and Russian reactor pressure vessel steels from the positron annihilation spectroscopy (PAS) point of view, having in mind knowledge obtained also from other techniques from the last decades. The second generations of Russian RPV steels seems to be fully comparable with German steels and their quality enables prolongation of NPP operating lifetime over projected 40 years. The embrittlement of CrMoV steel is very low due to the dynamic recovery of radiation-induced defects at reactor operating temperatures. © 2012 Elsevier Ltd. All rights reserved.
BibTeX:
	@article{Slugen2013,
	  author = {Sluge?, V. and Hein, H. and Sojak, S. and Egger, W. and Pavúk, M. and Veterníková, J. and Petriska, M. and Sabelová, V. and Hinca, R.},
	  title = {Microstructural examination of reactor pressure vessel steels by positron annihilation point of view},
	  journal = {Progress in Nuclear Energy},
	  year = {2013},
	  volume = {62},
	  pages = {1--7},
	  url = {http://www.sciencedirect.com/science/article/pii/S0149197012001023},
	  doi = {http://doi.org/10.1016/j.pnucene.2012.08.002}
	}
	
PLEPS study of ion implanted and annealed Fe-11.62% Cr alloys
S. Sojak, V. Sluge, W. Egger, L. Ravelli, M. Petriska, S. Stanek, M. Sahul, M. Skarba, P. Priputen, M. Stacho, J. Veterníková, R. Hinca and V. Sabelová; Journal of Physics: Conference Series 443 (1) (2013) 012036.
Abstract: Reduced activation ferritic/martensitic (RAFM) steels are one of the candidate structural materials for application in future nuclear facilities. These steels fulfill demands on radiation, thermal, and mechanical resistance during their operational lifetime. Our experiments were focused on study of microstructural changes of binary Fe-Cr alloys after annealing and irradiation, experimentally simulated by ion implantations. Alloys with 11.62% Cr were examined after helium ion implantations at different doses (0.1; 0.3; 0.5 C/cm2). Thermal annealing, motivated by literature and our previous work in the field of reactor steels [1,2], was performed at temperatures of 400, 475, 525 and 600 °C after implantations with aim to study changes of the defect size/amount. The Pulsed Low Energy Positron System (PLEPS) at FRM II reactor in Garching (Munich) was applied for lifetime studies [3]. Damage introduced into the microstructure caused problems with defect interpretations by the PLEPS technique and therefore SEM was applied as well. Positron lifetimes measured at annealing temperatures in range of 400-525 °C did not show an expected decrease. Only in the case of temperature of 600 °C the mean lifetime (MLT) decreased significantly and assumptions about the defect size decrease were made according to the literature [2].
BibTeX:
	@article{Sojak2013,
	  author = {Sojak, S. and Sluge, V. and Egger, W. and Ravelli, L. and Petriska, M. and Stanek, S. and Sahul, M. and Skarba, M. and Priputen, P. and Stacho, M. and Veterníková, J. and Hinca, R. and Sabelová, V.},
	  title = {PLEPS study of ion implanted and annealed Fe-11.62% Cr alloys},
	  journal = {Journal of Physics: Conference Series},
	  year = {2013},
	  volume = {443},
	  number = {1},
	  pages = {012036},
	  url = {http://iopscience.iop.org/article/10.1088/1742-6596/443/1/012036/meta},
	  doi = {http://doi.org/10.1088/1742-6596/443/1/012036}
	}
	
Thermal annealing influence on ions implanted Fe-Cr model alloys
S. Sojak, V. Sluge?, V. Kršjak, W. Egger, L. Ravelli, M. Petriska, S. Stan?ek, M. Skarba, P. Priputen, K. Vitázek, M. Stacho, J. Veterníková and V. Sabelová; Materials Science Forum 733 (2013) 274-277.
Abstract: Binary Fe-11.62wt%Cr alloys were investigated in as-received state as well as after a two step helium ion implantation at different energies (100 keV and 250 keV) with doses up to 3.12×1018 cm-2. In order to study changes in alloys in dependence on the temperature, thermal annealing was performed at temperatures of 400, 475, 525 and 600 oC and specimens were afterwards measured by a pulsed low energy positron system (PLEPS). Annealing out of defects at lower temperatures was not as significant as expected, and we also encountered difficulties with defect identification. However, an apparent decrease of defect size was observed in the specimen annealed at a temperature of 600 oC. © (2013) Trans Tech Publications, Switzerland.
BibTeX:
	@article{Sojak2013a,
	  author = {Sojak, S. and Sluge?, V. and Kršjak, V. and Egger, W. and Ravelli, L. and Petriska, M. and Stan?ek, S. and Skarba, M. and Priputen, P. and Vitázek, K. and Stacho, M. and Veterníková, J. and Sabelová, V.},
	  title = {Thermal annealing influence on ions implanted Fe-Cr model alloys},
	  booktitle = {POSITRON AND POSITRONIUM CHEMISTRY X},
	  journal = {Materials Science Forum},
	  year = {2013},
	  volume = {733},
	  pages = {274--277},
	  editor = {Kristiak, J and Kuriplach, J and Pujari, PK},
	  note = {10th International Workshop on Positron and Positronium Chemistry (PPC-10), Smolenice, SLOVAKIA, SEP 05-09, 2011},
	  url = {http://www.scientific.net/MSF.733.274},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.733.274}
	}
	
On the interplay of point defects and Cd in non-polar ZnCdO films
A. Zubiaga, F. Reurings, F. Tuomisto, F. Plazaola, J. García, A. Kuznetsov, W. Egger, J. Zúñiga-Pérez and V. Muñoz-Sanjosé; Journal of Applied Physics 113 (2) (2013) 023512.
Abstract: Non-polar ZnCdO films, grown over m- and r-sapphire with a Cd concentration ranging between 0.8% and 5%, have been studied by means of slow positron annihilation spectroscopy (PAS) combined with chemical depth profiling by secondary ion mass spectroscopy and Rutherford back-scattering. Vacancy clusters and Zn vacancies with concentrations up to 1017 cm-3 and 1018 cm-3, respectively, have been measured inside the films. Secondary ion mass spectroscopy results show that most Cd stays inside the ZnCdO film but the diffused atoms can penetrate up to 1.3 ?m inside the ZnO buffer. PAS results give an insight to the structure of the meta-stable ZnCdO above the thermodynamical solubility limit of 2%. A correlation between the concentration of vacancy clusters and Cd has been measured. The concentration of Zn vacancies is one order of magnitude larger than in as-grown non-polar ZnO films and the vacancy cluster are, at least partly, created by the aggregation of smaller Zn vacancy related defects. The Zn vacancy related defects and the vacancy clusters accumulate around the Cd atoms as a way to release the strain induced by the substitutional CdZn in the ZnO crystal.
BibTeX:
	@article{Zubiaga2013,
	  author = {Zubiaga, A. and Reurings, F. and Tuomisto, F. and Plazaola, F. and García, J.A. and Kuznetsov, A.Yu. and Egger, W. and Zúñiga-Pérez, J. and Muñoz-Sanjosé, V.},
	  title = {On the interplay of point defects and Cd in non-polar ZnCdO films},
	  journal = {Journal of Applied Physics},
	  year = {2013},
	  volume = {113},
	  number = {2},
	  pages = {023512},
	  url = {http://aip.scitation.org/doi/10.1063/1.4775396},
	  doi = {http://doi.org/10.1063/1.4775396}
	}
	

2012

Characterization of vacancy-type defects in heteroepitaxial GaN grown by low-energy plasma-enhanced vapor phase epitaxy
A. Calloni, R. Ferragut, A. Dupasquier, H. von Känel, A. Guiller, A. Rutz, L. Ravelli and W. Egger; Journal of Applied Physics 112 (2) (2012) 024510.
Abstract: The defect concentration in thin GaN layers was estimated by means of positron annihilation spectroscopy. Positron lifetime and Doppler broadening of the annihilation radiation were used. A comparative study of GaN films grown with different techniques was performed. Specific attention has been given to the new low energy plasma enhanced vapor phase epitaxy (LEPEVPE) growth technique. A very high Ga vacancy density (10 19 cm -3) was found in a thin GaN layer directly grown by LEPEVPE on a sapphire substrate. However, when a GaN substrate (commercial sample grown by Metal Organic Vapor Phase Epitaxy) is used as a template for LEPEVPE deposition, the vacancy density of the film is low (about 10 16 cm -3). This fact provides evidences that the LEPEVPE technique is able to produce high quality GaN layers.
BibTeX:
	@article{Calloni2012,
	  author = {Calloni, A. and Ferragut, R. and Dupasquier, A. and von Känel, H. and Guiller, A. and Rutz, A. and Ravelli, L. and Egger, W.},
	  title = {Characterization of vacancy-type defects in heteroepitaxial GaN grown by low-energy plasma-enhanced vapor phase epitaxy},
	  journal = {Journal of Applied Physics},
	  year = {2012},
	  volume = {112},
	  number = {2},
	  pages = {024510},
	  url = {http://aip.scitation.org/doi/10.1063/1.4737402},
	  doi = {http://doi.org/10.1063/1.4737402}
	}
	
Aging and free volume in a polymer of intrinsic microporosity (PIM-1)
S. Harms, K. Rätzke, F. Faupel, N. Chaukura, P. Budd, W. Egger and L. Ravelli; Journal of Adhesion 88 (7) (2012) 608-619.
Abstract: There is a growing market for polymeric gas separation membranes for applications such as air separation and carbon dioxide capture. One of the key properties dominating transport is the free volume between atoms, allowing gas diffusion. However, thin films, in particular, undergo aging, decreasing free volume, and, hence, performance with time. We have measured the change in free volume during aging of thin films of a polymer of intrinsic microporosity (PIM-1) by depth-resolved positron annihilation lifetime spectroscopy. For films with thickness, d, smaller than 1m, aging is nearly complete after 3 months, whereas for films with d>1 ?m aging continues even after several months. Aging is thickness-and time-dependent and the free volume diffuses through the film to the free surface.
BibTeX:
	@article{Harms2012,
	  author = {Harms, S. and Rätzke, K. and Faupel, F. and Chaukura, N. and Budd, P.M. and Egger, W. and Ravelli, L.},
	  title = {Aging and free volume in a polymer of intrinsic microporosity (PIM-1)},
	  journal = {Journal of Adhesion},
	  year = {2012},
	  volume = {88},
	  number = {7},
	  pages = {608--619},
	  url = {http://www.tandfonline.com/doi/abs/10.1080/00218464.2012.682902},
	  doi = {http://doi.org/10.1080/00218464.2012.682902}
	}
	
Niobium aggregation and vacancylike defect evolution in nanostructured Nb-doped Mg: Their role in the kinetics of the hydride-to-metal phase transformation
C. Macchi, C. Maurizio, R. Checchetto, S. Mariazzi, L. Ravelli, W. Egger, P. Mengucci, N. Bazzanella, A. Miotello, A. Somoza and R. Brusa; Physical Review B - Condensed Matter and Materials Physics 85 (21) (2012) 214117.
Abstract: The structural evolution of nanostructured Nb-doped magnesium film samples and its correlation with the change of the H 2 desorption kinetics after successive H 2 sorption cycles at 623 K was investigated by different techniques. The variation of the dispersed Nb fraction and the Nb clusterization was followed by extended x-ray absorption fine structure (EXAFS), while the progressive Mg nanostructuring was monitored by x-ray diffraction. The presence of vacancylike defects and their evolution was studied using positron annihilation lifetime spectroscopy and Doppler broadening spectroscopies. It was found that, with successive H 2 sorption cycles: (i) the H 2 desorption kinetics progressively becomes slower until stationary conditions are reached and (ii) the Nb dopant atoms, dispersed in the nanocrystalline Mg layers, aggregate, forming nanoclusters. Our results show that the progressive Nb aggregation drives the H 2 desorption kinetics. EXAFS analysis show that fast desorption kinetics is due to the presence of small (?1 nm) Nb aggregates rather than Nb atoms dispersed into the Mg matrix. With cycling, the Nb aggregates progressively grow, forming larger bcc Nb nanoclusters and the H 2 desorption kinetics becomes slower. In the as-deposited Nb-doped Mg samples, analysis of the positron data reveals the presence of intragranular vacancylike defects and of vacancy clusters which are inferred to be mainly located at the grain boundaries of the nanocrystalline Mg layers. With H 2 cycling: (i) a decrease of the atomic fraction of the intragranular vacancylike defects after the first two sorption cycles was observed, and (ii) an increase of the atomic fraction of vacancy clusters at grain boundaries and the appearance of vacancylike defects located at the interface between the Nb aggregates and the Mg matrix was probed. It was also found that the kinetics follows a nucleation and growth mechanism and, under stationary conditions, the Mg nucleation is controlled by vacancy-decorated bcc Nb nanoclusters rather than by vacancy clusters, as in undoped Mg samples.
BibTeX:
	@article{Macchi2012,
	  author = {Macchi, C. and Maurizio, C. and Checchetto, R. and Mariazzi, S. and Ravelli, L. and Egger, W. and Mengucci, P. and Bazzanella, N. and Miotello, A. and Somoza, A. and Brusa, R.S.},
	  title = {Niobium aggregation and vacancylike defect evolution in nanostructured Nb-doped Mg: Their role in the kinetics of the hydride-to-metal phase transformation},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2012},
	  volume = {85},
	  number = {21},
	  pages = {214117},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.214117},
	  doi = {http://doi.org/10.1103/PhysRevB.85.214117}
	}
	
Charakterisierung von Siliziumphotomultipliern
Johannes Schütze and Christoph Hager; Studienarbeit, Universität der Bundeswehr München, 2012.
BibTeX:
	@thesis{SchuetzeHager2012sa,
	  author = {Schütze, Johannes and Hager, Christoph},
	  title = {Charakterisierung von Siliziumphotomultipliern},
	  school = {Universität der Bundeswehr München},
	  year = {2012}
	}
	
Different Chromium Content and Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys
S. Sojak, V. Sluge?, V. Kršjak, W. Egger, L. Ravelli, M. Petriska, S. Stan?ek, M. Skarba, P. Priputen, K. Vitázek, M. Stacho, J. Veterníková and V. Sabelová; Physics Procedia 35 (2012) 80–85.
Abstract: Reduced activation ferritic/martensitic steels (RAFM) represented by binary Fe-Cr alloys, with different chromium content, were studied in as-received state as well as after helium ions implantation. In order to study changes in dependence on the temperature, thermal annealing of He ions implanted Fe-11.62%Cr specimens was performed. Measurements by Pulsed Low Energy Positron System (PLEPS) in Garching, Germany were performed afterwards. Annealing out of defects at lower temperatures was not significant as was expected and some uncertainties are present. Extensive decrease of positron lifetime of defects was observed in specimens annealed at temperature 600 degrees C.
BibTeX:
	@article{Sojak2012,
	  author = {Sojak, S. and Sluge?, V. and Kršjak, V. and Egger, W. and Ravelli, L. and Petriska, M. and Stan?ek, S. and Skarba, M. and Priputen, P. and Vitázek, K. and Stacho, M. and Veterníková, J. and Sabelová, V.},
	  title = {Different Chromium Content and Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys},
	  booktitle = {POSITRON STUDIES OF DEFECTS 2011},
	  journal = {Physics Procedia},
	  year = {2012},
	  volume = {35},
	  pages = {80–85},
	  editor = {Eijt, S and Schut, H},
	  note = {7th International Workshop on Positron Studies of Defects (PSD), Delft Univ Technol, Delft, NETHERLANDS, AUG 28-SEP 02, 2011},
	  url = {http://www.sciencedirect.com/science/article/pii/S1875389212019529},
	  doi = {http://doi.org/10.1016/j.phpro.2012.06.015}
	}
	

2011

Enhanced kinetics of hydride-metal phase transition in magnesium by vacancy clustering
R. Checchetto, N. Bazzanella, A. Kale, A. Miotello, S. Mariazzi, R. Brusa, P. Mengucci, C. Macchi, A. Somoza, W. Egger and L. Ravelli; Physical Review B - Condensed Matter and Materials Physics 84 (5) (2011) 054115.
Abstract: The relation between vacancies and vacancy clusters evolution and the H2 desorption kinetics was studied in nanocrystalline Mg samples submitted to successive H2 sorption cycles. Vacancy defects were detected by positron annihilation lifetime spectroscopy while the desorption process was monitored measuring the H2 desorption flux. During H 2 sorption cycles, vacancies disappear, the number of vacancy clusters increases, and the crystalline quality of the Mg grains increases. The disappearance of intragranular vacancies is followed by an acceleration of the H2 desorption process. This is attributed to the increase of vacancy clusters at grain boundaries which assist the Mg nucleation in the hydride to metal phase transition. For H2 sorption cycles, the values of vacancy and vacancy cluster concentrations were obtained into the frame of the positron diffusion trapping model and the size of the involved vacancy clusters was evaluated by ab initio calculations of positron annihilation rates in Mg.
BibTeX:
	@article{Checchetto2011,
	  author = {Checchetto, R. and Bazzanella, N. and Kale, A. and Miotello, A. and Mariazzi, S. and Brusa, R.S. and Mengucci, P. and Macchi, C. and Somoza, A. and Egger, W. and Ravelli, L.},
	  title = {Enhanced kinetics of hydride-metal phase transition in magnesium by vacancy clustering},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2011},
	  volume = {84},
	  number = {5},
	  pages = {054115},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.84.054115},
	  doi = {http://doi.org/10.1103/PhysRevB.84.054115}
	}
	
Layer-resolved study of the Mg to MgH2 transformation in Mg-Ti films with short-range chemical order
S. Eijt, H. Leegwater, H. Schut, A. Anastasopol, W. Egger, L. Ravelli, C. Hugenschmidt and B. Dam; Journal of Alloys and Compounds 509 (SUPPL. 2) (2011) S567-S571.
Abstract: Positron depth-profiling was applied to monitor the effects of hydrogenation on Mg1-yTiy thin films. S-W diagrams and VEPFIT analysis of the depth-profiles demonstrated the homogeneity of most metal and metal hydride films. In contrast, Mg0.90Ti0.10H x films consisted of a double layer, with a thin unloaded Mg 0.90Ti0.10 or Mg-Ti-Pd alloy layer on top of a hydrogenated bottom layer. The metal-to-metal-hydride transformation of Mg domains in the nanoscale phase-segregated Mg-Ti films was monitored exclusively, enabled by the large difference in positron affinity for Mg and Ti. The changes in the Doppler broadening parameters revealed that the metal-insulator transition for fluorite MgH2 is similar to that for rutile MgH 2. Positron lifetime spectroscopy showed the presence of di-vacancies in the metal sub-lattice of as-deposited and hydrogenated Mg-Ti metal films, which may induce the fast hydrogen sorption kinetics of the fluorite MgH 2 phase.
BibTeX:
	@article{Eijt2011,
	  author = {Eijt, S.W.H. and Leegwater, H. and Schut, H. and Anastasopol, A. and Egger, W. and Ravelli, L. and Hugenschmidt, C. and Dam, B.},
	  title = {Layer-resolved study of the Mg to MgH2 transformation in Mg-Ti films with short-range chemical order},
	  journal = {Journal of Alloys and Compounds},
	  year = {2011},
	  volume = {509},
	  number = {SUPPL. 2},
	  pages = {S567-S571},
	  url = {http://www.sciencedirect.com/science/article/pii/S0925838810023911},
	  doi = {http://doi.org/10.1016/j.jallcom.2010.09.157}
	}
	
Study of defects in an electroresistive Au/La2/3Sr 1/3MnO3/SrTiO3(001) heterostructure by positron annihilation
R. Ferragut, A. Dupaquier, S. Brivio, R. Bertacco and W. Egger; Journal of Applied Physics 110 (5) (2011) 053511.
Abstract: Defects in an ultrathin Au/La2/3Sr1/3MnO 3/SrTiO3 (Au/LSMO/STO) heterostructure displaying electroresistive behavior were studied using variable energy positron annihilation spectroscopy. Vacancy-like defects were found to be the dominant positron traps in the LSMO and STO thin perovskite oxides with a number density >1017 cm-3 and 2 × 1017 cm -3 in the STO substrate. High defect density was revealed by strong positron trapping at the Au/LSMO interface. Oxygen deficiency in LSMO would be the main source of these traps. Besides, a low density of sub-nano voids of ?6Å was found in the substrate and in the thin LSMO/STO films. © 2011 American Institute of Physics.
BibTeX:
	@article{Ferragut2011,
	  author = {Ferragut, R. and Dupaquier, A. and Brivio, S. and Bertacco, R. and Egger, W.},
	  title = {Study of defects in an electroresistive Au/La2/3Sr 1/3MnO3/SrTiO3(001) heterostructure by positron annihilation},
	  journal = {Journal of Applied Physics},
	  year = {2011},
	  volume = {110},
	  number = {5},
	  pages = {053511},
	  url = {http://aip.scitation.org/doi/10.1063/1.3631825},
	  doi = {http://doi.org/10.1063/1.3631825}
	}
	
Free volume changes on optical switching in azobenzene- polymethylmethacrylate blends studied by a pulsed low-energy positron beam
S. Harms, K. Rätzke, C. Pakula, V. Zaporojtchenko, T. Strunskus, W. Egger, P. Sperr and F. Faupel; Journal of Polymer Science, Part B: Polymer Physics 49 (6) (2011) 404-408.
Abstract: Polymers including chromophores, which can be switched by light, have been studied extensively during the last years due to a host of potential applications which arise from the marked changes in physical properties on switching. Even though there is clear evidence that the free volume has a significant influence on the isomerization kinetics, the question of free volume changes on switching was only addressed recently. Using a pulsed low-energy positron beam the ortho-positronium lifetime ?3 was taken as a very sensitive free volume probe, and no change in free volume was detected on isomerization in an azobenzene-polymethylmethacrylate (PMMA) copolymer containing about 8 wt % of the azobenzene moiety. Here, we report for the first time on free volume changes in an azobenzene-PMMA blend with an azobenzene moiety concentration as high as 40 wt %. Using the same pulsed low-energy positron beam, small but significant changes of ?3 were observed between the structurally relaxed dark and the UV-illuminated states suggesting a decrease in free volume of the order of 10%.
BibTeX:
	@article{Harms2011,
	  author = {Harms, S. and Rätzke, K. and Pakula, C. and Zaporojtchenko, V. and Strunskus, T. and Egger, W. and Sperr, P. and Faupel, F.},
	  title = {Free volume changes on optical switching in azobenzene- polymethylmethacrylate blends studied by a pulsed low-energy positron beam},
	  journal = {Journal of Polymer Science, Part B: Polymer Physics},
	  year = {2011},
	  volume = {49},
	  number = {6},
	  pages = {404--408},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/polb.22201/abstract;jsessionid=4D54F7AB4F5618E0910C9AE9CE32EE19.f04t03},
	  doi = {http://doi.org/10.1002/polb.22201}
	}
	
Free volume distribution at the Teflon AF®/silicon interfaces probed by a slow positron beam
S. Harms, K. Rätzke, V. Zaporojtchenko, F. Faupel, W. Egger and L. Ravelli; Polymer 52 (2) (2011) 505-509.
Abstract: We performed positron annihilation lifetime spectroscopy experiments at Teflon AF®/silicon interfaces as function of the positron implantation energy to determine the free volume hole size distribution in the interfacial region and to investigate the width of the interphase. While no interphase was detected in very short chained solvent-free, thermally evaporated Teflon AF®, an interphase of some tens of nm in extension was observed for high molecular weight spin-coated Teflon AF® films. Influences of the native oxide layer on the data evaluation could be ruled out.
BibTeX:
	@article{Harms2011a,
	  author = {Harms, S. and Rätzke, K. and Zaporojtchenko, V. and Faupel, F. and Egger, W. and Ravelli, L.},
	  title = {Free volume distribution at the Teflon AF®/silicon interfaces probed by a slow positron beam},
	  journal = {Polymer},
	  year = {2011},
	  volume = {52},
	  number = {2},
	  pages = {505--509},
	  url = {http://www.sciencedirect.com/science/article/pii/S0032386110010347},
	  doi = {http://doi.org/10.1016/j.polymer.2010.11.039}
	}
	
Vacancy defects in CdTe thin films
D. Keeble, J. Major, L. Ravelli, W. Egger and K. Durose; Physical Review B - Condensed Matter and Materials Physics 84 (17) (2011) 174122.
Abstract: Vacancy defects have been investigated in a series of CdTe thin films grown by close-space sublimation. Variable-energy positron annihilation lifetime spectroscopy measurements, performed with a high-intensity positron beam, were used to profile polycrystalline films with varying grain size. These were obtained by changing the nitrogen pressure used during deposition. Two vacancy defects were detected, with positron lifetimes of 321(7)ps and 450(30) ps, respectively. Density functional theory calculations support the assignment of the first to the Cd vacancy and provide evidence that the second is the divacancy defect.
BibTeX:
	@article{Keeble2011,
	  author = {Keeble, D.J. and Major, J.D. and Ravelli, L. and Egger, W. and Durose, K.},
	  title = {Vacancy defects in CdTe thin films},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2011},
	  volume = {84},
	  number = {17},
	  pages = {174122},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.84.174122},
	  doi = {http://doi.org/10.1103/PhysRevB.84.174122}
	}
	
Suppression of vacancy defects in epitaxial La-doped SrTiO 3 films
D. Keeble, B. Jalan, L. Ravelli, W. Egger, G. Kanda and S. Stemmer; Applied Physics Letters 99 (23) (2011) 232905.
Abstract: Variable energy positron annihilation lifetime spectroscopy of high-mobility La-doped SrTiO 3 grown by molecular beam epitaxy found that the films contained sufficiently low concentrations of Sr vacancies and vacancy cluster defects to allow the observation of positron annihilation events from the perfect lattice. This enabled the concentrations of charged cation vacancies to be estimated, and these were found to be at least an order of magnitude below the La-dopant concentrations.
BibTeX:
	@article{Keeble2011a,
	  author = {Keeble, D.J. and Jalan, B. and Ravelli, L. and Egger, W. and Kanda, G. and Stemmer, S.},
	  title = {Suppression of vacancy defects in epitaxial La-doped SrTiO 3 films},
	  journal = {Applied Physics Letters},
	  year = {2011},
	  volume = {99},
	  number = {23},
	  pages = {232905},
	  url = {http://aip.scitation.org/doi/10.1063/1.3664398},
	  doi = {http://doi.org/10.1063/1.3664398}
	}
	
Konstruktion und Aufbau einer positionierbaren, kollimierten Gammaquelle zur Kalibrierung eines gepixelten Germaniumdetektors für die Positronenannihilation
Ricardo Riedel; Studienarbeit, Universität der Bundeswehr München, 2011.
BibTeX:
	@thesis{Riedel2011sa,
	  author = {Ricardo Riedel},
	  title = {Konstruktion und Aufbau einer positionierbaren, kollimierten Gammaquelle zur Kalibrierung eines gepixelten Germaniumdetektors für die Positronenannihilation},
	  school = {Universität der Bundeswehr München},
	  year = {2011}
	}
	
Fe-Cr alloys behavior after helium implantation
V. Slugen, V. Krsjak, W. Egger, M. Petriska, S. Sojak and J. Veternikova; Journal of Nuclear Materials 409 (2) (2011) 163-166.
Abstract: The paper discusses our recent experiments focused on the chromium influence on the microstructural changes of iron based alloys under radiation treatment. Our experimental method - the positron annihilation lifetime spectroscopy (PALS) enables an observation of size and density changes of the vacancy type defects in the material microstructure. These defects have been created by implantation of charged particles (He2+). The cascade collisions in the crystal lattice and following Frenkel pair creation have been considered as possible approximation of the neutron flux damage up to 100 DPA in the region up to 1 ?m from the surface.
BibTeX:
	@article{Slugen2011,
	  author = {Slugen, V. and Krsjak, V. and Egger, W. and Petriska, M. and Sojak, S. and Veternikova, J.},
	  title = {Fe-Cr alloys behavior after helium implantation},
	  journal = {Journal of Nuclear Materials},
	  year = {2011},
	  volume = {409},
	  number = {2},
	  pages = {163--166},
	  url = {http://www.sciencedirect.com/science/article/pii/S002231151000499X},
	  doi = {http://doi.org/10.1016/j.jnucmat.2010.09.023}
	}
	

2010

Structural characterization of H plasma-doped ZnO single crystals by positron annihilation spectroscopies
W. Anwand, G. Brauer, T. Cowan, D. Grambole, W. Skorupa, J. ?ižek, J. Kuriplach, I. Procházka, W. Egger and P. Sperr; Physica Status Solidi (A) Applications and Materials Science 207 (11) (2010) 2415-2425.
Abstract: Nominally undoped, hydrothermally grown ZnO single crystals have been investigated before and after exposure to remote H plasma. Structural characterizations have been made by various positron annihilation spectroscopies (continuous and pulsed slow positron beams, conventional lifetime). The content of bound hydrogen (H-b) before and after the remote H plasma treatment at the polished side of the crystals was determined at depths of 100 and 600 nm, respectively, using nuclear reaction analysis. At a depth of 100 nm, H-b increased from (11.8 ± 2.5) to (48.7 ± 7.6) × 10 19 cm -3 after remote H plasma treatment, whereas at 600 nm no change in H-b was observed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
BibTeX:
	@article{Anwand2010,
	  author = {Anwand, W. and Brauer, G. and Cowan, T.E. and Grambole, D. and Skorupa, W. and ?ižek, J. and Kuriplach, J. and Procházka, I. and Egger, W. and Sperr, P.},
	  title = {Structural characterization of H plasma-doped ZnO single crystals by positron annihilation spectroscopies},
	  journal = {Physica Status Solidi (A) Applications and Materials Science},
	  year = {2010},
	  volume = {207},
	  number = {11},
	  pages = {2415-2425},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssa.200925609/abstract},
	  doi = {http://doi.org/10.1002/pssa.200925609}
	}
	
Study of defects in implanted silica glass by depth profiling Positron Annihilation Spectroscopy
R. Brusa, S. Mariazzi, L. Ravelli, P. Mazzoldi, G. Mattei, W. Egger, C. Hugenschmidt, B. Löwe, P. Pikart, C. Macchi and A. Somoza; Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 268 (19) (2010) 3186-3190.
Abstract: Positron Annihilation Spectroscopy (PAS) performed with continuous and pulsed positron beams allows to characterize the size of the intrinsic nano-voids in silica glass, their in depth modification after ion implantation and their decoration by implanted ions. Three complementary PAS techniques, lifetime spectroscopy (LS), Doppler broadening spectroscopy (DBS) and coincidence Doppler broadening spectroscopy (CDBS) will be illustrated by presenting, as a case study, measurements obtained on virgin and gold implanted silica glass. © 2010 Elsevier B.V. All rights reserved.
BibTeX:
	@article{Brusa2010,
	  author = {Brusa, R.S. and Mariazzi, S. and Ravelli, L. and Mazzoldi, P. and Mattei, G. and Egger, W. and Hugenschmidt, C. and Löwe, B. and Pikart, P. and Macchi, C. and Somoza, A.},
	  title = {Study of defects in implanted silica glass by depth profiling Positron Annihilation Spectroscopy},
	  journal = {Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms},
	  year = {2010},
	  volume = {268},
	  number = {19},
	  pages = {3186-3190},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168583X10005343},
	  doi = {http://doi.org/10.1016/j.nimb.2010.05.084}
	}
	
Positron annihilation spectroscopy: A technique to probe the molecular level of nanofiltration membranes
A. Cano-Odena, P. Vandezande, K. Hendrix, R. Zaman, K. Mostafa, W. Egger, P. Sperr, J. De Baerdemaeker and I. Vankelecom; JOURNAL MISSING! (2010) 320-322.
BibTeX:
	@conference{Cano-Odena2010,
	  author = {Cano-Odena, A. and Vandezande, P. and Hendrix, K. and Zaman, R. and Mostafa, K. and Egger, W. and Sperr, P. and De Baerdemaeker, J. and Vankelecom, I.F.J.},
	  title = {Positron annihilation spectroscopy: A technique to probe the molecular level of nanofiltration membranes},
	  booktitle = {20th Annual Meeting of the North American Membrane Society and 11th International Conference on Inorganic Membranes 2010, NAMS/ICIM 2010},
	  publisher = {North American Membrane Society ( NAMS )},
	  year = {2010},
	  pages = {320--322}
	}
	
Pulsed low-energy positron beams: A useful tool to investigate defect structures in deformed metals and alloys
W. Egger, P. Sperr, G. Kögel and H.-J. Gudladt; Journal of Physics: Conference Series 240 (2010) 012164.
Abstract: To understand in more detail the behaviour of deformed metallic materials, the knowledge of defects and defect distributions at an atomistic level is important. To investigate the plastic zone in front of a crack tip, the positron annihilation lifetime spectroscopy with a pulsed monoenergetic positron beam of variable energy allows the detection of vacancies, dislocations, vacancy clusters and micro voids in the crack surface near region. Moreover, for a given defect type it is possible to determine its concentration. The positron lifetime measurements in samples of different materials (aluminium, copper) showed different defect profiles for crack surfaces produced by monotonic and cyclic deformation. In addition, this technique was able to characterize the kind of damage (monotonic or cyclic) by analysing the different positron lifetimes. © 2010 IOP Publishing Ltd.
BibTeX:
	@article{Egger2010,
	  author = {Egger, W. and Sperr, P. and Kögel, G. and Gudladt, H.-J.},
	  title = {Pulsed low-energy positron beams: A useful tool to investigate defect structures in deformed metals and alloys},
	  journal = {Journal of Physics: Conference Series},
	  year = {2010},
	  volume = {240},
	  pages = {012164},
	  url = {http://iopscience.iop.org/article/10.1088/1742-6596/240/1/012164/meta;jsessionid=43E8EFB1F3F805A71643DA6B40BEEFE9.ip-10-40-1-105},
	  doi = {http://doi.org/10.1088/1742-6596/240/1/012164}
	}
	
Pulsed low-energy positron beams in materials sciences
W. Egger; In: R. Brusa, A. Dupasquier and A.P. Mills jr. (Eds.), Physics with many Positrons , Vol. 174 , p. 419-449 , IOS Press , 2010.
Abstract: Pulsed low-energy positron beams of variable energy are powerful tools for defect profiling in materials. In this lecture we will at first describe two pulsed-beam systems developed over the last two decades: The Pulsed Low Energy Positron System (PLEPS) for depth-resolved defect profiling and the Scanning Positron Microscope (SPM), which in addition offers lateral resolution. We then consider some examples of applications of those pulsed beams to condensed matter problems. Next, the limits of those systems are discussed. Finally, we will give an outlook how pulsing with many positrons may be achieved and used for the purposes of materials sciences by combining existing experimental equipment with a strong positron source. © 2010 by Società Italiana di Fisica.
BibTeX:
	@incollection{Egger2010a,
	  author = {Egger, Werner},
	  title = {Pulsed low-energy positron beams in materials sciences},
	  booktitle = {Physics with many Positrons},
	  publisher = {IOS Press},
	  year = {2010},
	  volume = {174},
	  pages = {419--449},
	  editor = {Brusa, R. and Dupasquier, A. and Mills jr., A. P.},
	  url = {http://ebooks.iospress.nl/publication/26841},
	  doi = {http://doi.org/10.3254/978-1-60750-647-8-419}
	}
	
Positronium formation in porous materials for antihydrogen production
R. Ferragut, A. Calloni, A. Dupasquier, G. Consolati, F. Quasso, M. Giammarchi, D. Trezzi, W. Egger, L. Ravelli, M. Petkov, S. Jones, B. Wang, O. Yaghi, B. Jasinska, N. Chiodini and A. Paleari; Journal of Physics: Conference Series 225 (2010) 012007.
Abstract: Positronium (Ps) formation measurements in several porous materials as: Vycor, germanate Xerogel, Metal-Organic Frameworks MOF-177 and Aerogel with two densities (20 and 150 mg/cm3), were performed by means of a variable energy positron beam provided with a Ge detector and a positron lifetime spectrometer. An efficient formation of cooled Ps atoms is a requisite for the production of antihydrogen, with the aim of a direct measurement of the Earth gravitational acceleration g of antimatter, which is a primary scientific goal of AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy). Porous materials are necessary to form a high yield of Ps atoms as well as to cool Ps through collisions with the inner walls of the pores. The different materials were characterized and produce Ps into the pores. Lifetime measurements give an estimation of the typical pores dimension of the substances. A comparative study of the positron lifetime and the Ps fraction values in the above mentioned materials indicates that silica Aerogel, with the appropriate density, is an excellent candidate for an efficient formation of cold Ps atoms for the AEGIS project. © 2010 IOP Publishing Ltd.
BibTeX:
	@article{Ferragut2010,
	  author = {Ferragut, R. and Calloni, A. and Dupasquier, A. and Consolati, G. and Quasso, F. and Giammarchi, M.G. and Trezzi, D. and Egger, W. and Ravelli, L. and Petkov, M.P. and Jones, S.M. and Wang, B. and Yaghi, O.M. and Jasinska, B. and Chiodini, N. and Paleari, A.},
	  title = {Positronium formation in porous materials for antihydrogen production},
	  journal = {Journal of Physics: Conference Series},
	  year = {2010},
	  volume = {225},
	  pages = {012007},
	  url = {http://iopscience.iop.org/article/10.1088/1742-6596/225/1/012007/meta},
	  doi = {http://doi.org/10.1088/1742-6596/225/1/012007}
	}
	
Free volume and swelling in thin films of poly(N-isopropylacrylamide) end-capped with N-butyltrithiocarbonate
S. Harms, K. Rätzke, F. Faupel, W. Egger, L. Ravello, A. Laschewsky, W. Wang and P. Müller-Buschbaum; Macromolecular Rapid Communications 31 (15) (2010) 1364-1367.
Abstract: The free volume in thin films of poly(N-isopropylacrylamid) end-capped with n-butyltriocarbonate (nbc-PNIPAM) is probed with positron annihilation lifetime spectroscopy (PALS). The PALS measurements are performed as function of energy to obtain depth profiles of the free volume of nbc-PNIPAM films. The range of nbc-PNIPAM films with thicknesses from 40 to 200 nm is focused. With decreasing film thickness the free volume increases in good agreement with an increase in the maximum swelling capability of the nbc-PNIPAM films. Thus in thin hydrogel films the sorption and swelling behavior is governed by free volume. (Figure Presented) © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
BibTeX:
	@article{Harms2010,
	  author = {Harms, S. and Rätzke, K. and Faupel, F. and Egger, W. and Ravello, L. and Laschewsky, A. and Wang, W. and Müller-Buschbaum, P.},
	  title = {Free volume and swelling in thin films of poly(N-isopropylacrylamide) end-capped with N-butyltrithiocarbonate},
	  journal = {Macromolecular Rapid Communications},
	  year = {2010},
	  volume = {31},
	  number = {15},
	  pages = {1364-1367},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/marc.201000067/abstract},
	  doi = {http://doi.org/10.1002/marc.201000067}
	}
	
Identification of vacancy defects in a thin film perovskite oxide
D. Keeble, R. MacKie, W. Egger, B. Löwe, P. Pikart, C. Hugenschmidt and T. Jackson; Physical Review B - Condensed Matter and Materials Physics 81 (6) (2010) 064102.
Abstract: Vacancies are the dominant point defects in perovskite oxides, however, detecting and identifying the nature of vacancy defects in thin films remains challenging. This can be achieved using electron-beam methods but concentrations of several percent are required. Here we use a high-flux positron beam, providing high statistics positron lifetime measurements, to identify vacancies in laser ablated SrTiO3 on SrTiO3. The method is capable of subparts per million sensitivity and when combined with density-functional theory provides local structure information. The positron lifetime spectrum depth profile detects the presence of large vacancy clusters in a surface layer, a uniform distribution of Sr vacancies through the bulk of the film and resolves the interface with the substrate. © 2010 The American Physical Society.
BibTeX:
	@article{Keeble2010,
	  author = {Keeble, D.J. and MacKie, R.A. and Egger, W. and Löwe, B. and Pikart, P. and Hugenschmidt, C. and Jackson, T.J.},
	  title = {Identification of vacancy defects in a thin film perovskite oxide},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2010},
	  volume = {81},
	  number = {6},
	  pages = {064102},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.81.064102},
	  doi = {http://doi.org/10.1103/PhysRevB.81.064102}
	}
	
Identification of A- and B-site cation vacancy defects in perovskite oxide thin films
D. Keeble, S. Wicklein, R. Dittmann, L. Ravelli, R. MacKie and W. Egger; Physical Review Letters 105 (22) (2010) 226102.
Abstract: Cation vacancies on both sublattices (VTi, VSr) have been identified in homoepitaxial pulsed laser deposited SrTiO3 films using high intensity variable energy positron annihilation lifetime spectroscopy (PALS) measurements. Film nonstoichiometry was varied by varying laser fluence. PALS showed that on increasing the fluence above the Ti/Sr?1 value, the concentration ratio [VSr]/[VTi] systematically increased. Reducing the fluence into the Ti-poor region below resulted in additional vacancy cluster defect formation. Vacancy concentrations greater than ?50ppm were observed in all films. © 2010 The American Physical Society.
BibTeX:
	@article{Keeble2010a,
	  author = {Keeble, D.J. and Wicklein, S. and Dittmann, R. and Ravelli, L. and MacKie, R.A. and Egger, W.},
	  title = {Identification of A- and B-site cation vacancy defects in perovskite oxide thin films},
	  journal = {Physical Review Letters},
	  year = {2010},
	  volume = {105},
	  number = {22},
	  pages = {226102},
	  url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.105.226102},
	  doi = {http://doi.org/10.1103/PhysRevLett.105.226102}
	}
	
Divacancies and the hydrogenation of Mg-Ti films with short range chemical order
H. Leegwater, H. Schut, W. Egger, A. Baldi, B. Dam and S. Eijt; Applied Physics Letters 96 (12) (2010) 121902.
Abstract: We obtained evidence for the partial chemical segregation of as-deposited and hydrogenated Mg1-yTiy films (0?y?0.30) into nanoscale Ti and Mg domains using positron Doppler-broadening. We exclusively monitor the hydrogenation of Mg domains, owing to the large difference in positron affinity for Mg and Ti. The electron momentum distribution broadens significantly upon transformation to the MgH2 phase over the whole compositional range. This reveals the similarity of the metal-insulator transition for rutile and fluorite MgH2. Positron lifetime studies show the presence of divacancies in the as-deposited and hydrogenated Mg-Ti metal films. In conjunction with the relatively large local lattice relaxations we deduce to be present in fluorite MgH2, these may be responsible for the fast hydrogen sorption kinetics in this MgH2 phase. © 2010 American Institute of Physics.
BibTeX:
	@article{Leegwater2010,
	  author = {Leegwater, H. and Schut, H. and Egger, W. and Baldi, A. and Dam, B. and Eijt, S.W.H.},
	  title = {Divacancies and the hydrogenation of Mg-Ti films with short range chemical order},
	  journal = {Applied Physics Letters},
	  year = {2010},
	  volume = {96},
	  number = {12},
	  pages = {121902},
	  url = {http://aip.scitation.org/doi/10.1063/1.3368698},
	  doi = {http://doi.org/10.1063/1.3368698}
	}
	
Ordered nanoporous membranes based on diblock copolymers with high chemical stability and tunable separation properties
X. Li, C.-A. Fustin, N. Lefèvre, J.-F. Gohy, S. Feyter, J. Baerdemaeker, W. Egger and I. Vankelecom; Journal of Materials Chemistry 20 (21) (2010) 4333-4339.
Abstract: Block copolymers, having the ability to self-assemble into arrays of well-defined nanostructures, can be turned into thin films. Their application as membrane was so far strongly limited by the fact that these thin films have to be transferred manually onto a porous membrane support, or because of the very strict preparation conditions. A simple method is reported here to directly produce ordered nanoporous membranes on porous supports. Well-ordered membrane structures were prepared via a simple strategy exploiting blending of a block copolymer (PS-b-PEO) with a homopolymer (PAA), involving no thermal or solvent treatment. The membranes could be directly introduced on several types of porous membrane supports via straightforward spin or dip coating of very small amounts of polymer. Moreover, the permeability of the membranes could be readily tuned by removal of the PAA without changing membrane morphology. While already inherently stable in e.g. sodium hypochlorite solutions, the chemical stability of the membranes could be further enhanced via simple UV-radiation, clearly widening their potential application field. © The Royal Society of Chemistry 2010.
BibTeX:
	@article{Li2010,
	  author = {Li, X. and Fustin, C.-A. and Lefèvre, N. and Gohy, J.-F. and Feyter, S.D. and Baerdemaeker, J.D. and Egger, W. and Vankelecom, I.F.J.},
	  title = {Ordered nanoporous membranes based on diblock copolymers with high chemical stability and tunable separation properties},
	  journal = {Journal of Materials Chemistry},
	  year = {2010},
	  volume = {20},
	  number = {21},
	  pages = {4333-4339},
	  url = {http://pubs.rsc.org/en/Content/ArticleLanding/2010/JM/b926774c#!divAbstract},
	  doi = {http://doi.org/10.1039/b926774c}
	}
	
Experimental elucidation of vacancy complexes associated with hydrogen ion-induced splitting of bulk GaN
O. Moutanabbir, R. Scholz, U. Gösele, A. Guittoum, M. Jungmann, M. Butterling, R. Krause-Rehberg, W. Anwand, W. Egger and P. Sperr; Physical Review B - Condensed Matter and Materials Physics 81 (11) (2010) 115205.
Abstract: We present a detailed study of the thermal evolution of H ion-induced vacancy related complexes and voids in bulk GaN implanted under ion-cut conditions. By using transmission electron microscopy, we found that the damage band in as-implanted GaN is decorated with a high density of nanobubbles of ?1-2nm in diameter. Variable energy Doppler broadening spectroscopy showed that this band contains vacancy clusters and voids. In addition to vacancy clusters, the presence of VGa, VGa -H2, and VGaVN complexes was evidenced by pulsed low-energy positron lifetime spectroscopy. Subtle changes upon annealing in these vacancy complexes were also investigated. As a general trend, a growth in open-volume defects is detected in parallel to an increase in both size and density of nanobubbles. The observed vacancy complexes appear to be stable during annealing. However, for temperatures above 450°C, unusually large lifetimes were measured. These lifetimes are attributed to the formation of positronium in GaN. Since the formation of positronium is not possible in a dense semiconductor, our finding demonstrates the presence of sufficiently large open-volume defects in this temperature range. Based on the Tao-Eldrup model, the average lattice opening during thermal annealing was quantified. We found that a void diameter of 0.4 nm is induced by annealing at 600°C. The role of these complexes in the subsurface microcracking is discussed. © 2010 The American Physical Society.
BibTeX:
	@article{Moutanabbir2010,
	  author = {Moutanabbir, O. and Scholz, R. and Gösele, U. and Guittoum, A. and Jungmann, M. and Butterling, M. and Krause-Rehberg, R. and Anwand, W. and Egger, W. and Sperr, P.},
	  title = {Experimental elucidation of vacancy complexes associated with hydrogen ion-induced splitting of bulk GaN},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2010},
	  volume = {81},
	  number = {11},
	  pages = {115205},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.81.115205},
	  doi = {http://doi.org/10.1103/PhysRevB.81.115205}
	}
	
Open volume in bioadhesive detected by positron annihilation lifetime spectroscopy
K. Rätzke, M. Wiegemann, M. Shaikh, S. Harms, R. Adelung, W. Egger and P. Sperr; Acta Biomaterialia 6 (7) (2010) 2690-2694.
Abstract: Barnacles attach to a wide variety of surfaces underwater and show substrate-specific adhesion mechanisms. Investigating and understanding these mechanisms is a key for developing new technical adhesives. We expected open volume (porosity) at the sub-nanometre scale to occur in barnacle adhesive. With positron annihilation lifetime spectroscopy (PALS) it is possible to detect porosity at the nanometre scale by determining the lifetime of positrons. This method has not been applied to bioadhesives so far. We showed that PALS is a suitable technique for the investigation of the barnacle base and its adhesive with respect to open volume. The results were interpreted using a standard model adapted from polymers. We thereby estimated pore sizes of 0.5 nm. © 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
BibTeX:
	@article{Raetzke2010,
	  author = {Rätzke, K. and Wiegemann, M. and Shaikh, M.Q. and Harms, S. and Adelung, R. and Egger, W. and Sperr, P.},
	  title = {Open volume in bioadhesive detected by positron annihilation lifetime spectroscopy},
	  journal = {Acta Biomaterialia},
	  year = {2010},
	  volume = {6},
	  number = {7},
	  pages = {2690-2694},
	  url = {http://www.sciencedirect.com/science/article/pii/S1742706109005807},
	  doi = {http://doi.org/10.1016/j.actbio.2009.12.039}
	}
	
In-vacancies in Si-doped InN
C. Rauch, F. Reurings, F. Tuomisto, T. Veal, C. McConville, H. Lu, W. Schaff, C. Gallinat, G. Koblmüller, J. Speck, W. Egger, B. Löwe, L. Ravelli and S. Sojak; Physica Status Solidi (A) Applications and Materials Science 207 (5) (2010) 1083-1086.
Abstract: The introduction of vacancy type point defects by Si doping in InN grown by plasma-assisted molecular beam epitaxy was studied using a monoenergetic positron beam. With the combination of positron lifetime and Doppler broadening measurements, compensating In-vacancy (V In) acceptors were identified in the material. For increasing Si doping an enhanced formation of VIn defects was observed, up to a concentration of c V = 7 × 10 17 cm -3 in the highest doped sample (ne = 6:6 × 10 20 cm -3). A strong inhomogeneity of the defect profile with a significant increase of the V In concentration toward the layer/substrate interface could be detected. Additionally, larger vacancy clusters containing several V In are formed in the proximity of the interface. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
BibTeX:
	@article{Rauch2010,
	  author = {Rauch, C. and Reurings, F. and Tuomisto, F. and Veal, T.D. and McConville, C.F. and Lu, H. and Schaff, W.J. and Gallinat, C.S. and Koblmüller, G. and Speck, J.S. and Egger, W. and Löwe, B. and Ravelli, L. and Sojak, S.},
	  title = {In-vacancies in Si-doped InN},
	  journal = {Physica Status Solidi (A) Applications and Materials Science},
	  year = {2010},
	  volume = {207},
	  number = {5},
	  pages = {1083-1086},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssa.200983120/abstract},
	  doi = {http://doi.org/10.1002/pssa.200983120}
	}
	
Irradiation-induced defects in InN and GaN studied with positron annihilation
F. Reurings, F. Tuomisto, W. Egger, B. Löwe, L. Ravelli, S. Sojak, Z. Liliental-Weber, R. Jones, K. Yu, W. Walukiewicz and W. Schaff; Physica Status Solidi (A) Applications and Materials Science 207 (5) (2010) 1087-1090.
Abstract: We use positron annihilation to study 2-MeV 4He + irradiated and subsequently rapid-thermal-annealed InN grown by molecular-beam epitaxy and GaN grown by metal-organic chemical-vapour deposition. The irradiation fluences were in the range 5×10 14-2×10 16 cm -2. In vacancies are introduced in the irradiation at a low rate of 100 cm -1, with their concentration saturating in the mid-1017 cm -3 range at an irradiation fluence of 2×10 15 cm -2. The annealing, performed at temperatures between 425 and 475 °C, is observed to result in an inhomogeneous redistribution of the In vacancies. The behaviour is opposite to GaN, where Ga vacancies are introduced at a much higher rate of 3600 cm -1 showing no detectable saturation. About half of the Ga vacancies are found to recover in the annealing, in agreement with previous studies, while the remaining Ga vacancies undergo no spatial redistribution. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
BibTeX:
	@article{Reurings2010,
	  author = {Reurings, F. and Tuomisto, F. and Egger, W. and Löwe, B. and Ravelli, L. and Sojak, S. and Liliental-Weber, Z. and Jones, R.E. and Yu, K.M. and Walukiewicz, W. and Schaff, W.J.},
	  title = {Irradiation-induced defects in InN and GaN studied with positron annihilation},
	  journal = {Physica Status Solidi (A) Applications and Materials Science},
	  year = {2010},
	  volume = {207},
	  number = {5},
	  pages = {1087-1090},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssa.200983111/abstract},
	  doi = {http://doi.org/10.1002/pssa.200983111}
	}
	

2009

Characterization of ZnO nanostructures: A challenge to positron annihilation spectroscopy and other methods
G. Brauer, W. Anwand, D. Grambole, W. Egger, P. Sperr, I. Beinik, L. Wang, C. Teichert, J. Kuriplach, J. Lang, S. Zviagins, E. Cizmar, C.C. Ling, Y.F. Hsu, Y.Y. Xi, X. Chen, A.B. Djurisic and W. Skorupa; Physica Status Solidi (C) Current Topics in Solid State Physics 6 (11) (2009) 2556-2560.
Abstract: ZnO nanostructures are of special interest for device applications. However, their structural characterization remains an ongoing challenge. This paper reviews recent efforts and latest achievements in this direction. Results comprise PAS in the form of Slow Positron Implantation Spectroscopy (SPIS) and Pulsed Low Energy Positron Lifetime Spectroscopy (PLEPS), Nuclear Reaction Analysis (NRA), Atomic Force Microscopy (AFM), conductive AFM (C-AFM), Nuclear Magnetic Resonance (NMR), Electron Spin Resonance (ESR), Photoluminescence (PL) spectroscopy, and latest theoretical investigations of structure-related and positron properties of selected defects. The fundamental importance of a relationship between fabrication conditions, native defect formation, and resulting optical and electronic properties is demonstrated by getting either inferior (nanorods) or significantly improved (tetrapods) optical properties compared to single crystal samples, depending on the nanostructure fabrication method.
BibTeX:
	@article{Brauer2009,
	  author = {Brauer, Gerhard and Anwand, Wolfgang and Grambole, Dieter and Egger, Werner and Sperr, Peter and Beinik, Igor and Wang, Lin and Teichert, Christian and Kuriplach, Jan and Lang, Jan and Zviagins, Sergei and Cizmar, Erik and Ling, Chi Chung and Hsu, Yuk Fan and Xi, Yan Yan and Chen, Xinyi and Djurisic, Aleksandra B. and Skorupa, Wolfgang},
	  title = {Characterization of ZnO nanostructures: A challenge to positron annihilation spectroscopy and other methods},
	  journal = {Physica Status Solidi (C) Current Topics in Solid State Physics},
	  year = {2009},
	  volume = {6},
	  number = {11},
	  pages = {2556-2560},
	  note = {ICPA 15},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssc.200982081/abstract},
	  doi = {http://doi.org/10.1002/pssc.200982081}
	}
	
Probing the Molecular Level of Polyimide-Based Solvent Resistant Nanofiltration Membranes with Positron Annihilation Spectroscopy
A. Cano-Odena, P. Vandezande, K. Hendrix, R. Zaman, K. Mostafa, W. Egger, P. Sperr, J. De Baerdemaeker and I.F.J. Vankelecom; Journal of Physical Chemistry B 113 (30) (2009) 10170–10176.
Abstract: Positron annihilation spectroscopy (PAS) has been performed to link fundamental polymer properties to membrane performance, more specifically for polyimide (PI)-based solvent-resistant nanofiltration membranes. Laboratory-made membranes with well-known properties were applied first to define proper pretreatment conditions for the membrane to allow PAS-analysis and to allow more correct linking of PAS results to membrane properties. This knowledge was then applied to probe the structure of commercial PI-based Starmem membranes.
BibTeX:
	@article{Cano-Odena2009,
	  author = {Cano-Odena, Angels and Vandezande, Pieter and Hendrix, Katrien and Zaman, Rolph and Mostafa, Khaled and Egger, Werner and Sperr, Peter and De Baerdemaeker, Jeremie and Vankelecom, Ivo F. J.},
	  title = {Probing the Molecular Level of Polyimide-Based Solvent Resistant Nanofiltration Membranes with Positron Annihilation Spectroscopy},
	  journal = {Journal of Physical Chemistry B},
	  year = {2009},
	  volume = {113},
	  number = {30},
	  pages = {10170–10176},
	  url = {http://pubs.acs.org/doi/abs/10.1021/jp9012653},
	  doi = {http://doi.org/10.1021/jp9012653}
	}
	
Hydrogen-induced defects in Pd films
J. Cizek, I. Prochazka, O. Melikhova, M. Vlach, N. Zaludova, G. Brauer, W. Anwand, W. Egger, P. Sperr, C. Hugenschmidt, R. Gemma, A. Pundt and R. Kirchheim; Physica Status Solidi (C) Current Topics in Solid State Physics 6 (11) (2009) 2364-2366.
Abstract: Hydrogen absorbed in crystalline solids causes a lattice expansion and the formation of hydride phases. Contrary to free standing bulk samples, thin films are fixed at substrates, which prevent their in-plane expansion. This makes hydrogen-induced expansion of thin films highly anisotropic and leads to the formation of high stresses in hydrogen loaded thin films. As a consequence, lattice defects may be created in thin films loaded with hydrogen. This work reports about defects created by hydrogen loading in epitaxial Pd films deposited on Al(2)O(3) substrates by cold cathode beam sputtering. Hydrogen-induced defects are characterized by positron annihilation spectroscopy performed with variable energy slow positron beams. Extended studies of defect depth profile and its development with increasing concentration of hydrogen are performed by measurement of Doppler broadening of annihilation profile using a continuous positron beam. Selected states are investigated also by positron lifetime spectroscopy on an intense pulsed positron beam. Firstly, the microstructure of virgin films is characterized. Subsequently, the hydrogen concentration in the films is increased step-by-step by electrochemical charging. The development of the film microstructure and the evolution of defects are investigated.
BibTeX:
	@article{Cizek2009,
	  author = {Cizek, Jakub and Prochazka, Ivan and Melikhova, Oksana and Vlach, Martin and Zaludova, Nada and Brauer, Gerhard and Anwand, Wolfgang and Egger, Werner and Sperr, Peter and Hugenschmidt, Christoph and Gemma, Ryota and Pundt, Astrid and Kirchheim, Reiner},
	  title = {Hydrogen-induced defects in Pd films},
	  journal = {Physica Status Solidi (C) Current Topics in Solid State Physics},
	  year = {2009},
	  volume = {6},
	  number = {11},
	  pages = {2364-2366},
	  note = {ICPA 15},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssc.200982064/abstract},
	  doi = {http://doi.org/10.1002/pssc.200982064}
	}
	
Nature of defects induced by Au implantation in hexagonal silicon carbide single crystals
A. Gentils, M.-F. Barthe, W. Egger and P. Sperr; AIP Conference Proceedings 1099 (2009) 891-895.
Abstract: Pulsed-slow-positron-beam-based positron lifetime spectroscopy was used to investigate the nature of vacancy defects induced by 20 MeV An implantation in single crystals 6H-SiC. Preliminary analysis of the data shows that at lower fluence, below 10(14) cm(-2), a positron lifetime of 220 ps has been obtained: it could be associated with the divacancy V(Si)-V(C) in comparison with the literature. At higher fluence, above 10(15) cm(-2), a positron lifetime of 260-270 ps, increasing with the incident positron energy, has been observed after decomposition of the lifetime spectra. By comparison with lifetime calculations, open-volumes such as quadrivacancy (V(Si)-V(C))(2) clusters could be associated with this value.
BibTeX:
	@article{Gentils2009,
	  author = {Gentils, Aurelie and Barthe, Marie-France and Egger, Werner and Sperr, Peter},
	  title = {Nature of defects induced by Au implantation in hexagonal silicon carbide single crystals},
	  journal = {AIP Conference Proceedings},
	  year = {2009},
	  volume = {1099},
	  pages = {891--895},
	  note = {20th International Conference on Application of Accelerators in Research and Industry, Ft Worth, TX, AUG 10-15, 2008},
	  url = {http://aip.scitation.org/doi/abs/10.1063/1.3120183},
	  doi = {http://doi.org/10.1063/1.3120183}
	}
	
Microstructural study of He-implanted Fe-Cr alloys with the use of conventional lifetime technique and pulsed low energy positron beam
V. Kršjak, V. Sluge?, M. Petriska, S. Sojak and W. Egger; Physica Status Solidi (C) Current Topics in Solid State Physics 6 (11) (2009) 2339-2341.
Abstract: Experimental simulation of the radiation damage using He+ implantation has been performed in research of Fe-Cr model alloys. Different chromium content in the studied materials enables investigations of the effect of this element on the microstructure of radiation treated materials. The damaged region was investigated with the positron lifetime techniques with focus on the size and distribution of the defects. Our measurements show that not only pulsed low energy positron beam spectroscopy (PLEPS) but also conventional lifetime spectroscopy can study the behaviour of vacancy type defects induced by implantation of charged particles. Our results show that initial microstructure of low Cr alloys with significant presence of vacancy type defects is less resistant to creation of defect agglomeration in comparison with higher Cr alloys.
BibTeX:
	@article{Krsjak2009,
	  author = {Kršjak, Vladimir and Sluge?, Vladimir and Petriska, Martin and Sojak, Stanislav and Egger, Werner},
	  title = {Microstructural study of He-implanted Fe-Cr alloys with the use of conventional lifetime technique and pulsed low energy positron beam},
	  journal = {Physica Status Solidi (C) Current Topics in Solid State Physics},
	  year = {2009},
	  volume = {6},
	  number = {11},
	  pages = {2339-2341},
	  note = {ICPA 15},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssc.200982117/abstract},
	  doi = {http://doi.org/10.1002/pssc.200982117}
	}
	
Helium implanted FeCr alloys studied by positron annihilation lifetime technique
V. Kršjak, W. Egger, M. Petriska and S. Sojak; Problems of Atomic Science and Technology 4 (1) (2009) 109-115.
Abstract: The influence of chromium on the radiation damage resistance of iron based alloys has been studied using conventional positron lifetime technique and a pulsed low energy positron beam. To simulate high neutron flux, the helium implantation has been used. Different levels of helium doses ( 6.24.10(17)-3.12.10(18) cm(-2)) corresponding to a local damage of up to 90 dpa were accumulated in a thin 1 nm) and small vacancy clusters together with the initial dislocations and small point defects.
BibTeX:
	@article{Krsjak2009a,
	  author = {Kršjak, Vladimir and Egger, Werner and Petriska, Martin and Sojak, Stanislav},
	  title = {Helium implanted FeCr alloys studied by positron annihilation lifetime technique},
	  journal = {Problems of Atomic Science and Technology},
	  year = {2009},
	  volume = {4},
	  number = {1},
	  pages = {109--115},
	  note = {IAEA Technical Meeting on Accelerator Simulation and Theoretical Medeling of Radiation Effect, Kharkov Inst Phys & Technol, Natl Res Ctr, Kharkov, UKRAINE, JUN 09-13, 2008},
	  url = {http://vant.kipt.kharkov.ua/ANNOTAZII_2009/annotazii_2009_4_109.html}
	}
	
Positron annihilation studies on the nature and thermal behaviour of irradiation induced defects in tungsten
P.E. Lhuillier, M.F. Barthe, P. Desgardin, W. Egger and P. Sperr; Physica Status Solidi (C) Current Topics in Solid State Physics 6 (11) (2009) 2329-2332.
Abstract: Positron Annihilation Lifetime Spectroscopy has been performed with a pulsed positron beam to investigate the nature and evolution of implantation-induced defects created in the track region (TR) of 800 keV (3)He ions at different fluences. At high fluence - 5x10(16) cm(-2) - lifetime decomposition exhibits a predominant (98%) positron lifetime of 200 ps which is attributed to irradiation induced monovacancy. The increasing average lifetime as a function of the post-implantation annealing temperature has allowed to identify vacancy clustering due to vacancy migration (that occurs from 473 K).
BibTeX:
	@article{Lhuillier2009,
	  author = {Lhuillier, P. E. and Barthe, M. F. and Desgardin, P. and Egger, W. and Sperr, P.},
	  title = {Positron annihilation studies on the nature and thermal behaviour of irradiation induced defects in tungsten},
	  journal = {Physica Status Solidi (C) Current Topics in Solid State Physics},
	  year = {2009},
	  volume = {6},
	  number = {11},
	  pages = {2329-2332},
	  note = {ICPA 15},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssc.200982114/abstract},
	  doi = {http://doi.org/10.1002/pssc.200982114}
	}
	
Direct evidence by positron annihilation spectroscopy of defect distributions deeper than Rp in Ar+ implanted silica glass
P. Mazzoldi, G. Mattei, L. Ravelli, W. Egger, S. Mariazzi and R.S. Brusa; Journal of Physics D: Applied Physics 42 (11) (2009) 115418.
Abstract: Positron annihilation spectroscopy was used to depth profile the modification of intrinsic structural nanovoids in silica glass implanted with Ar+ ions at different fluences and implantation energies. Beyond an expected defect distribution below the ion projected range R-p, a second defect distribution extending more than two times deeper than R-p was revealed. This second defective layer was found to be related to recoiled oxygen atoms whose diffusion is probably increased by the stress gradient induced by the compaction of the first layer.
BibTeX:
	@article{Mazzoldi2009,
	  author = {Mazzoldi, P. and Mattei, G. and Ravelli, L. and Egger, W. and Mariazzi, S. and Brusa, R. S.},
	  title = {Direct evidence by positron annihilation spectroscopy of defect distributions deeper than Rp in Ar+ implanted silica glass},
	  journal = {Journal of Physics D: Applied Physics},
	  year = {2009},
	  volume = {42},
	  number = {11},
	  pages = {115418},
	  url = {http://iopscience.iop.org/0022-3727/42/11/115418/},
	  doi = {http://doi.org/10.1088/0022-3727/42/11/115418}
	}
	
Advanced Fe-Cr alloys studied by pulsed low energy positron system before and after helium ions implantation
S. Sojak, V. Kršjak and W. Egger; American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP 5 (2009) 469-472.
Abstract: Positron annihilation spectroscopy (PAS) is a non-destructive technique which provides information about microstructural damage of structural materials. In this paper, the Pulsed Low Energy Positron System (PLEPS) at the research reactor FRM-II at TU Munich was used to study depth profiling of binary Fe-Cr alloys. Fe-Cr model alloys with different chromium content were investigated in the as-received state as well as after helium ion implantation (dose up to 6.24x1017 ions/cm-2). Measured results show changes in the size of defects after implantation and also in non-implanted specimens depending on the Cr content.
BibTeX:
	@article{Sojak2009,
	  author = {Sojak, S. and Kršjak, V. and Egger, W.},
	  title = {Advanced Fe-Cr alloys studied by pulsed low energy positron system before and after helium ions implantation},
	  journal = {American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP},
	  year = {2009},
	  volume = {5},
	  pages = {469--472},
	  url = {http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1635396},
	  doi = {http://doi.org/10.1115/PVP2009-77537}
	}
	

2008

Investigations of epoxy-based adhesives with PLEPS
W. Egger, P. Sperr, G. Kögel, M. Wetzel and H.-J. Gudladt; Applied Surface Science 255 (1) (2008) 209-212.
Abstract: Contamination-tolerant adhesives are of ever increasing importance in industrial applications. The possible failure mechanisms of adhesive bonds in these adhesives are however still poorly understood. Results of a series of investigations with our pulsed low energy positron beam system (PLEPS) in an epoxy-based contamination-tolerant adhesive are presented.
BibTeX:
	@article{Egger2008,
	  author = {Egger, W. and Sperr, P. and Kögel, G. and Wetzel, M. and Gudladt, H.-J.},
	  title = {Investigations of epoxy-based adhesives with PLEPS},
	  journal = {Applied Surface Science},
	  year = {2008},
	  volume = {255},
	  number = {1},
	  pages = {209--212},
	  note = {SLOPOS 11},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433208012257},
	  doi = {http://doi.org/10.1016/j.apsusc.2008.05.205}
	}
	
On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range
P.M. Gordo, L. Liszkay, Z. Kajcsos, K. Havancsák, V.A. Skuratov, G. Kögel, P. Sperr, W. Egger, A.P. de Lima and M.F.F. Marques; Applied Surface Science 255 (1) (2008) 254-256.
Abstract: Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of similar to 1.5 nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids.
BibTeX:
	@article{Gordo2008,
	  author = {Gordo, P. M. and Liszkay, L. and Kajcsos, Zs. and Havancsák, K. and Skuratov, V. A. and Kögel, G. and Sperr, P. and Egger, W. and de Lima, A. P. and Marques, M. F. Ferreira},
	  title = {On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range},
	  journal = {Applied Surface Science},
	  year = {2008},
	  volume = {255},
	  number = {1},
	  pages = {254--256},
	  note = {SLOPOS 11},
	  url = {http://www.scientific.net/MSF.445-446.93},
	  doi = {http://doi.org/10.1016/j.apsusc.2008.05.185}
	}
	
Surface and bulk investigations at the high intensity positron beam facility NEPOMUC
C. Hugenschmidt, G. Dollinger, W. Egger, G. Kögel, B. Löwe, J. Mayer, P. Pikart, C. Piochacz, R. Repper, K. Schreckenbach, P. Sperr and M. Stadlbauer; Applied Surface Science 255 (1) (2008) 29-32.
Abstract: The NEutron-induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II delivers a low-energy positron beam (E = 15-1000 eV) of high intensity in the range between 4 × 107 and 5 × 108 moderated positrons per second. At present four experimental facilities are in operation at NEPOMUC: a coincident Doppler-broadening spectrometer (CDBS) for defect spectroscopy and investigations of the chemical vicinity of defects, a positron annihilation-induced Auger-electron spectrometer (PAES) for surface studies and an apparatus for the production of the negatively charged positronium ion Ps-. Recently, the pulsed low-energy positron system (PLEPS) has been connected to the NEPOMUC beam line, and first positron lifetime spectra were recorded within short measurement times. A positron remoderation unit which is operated with a tungsten single crystal in back reflection geometry has been implemented in order to improve the beam brilliance. An overview of NEPOMUC's status, experimental results and recent developments at the running spectrometers are presented.
BibTeX:
	@article{Hugenschmidt2008,
	  author = {Hugenschmidt, C. and Dollinger, G. and Egger, W. and Kögel, G. and Löwe, B. and Mayer, J. and Pikart, P. and Piochacz, C. and Repper, R. and Schreckenbach, K. and Sperr, P. and Stadlbauer, M.},
	  title = {Surface and bulk investigations at the high intensity positron beam facility NEPOMUC},
	  journal = {Applied Surface Science},
	  year = {2008},
	  volume = {255},
	  number = {1},
	  pages = {29--32},
	  note = {SLOPOS 11},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433208012269},
	  doi = {http://doi.org/10.1016/j.apsusc.2008.05.304}
	}
	
A positron remoderator for the high intensity positron source NEPOMUC
C. Piochacz, G. Kögel, W. Egger, C. Hugenschmidt, J. Mayer, K. Schreckenbach, P. Sperr, M. Stadlbauer and G. Dollinger; Applied Surface Science 255 (1) (2008) 98-100.
Abstract: A remoderator for the high intensity positron source NEPOMUC was developed and installed at the beam facility. A beam of remoderated positrons could be produced with different energies and a diameter of less than 2 mm was obtained. The efficiency of the remoderation setup was determined to be 5%. Due to the brilliance of the remoderated beam, the measurements at the coincidence Doppler broadening spectrometer (CDBS) and at the positron annihilation induced Auger electron spectrometer (PAES) could be improved. The setup and functionality of the remoderation device is presented as well as the first measurements at the remoderator, CDBS and PAES.
BibTeX:
	@article{Piochacz2008,
	  author = {Piochacz, C. and Kögel, G. and Egger, W. and Hugenschmidt, C. and Mayer, J. and Schreckenbach, K. and Sperr, P. and Stadlbauer, M. and Dollinger, G.},
	  title = {A positron remoderator for the high intensity positron source NEPOMUC},
	  journal = {Applied Surface Science},
	  year = {2008},
	  volume = {255},
	  number = {1},
	  pages = {98--100},
	  note = {SLOPOS 11},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433208011975},
	  doi = {http://doi.org/10.1016/j.apsusc.2008.05.286}
	}
	
Status of the pulsed low energy positron beam system (PLEPS) at the Munich Research Reactor FRM-II
P. Sperr, W. Egger, G. Kögel, G. Dollinger, C. Hugenschmidt, R. Repper and C. Piochacz; Applied Surface Science 255 (1) (2008) 35-38.
Abstract: The Munich pulsed low energy positron beam system (PLEPS) is now installed at the high intensity positron source (NEPOMUC) at the Munich Research Reactor FRM-II. In order to enhance the performance of the system several improvements have been implemented: two additional collinear detector ports have been installed. Therefore in addition to the normal lifetime measurements it is now possible to simultaneously perform Doppler-broadening, coincident Doppler-broadening and age momentum correlation experiments. An additional chopper was included to periodically suppress pulses and therefore to extend the standard time window of 20 ns for precise measurements of longer lifetimes. First test-experiments have been performed in May and July 2007. With all pulsing components in operation we achieved a count-rate of 1.4 × 104 counts per second. The total time resolution (pulsing and detector) was about 240 ps (FWHM) with a peak to background ratio up to 6 × 103:1.
BibTeX:
	@article{Sperr2008,
	  author = {Sperr, P. and Egger, W. and Kögel, G. and Dollinger, G. and Hugenschmidt, C. and Repper, R. and Piochacz, C.},
	  title = {Status of the pulsed low energy positron beam system (PLEPS) at the Munich Research Reactor FRM-II},
	  journal = {Applied Surface Science},
	  year = {2008},
	  volume = {255},
	  number = {1},
	  pages = {35--38},
	  note = {SLOPOS 11},
	  url = {http://www.sciencedirect.com/science/article/pii/S016943320801218X},
	  doi = {http://doi.org/10.1016/j.apsusc.2008.05.307}
	}
	

2007

Pulsed low energy positron system (PLEPS) at the Munich research reactor FRMII
W. Egger, P. Sperr, G. Kögel and G. Dollinger; Physica Status Solidi (C) Current Topics in Solid State Physics 4 (10) (2007) 3969-3972.
Abstract: Currently, the Munich pulsed low energy positron beam-system (PLEPS) is transferred to the high-intensity positron source (NEPOMUC) at the Munich research Reactor FRM II. We expect count-rates up to 106 cps. Several improvements to enhance the performance of the system have been implemented. Until now, PLEPS was employed exclusively for lifetime measurements. To enable also Doppler-broadening, coincident Doppler-broadening and AMOC experiments, two additional detector ports have been installed. An additional chopper allows to suppress pulses and, therefore, to extend the standard time window of 20 ns for precise measurements of longer lifetimes. The high event-rate supports the use of smaller scintillators. Consequently an improvement in time resolution is expected. Also envisaged is the use of new detector materials. Various measures to further reduce background from back-scattered positrons and a reduction of the beam diameter down to 1 mm will improve the overall performance of the system.
BibTeX:
	@article{Egger2007,
	  author = {Egger, W. and Sperr, P. and Kögel, G. and Dollinger, G.},
	  title = {Pulsed low energy positron system (PLEPS) at the Munich research reactor FRMII},
	  journal = {Physica Status Solidi (C) Current Topics in Solid State Physics},
	  year = {2007},
	  volume = {4},
	  number = {10},
	  pages = {3969--3972},
	  note = {ICPA 14},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssc.200675812/abstract},
	  doi = {http://doi.org/10.1002/pssc.200675812}
	}
	
Near surface stress determination in Kr-implanted polycrystalline titanium by the X-ray sin2?-method
M. Härting, S. Nsengiyumva, A. Raji, G. Dollinger, P. Sperr, S. Naidoo, T. Derry, C. Comrie and D. Britton; Surface and Coatings Technology 201 (19-20 SPEC. ISS.) (2007) 8237-8241.
Abstract: Ion implantation has been performed on polycrystalline titanium samples with 180 keV Kr+ ions at various doses from 1 × 1015 to 5 × 1016 ions cm- 2 at room temperature. The samples where characterised by Rutherford backscattering spectrometry, positron annihilation lifetime spectroscopy and X-ray diffraction. By means of the sin2? technique the near surface stress has been determined for both unimplanted and implanted samples. The initial stress state has been shown to be strongly tensile in the first 75 nm below the surface, and weakly compressive deeper inside. The main effect of the implantation process was to relax the pre-existing tensile stress in the track region. An additional compressive stress was introduced deeper in the sample and could be attributed to the presence of larger defect clusters.
BibTeX:
	@article{Haerting2007,
	  author = {Härting, M. and Nsengiyumva, S. and Raji, A.T. and Dollinger, G. and Sperr, P. and Naidoo, S.R. and Derry, T.E. and Comrie, C.M. and Britton, D.T.},
	  title = {Near surface stress determination in Kr-implanted polycrystalline titanium by the X-ray sin2?-method},
	  journal = {Surface and Coatings Technology},
	  year = {2007},
	  volume = {201},
	  number = {19-20 SPEC. ISS.},
	  pages = {8237--8241},
	  note = {14th International Conference on Surface Modification by Ion Beams (SMMIB 05), Kusadasi, TURKEY, SEP 04-09, 2005},
	  url = {http://www.sciencedirect.com/science/article/pii/S0257897207002587},
	  doi = {http://doi.org/10.1016/j.surfcoat.2006.02.072}
	}
	
Implementation of the Munich scanning positron microscope at the positron source NEPOMUC
C. Piochacz, W. Egger, C. Hugenschmidt, G. Kögel, K. Schreckenbach, P. Sperr and G. Dollinger; Physica Status Solidi (C) Current Topics in Solid State Physics 4 (10) (2007) 4028-4031.
Abstract: The Munich scanning positron microscope (SPM) permits positron lifetime measurements with a lateral resolution down to 2 um within an energy range of 1-20 keV. One practical limitation of the SPM is set by the long measurement times of several days per a 2D-scan due to the low intensity positron beam produced by standard 22Na sources. This disadvantage will be overcome by installing the SPM at the high intense positron beam facility NEPOMUC at the research reactor FRM II in Garching. Thus the time for one measurement will be shortened by a factor of 60. In addition it is expected to reduce the lateral resolution to about 100 nm. Due to the beam characteristics of the NEPOMUC facility an interface is needed, which enhances the phase space density of the beam. The requirements, which have to be fulfilled by the interface, will be described and an overview of the different components such as bunching units, remoderation stages and rf-elevator will be given.
BibTeX:
	@article{Piochacz2007,
	  author = {Piochacz, C. and Egger, W. and Hugenschmidt, C. and Kögel, G. and Schreckenbach, K. and Sperr, P. and Dollinger, G.},
	  title = {Implementation of the Munich scanning positron microscope at the positron source NEPOMUC},
	  journal = {Physica Status Solidi (C) Current Topics in Solid State Physics},
	  year = {2007},
	  volume = {4},
	  number = {10},
	  pages = {4028--4031},
	  note = {ICPA 14},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssc.200675824/abstract},
	  doi = {http://doi.org/10.1002/pssc.200675824}
	}
	

2006

Microstructural defect characterisation of a-Si:H deposited by low temperature HW-CVD on paper substrates
D.T. Britton, M. Härting, D. Knoesen, Z. Sigcau, F.P. Nemalili, T.P. Ntsoane, P. Sperr, W. Egger and M. Nippus; Thin Solid Films 501 (1-2) (2006) 79-83.
Abstract: Hydrogenated amorphous silicon has been deposited on 80 g m(-2) wood-free paper, with and without an intermediate metallic interlayer, using low temperature hot wire chemical vapor deposition (HW-CVD). Electrical measurements show these layers to be of good quality. In this paper we cornpare the differences in microstructural properties of the two types of layer, concentrating on the influence of the substrates, including their effect on the deposition rate of the material and substrate temperature. During the deposition process, the metallized substrates reach a higher temperature than plain paper. Both X-diffiraction and positron annihilation lifetime studies indicate that the growth rate on the uncoated substrate is slightly higher than with prior metallization. There is no evidence of a crystalline phase or voids in the a-Si:H layers, and the internal defect structure is similar, with a dominant dangling-bond complex of similar size.
BibTeX:
	@article{Britton2006,
	  author = {Britton, D. T. and Härting, M. and Knoesen, D. and Sigcau, Z. and Nemalili, F. P. and Ntsoane, T. P. and Sperr, P. and Egger, W. and Nippus, M.},
	  title = {Microstructural defect characterisation of a-Si:H deposited by low temperature HW-CVD on paper substrates},
	  journal = {Thin Solid Films},
	  year = {2006},
	  volume = {501},
	  number = {1-2},
	  pages = {79--83},
	  note = {3rd International Conference on Hot-Wire CVD Process, Utrecht Univ, Utrecht, NETHERLANDS, AUG 23-27, 2004},
	  url = {http://www.sciencedirect.com/science/article/pii/S0040609005009764},
	  doi = {http://doi.org/10.1016/j.tsf.2005.07.111}
	}
	
Decoration of buried surfaces in Si detected by positron annihilation spectroscopy
R. Brusa, C. Macchi, S. Mariazzi, G. Karwasz, W. Egger, P. Sperr and G. Kögel; Applied Physics Letters 88 (1) (2006) 011920.
Abstract: The terminations of buried surfaces of two different cavity types (nano- and microcavities) produced in the same He+-H+ co-implanted p-type Si (100) sample annealed at 900 degrees C, are studied and characterized by positron annihilation spectroscopy. The characterization was carried out by means of three complementary positron techniques: Doppler broadening and coincidence-Doppler broadening spectroscopy with a continuous slow positron beam, and lifetime spectroscopy with a pulsed slow positron beam. It was found that the nanocavities have a pristine surface of Si, while the surfaces of the microcavities, formed below protruding blisters, are oxygen decorated. This case study opens the interesting use of the positron spectroscopy tool in the topical subject of empty space for microelectronics applications.
BibTeX:
	@article{Brusa2006,
	  author = {Brusa, R.S. and Macchi, C. and Mariazzi, S. and Karwasz, G.P. and Egger, W. and Sperr, P. and Kögel, G.},
	  title = {Decoration of buried surfaces in Si detected by positron annihilation spectroscopy},
	  journal = {Applied Physics Letters},
	  year = {2006},
	  volume = {88},
	  number = {1},
	  pages = {011920},
	  url = {http://scitation.aip.org/content/aip/journal/apl/88/1/10.1063/1.2162691},
	  doi = {http://doi.org/10.1063/1.2162691}
	}
	
Analysis of defect configurations with positron lifetime measurements by pulsed low energy beams
W. Egger, G. Kögel, P. Sperr and H.-J. Gudladt; International Journal of Materials Research 97 (12) (2006) 1633-1641.
Abstract: To understand the damage behavior of mechanically deformed metallic materials in more detail, the kind of defect and its concentration have to be known. In addition, the kinetics of decomposition and of precipitation hardening are influenced by the presence of defects and the corresponding concentration. Consequently, an analysis of dominating defects would be helpful. Compared with well known techniques, positron annihilation spectroscopy offers the opportunity for lifetime measurements that are characteristic for special kinds of defects, e. g. dislocations, small vacancy clusters and micro-voids. To detect the spatial distribution and to determine the concentration of defects, the low energy pulsed positron beam technique can be used. This technique, in combination with the scanning positron microscope, opens a broad field of applications for defect analysis in metallic and ceramic materials.
BibTeX:
	@article{Egger2006,
	  author = {Egger, W. and Kögel, G. and Sperr, P. and Gudladt, H.-J.},
	  title = {Analysis of defect configurations with positron lifetime measurements by pulsed low energy beams},
	  journal = {International Journal of Materials Research},
	  year = {2006},
	  volume = {97},
	  number = {12},
	  pages = {1633-1641},
	  url = {http://www.hanser-elibrary.com/doi/abs/10.3139/146.101394},
	  doi = {http://doi.org/10.3139/146.101394}
	}
	
Positron lifetime and microstructural characterisation of a-Si:H deposited by low temperature HW-CVD on paper substrates
M. Härting, D. Britton, D. Knoesen and W. Egger; Applied Surface Science 252 (9) (2006) 3188-3193.
Abstract: In thin film electronic applications, the limiting factor, in terms of cost and usability, is generally the substrate material. As a consequence, different materials are being investigated as potential lightweight, inexpensive and flexible substrates. In this respect, we have been the first research collaboration to produce silicon-based electronics on paper substrates. Here we present structural characterisation of hydrogenated amorphous silicon (a-Si:H) layers deposited on 80 g m-2 wood-free paper, with and without an intermediate metallic interlayer, using low temperature hot wire chemical vapour deposition (HW-CVD). Both pulsed positron beam profiling and X-ray diffraction studies indicate that the growth rate on the uncoated substrate is slightly higher than with prior metallization. There is no evidence of a crystalline phase or voids in the a-Si:H layers. The internal defect structure is similar, with a dominant dangling bond complex of similar size, which has a slightly longer lifetime than in layers grown at higher temperatures on conventional substrates.
BibTeX:
	@article{Haerting2006,
	  author = {Härting, M. and Britton, D.T. and Knoesen, D. and Egger, W.},
	  title = {Positron lifetime and microstructural characterisation of a-Si:H deposited by low temperature HW-CVD on paper substrates},
	  journal = {Applied Surface Science},
	  year = {2006},
	  volume = {252},
	  number = {9},
	  pages = {3188-3193},
	  note = {SLOPOS 10},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433205012018},
	  doi = {http://doi.org/10.1016/j.apsusc.2005.08.068}
	}
	
Czochralski-grown nitrogen-doped silicon: Electrical properties of MOS structures; A positron annihilation study
L. Harmatha, M. ?apajna, V. Sluge?, P. Ballo, P. Píse?ný, J. Šik and G. Kögel; Microelectronics Journal 37 (4) (2006) 283-289.
Abstract: Czochralski-grown nitrogen-doped (NCZ) silicon was studied using different methods. Measurements of interface traps density, effective generation lifetime and effective surface generation velocity were performed on selected Metal-Oxide-Semiconductor (MOS) structures. Application of the positron annihilation technique (PAS) - pulsed low energy positron system (PLEPS) - was focused on the detection of nitrogen-related defects in NCZ silicon in the near surface region. PAS - PLEPS technique gave relevant results on p-type NCZ silicon. Low sensitivity in the application to n-type NCZ silicon discriminates the PAS - PLEPS technique and should be alternated by other experimental technique. On the other hand, more pertinent measurement of generation lifetime was performed on MOS structures with n-type Si. Although the generation lifetime decreases in NCZ silicon, considerable lateral homogenization of the relaxation time was observed on the wafer.
BibTeX:
	@article{Harmatha2006,
	  author = {Harmatha, L. and ?apajna, M. and Sluge?, V. and Ballo, P. and Píse?ný, P. and Šik, J. and Kögel, G.},
	  title = {Czochralski-grown nitrogen-doped silicon: Electrical properties of MOS structures; A positron annihilation study},
	  journal = {Microelectronics Journal},
	  year = {2006},
	  volume = {37},
	  number = {4},
	  pages = {283--289},
	  url = {http://www.sciencedirect.com/science/article/pii/S0026269205002387},
	  doi = {http://doi.org/10.1016/j.mejo.2005.04.059}
	}
	
Depth-Resolved Analysis of the Aging Behavior of Epoxy Thin Films by Positron Spectroscopy
J. Kanzow, F. Faupel, W. Egger, P. Sperr, G. Kögel, C. Wehlack, A. Meiser and W. Possart; In: W. Possart (Ed.), Adhesion: Current Research and Applications , Chapter 29 , p. 465-477 , Wiley-VCH , 2006.
Abstract: During recent decades positron annihilation spectroscopy has become a very powerful tool for the investigation of polymers. In particular, positron annihilation lifetime spectroscopy (PALS) yields valuable information about free volume and related properties. Moreover, special chemical information can be obtained. Now advances in positron beam technology also allow investigations of thin polymer films and surface regions. In this paper, we report the use, for the first time, of PALS to elucidate aging mechanisms in thin epoxy films, based on depth-resolved investigations of the epoxy films exposed to two different aging conditions. We also consider the results of IR external reflection absorption spectroscopy (IR-ERAS) and X-ray photoelectron spectroscopy (XPS) depth-profiling of the elemental composition. This additional information enables us to clarify structural modifications due to aging. Nitrogen depletion and a decrease in free volume were observed, especially in the near-surface region of the thin epoxy films.
BibTeX:
	@incollection{Kanzow2006,
	  author = {Kanzow, J. and Faupel, F. and Egger, W. and Sperr, P. and Kögel, G. and Wehlack, C. and Meiser, A. and Possart, W.},
	  title = {Depth-Resolved Analysis of the Aging Behavior of Epoxy Thin Films by Positron Spectroscopy},
	  booktitle = {Adhesion: Current Research and Applications},
	  publisher = {Wiley-VCH},
	  year = {2006},
	  pages = {465--477},
	  editor = {Wulff Possart},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/3527607307.ch29/summary},
	  doi = {http://doi.org/10.1002/3527607307.ch29}
	}
	
Planned positron experiments at FRM-II
G. Kögel and G. Dollinger; Applied Surface Science 252 (9) (2006) 3111-3120.
Abstract: The new research reactor FRM-II near Munich has a strong positron source, which delivers an intense, nearly monoenergetic positron beam. Our positron systems, the pulsed low energy positron source (PLEPS) and the scanning positron microscope (SPM) will be operated at this beam. Some aspects of matching these systems to the new positron source will be discussed. Considerable improvements are expected, e.g. more than 105 s-1 recorded events at PLEPS and sub-micrometre resolution at SPM. They will enable investigations in so far inaccessible problems like the evaluation of annihilation characteristics and trapping constants of individual defects or studies of fast dynamical processes. In applied materials science complex defect structures will be studied which demand a resolution into many differing lifetimes, e.g. fractured specimens, wear, corrosion, etc. Also large series of measurements at small systematic modifications are planned. There is also the opportunity to analyse in addition the chemical microstructure of the specimens by means of a hydrogen microprobe and other ion beam techniques available close to FRM-II at the Technical University of Munich.
BibTeX:
	@article{Koegel2006,
	  author = {Kögel, G. and Dollinger, G.},
	  title = {Planned positron experiments at FRM-II},
	  journal = {Applied Surface Science},
	  year = {2006},
	  volume = {252},
	  number = {9},
	  pages = {3111-3120},
	  editor = {Al-Qaradawi I.Y., Coleman P.G.},
	  note = {SLOPOS 10},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433205012158},
	  doi = {http://doi.org/10.1016/j.apsusc.2005.08.048}
	}
	
Electrical properties of MOS structures on nitrogen-doped Czochralski-grown silicon: A positron annihilation study
V. Sluge?, L. Harmatha, M. ?apajna, P. Ballo, P. Píse?ný, J. Šik, G. Kögel and V. Kršjak; Applied Surface Science 252 (9) (2006) 3201-3208.
Abstract: Measurements of interface trap density, effective generation lifetime (GL) and effective surface generation velocity have been performed using differen: methods on selected MOS structures prepared on nitrogen-doped Czochralski-grown (NCz) silicon. The application of the positron annihilation technique using a pulsed low energy positron system (PLEPS) focused on the detection of nitrogen-related defects in NCz silicon in the near surface region. In the case of p-type Cz silicon, all the results could be used for the testing of homogeneity. In n-type Cz silicon, positron annihilation was found insensitive to nitrogen doping.
BibTeX:
	@article{Slugen2006,
	  author = {Sluge?, V. and Harmatha, L. and ?apajna, M. and Ballo, P. and Píse?ný, P. and Šik, J. and Kögel, G. and Kršjak, V.},
	  title = {Electrical properties of MOS structures on nitrogen-doped Czochralski-grown silicon: A positron annihilation study},
	  journal = {Applied Surface Science},
	  year = {2006},
	  volume = {252},
	  number = {9},
	  pages = {3201-3208},
	  note = {SLOPOS 10},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433205012031},
	  doi = {http://doi.org/10.1016/j.apsusc.2005.08.071}
	}
	

2005

Absence of positronium formation in clean buried nanocavities in p-type silicon
R.S. Brusa, C. Macchi, S. Mariazzi, G.P. Karwasz, W. Egger, P. Sperr and G. Kögel; Physical Review B - Condensed Matter and Materials Physics 71 (24) (2005) 245320.
Abstract: Buried nanocavities at about 350 nm depth in Si were produced by thermal treatment of He implanted p-type (100) Si. The internal surfaces of the nanocavities were found free of impurity decorations by examining the high-momentum part of the Doppler-broadened positron annihilation spectra. Positron lifetime measurements with a pulsed slow positron beam show neither a short lifetime (125-150 ps) ascribable to parapositronium nor a longer lifetime (2-4 ns) ascribable to pick-off annihilation of orthopositronium. The lifetime of positrons trapped into nanocavities was found to be about 500 ps. The absence of positronium formation could be explained by an insufficient electron density and a lack of electron states in the band gap at the nanocavities internal surfaces produced in the p-type silicon.
BibTeX:
	@article{Brusa2005,
	  author = {Brusa, R. S. and Macchi, C. and Mariazzi, S. and Karwasz, G. P. and Egger, W. and Sperr, P. and Kögel, G.},
	  title = {Absence of positronium formation in clean buried nanocavities in p-type silicon},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2005},
	  volume = {71},
	  number = {24},
	  pages = {245320},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.71.245320},
	  doi = {http://doi.org/10.1103/PhysRevB.71.245320}
	}
	
Specific defects and thermomechanical properties of electrodeposited Cu foils
V. Gröger, G. Khatabi, A. Kotas-Betzwar, P. Zimprich, B. Mikulowski, G. Boczkal, W. Egger, H.D. Merchant and B. Weiss; Archives of Metallurgy and Materials 50 (1) (2005) 167-174.
Abstract: Electrodeposition of copper foils is a commercially widely used technique whose potential for producing functionally graded materials by deliberate time variation of the deposition parameters has been shown. Due to the presence of superabundant vacancies (stabilized by hydrogen) structural instabilities are strongly enhanced. More detailed knowledge of microstructural details (especially defect changes during annealing and stability at elevated temperatures) is needed for a basic understanding. Electrical residual resistivity isochrones, positron annihilation, Young's modulus and linear thermal expansion of copper foils of 35 ?m thickness of different grain size electrodeposited at commercially usual rates are investigated. For all samples structural changes have been observed during the measurements, the strongest influence seems to be due to the annealing out of single vacancies (presumably by releasing hydrogen) and to grain coarsening.
BibTeX:
	@article{Groeger2005,
	  author = {Gröger, V. and Khatabi, G. and Kotas-Betzwar, A. and Zimprich, P. and Mikulowski, B. and Boczkal, G. and Egger, W. and Merchant, H. D. and Weiss, B.},
	  title = {Specific defects and thermomechanical properties of electrodeposited Cu foils},
	  journal = {Archives of Metallurgy and Materials},
	  year = {2005},
	  volume = {50},
	  number = {1},
	  pages = {167--174},
	  note = {Symposium on Texture and Microstructure Analysis of Functionally Graded Materials, Cracow, POLAND, OCT 03-07, 2004},
	  url = {http://www.imim.pl/archives/volume-50-issue-1-2005}
	}
	

2004

Free volume determination of azobenzene-PMMA copolymer by a pulsed low-energy positron lifetime beam with in-situ UV illumination
J. Algers, P. Sperr, W. Egger, L. Liszkay, G. Kögel, J. de Baerdemaeker and F.H.J. Maurer; Macromolecules 37 (21) (2004) 8035-8042.
Abstract: The free volume properties of a poly(methyl methacrylate)-azobenzene copolymer were for the first time directly probed by use of a low-energy positron lifetime beam with in-situ excitation capabilities, showing that the free volume cavity size was not appreciably influenced by photoisomerization and thermal isomerization in the temperature range 34-180°C. Isomerization is therefore suggested to occur without any molecular rearrangement of the glassy polymer matrix, which would also account for the lack of any shift to shorter wavelengths for the photoisomerization occurring in a glassy polymer in comparison to a chloroform solution. A decrease in the thermal isomerization rate at room temperature caused by the glassy polymer is explained in terms of a model in which only a fraction of the azobenzene is free to isomerize. The cis-azobenzene was found to be an efficient inhibitor of positronium formation, which enabled measurements of thermal isomerization rates and changes in the steady-state concentration of cis-azobenzene for an illuminated sample as a function of temperature.
BibTeX:
	@article{Algers2004,
	  author = {Algers, J. and Sperr, P. and Egger, W. and Liszkay, L. and Kögel, G. and de Baerdemaeker, J. and Maurer, F. H. J.},
	  title = {Free volume determination of azobenzene-PMMA copolymer by a pulsed low-energy positron lifetime beam with in-situ UV illumination},
	  journal = {Macromolecules},
	  year = {2004},
	  volume = {37},
	  number = {21},
	  pages = {8035-8042},
	  url = {http://pubs.acs.org/doi/abs/10.1021/ma0486086},
	  doi = {http://doi.org/10.1021/ma0486086}
	}
	
Thermal Isomerization of Azo-Benzene in PMMA Probed by a Pulsed Low-Energy Positron Beam
J. Algers, V. Skeppstedt, P. Sperr, W. Egger and F.H.J. Maurer; Materials Science Forum 445-446 (2004) 262-264.
Abstract: Polymers containing the Azo-benzene group have attracted attention due to anticipated uses such as materials for data storage, membranes with controllable permeability and materials with changeable solubility [1,2]. These expectations are due to the change in shape and dipole moment of the Azo-compound when excited by photo isomerization from the trans-state to the cis-state by UV light of ca 360 nm. Films of copolymers of poly-methyl methacrylate (PMMA) containing 8 wt-% Azo-benzene were photo isomerized with a UV lamp and then measured as a function of time in a low-energy positron beam. The observed changes in the lifetime parameters as a function of time were in agreement with the thermal isomerization (decay) of the cis-isomer as measured by conventional UV spectroscopy, The results show that positron lifetime spectra at low energy are sensitive for the cis-trans isomerization of the Azo-compound, suggesting trapping and annihilation of positrons at free volume sites around the Azo-benzene groups.
BibTeX:
	@article{Algers2004a,
	  author = {Algers, J. and Skeppstedt, V. and Sperr, P. and Egger, W. and Maurer, F. H. J.},
	  title = {Thermal Isomerization of Azo-Benzene in PMMA Probed by a Pulsed Low-Energy Positron Beam},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {262--264},
	  note = {ICPA 13},
	  url = {https://www.scientific.net/MSF.445-446.262},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.262}
	}
	
Defect Characterization of the Structure-Growth Zone-Model for Sputter Deposited Cu Films
J. De Baerdemaeker, C. Dauwe, D. Segers, C. Detavernier, D. Deduytsche, W. Egger and P. Sperr; Materials Science Forum 445-446 (2004) 69-71.
Abstract: The 'zone-model' for sputter deposited Cu films was analyzed by positron annihilation spectroscopy to give a valuable insight in the nature of the defects present in the different zones of the model. Both depth selective Doppler broadening and positron lifetime spectroscopy were applied using slow positron beams. Room temperature grain growth for films sputtered in the zoneT regime was also analyzed for the first time with positron annihilation spectroscopy.
BibTeX:
	@article{DeBaerdemaeker2004,
	  author = {De Baerdemaeker, J. and Dauwe, C. and Segers, D. and Detavernier, C. and Deduytsche, D. and Egger, W. and Sperr, P.},
	  title = {Defect Characterization of the Structure-Growth Zone-Model for Sputter Deposited Cu Films},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {69--71},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.69},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.69}
	}
	
Studies of light alloys by positron annihilation techniques
A. Dupasquier, G. Kögel and A. Somoza; Acta Materialia 52 (16) (2004) 4707-4726.
Abstract: The potential of positron annihilation spectroscopy (PAS) in the study of light alloys is illustrated with special regards to age-hardening, severe plastic deformation, fatigue and fracture in aluminium- and magnesium-based alloys. First, the physical grounds of PAS sensitivity to open-volume defects are explained. Then the main conventional variants of PAS, lifetime spectroscopy and Doppler-broadening spectroscopy, are introduced. State-of-the-art equipment, based on intense positron sources and energy-controlled beams, is also described, in view of applications where microscopic spatial resolution and sub-nanosecond time resolution are combined. Various examples of PAS studies in the field of light alloys, mainly based on the latest experience of the authors, are presented. It is shown how PAS detects structural changes in age-hardenable alloys, helps to describe the solute aggregation kinetics and gives information on vacancy-solute interactions. PAS characterisation of internal surfaces (misfit interfaces and grain boundaries) in terms of local structure (degree of disorder, chemistry) is also discussed. Lastly, recent advances in the study of fatigue by positron microscopy are reported.
BibTeX:
	@article{Dupasquier2004,
	  author = {Dupasquier, A and Kögel, G and Somoza, A},
	  title = {Studies of light alloys by positron annihilation techniques},
	  journal = {Acta Materialia},
	  year = {2004},
	  volume = {52},
	  number = {16},
	  pages = {4707-4726},
	  url = {http://www.sciencedirect.com/science/article/pii/S1359645404004033},
	  doi = {http://doi.org/10.1016/j.actamat.2004.07.004}
	}
	
Measurements of defect structures of a cyclically deformed Al-Mg-Si alloy by positron annihilation techniques
W. Egger, G. Kögel, P. Sperr, W. Triftshäuser, J. Bär, S. Rödling and H.-J. Gudladt; Materials Science and Engineering A 387-389 (1-2 SPEC. ISS.) (2004) 317-320.
Abstract: Defect distributions close to cracks created by monotonic and fatigue tests in the Al-Mg-Si alloy Al 6013 have been investigated by positron lifetime studies. The defect structure close to the crack surface was studied at sub-mum resolution with a pulsed positron beam, whereas the lateral defect distribution in the crack-tip near region was determined using a scanning positron microscope with a few mum resolution. A high concentration of large vacancy clusters of up to 30 vacancies and voids close to the fatigue crack, as well as a high dislocation density in a region with an extension of similar to100 mum around the crack tip, were detected in the fatigued samples. Moreover, the generation of vacancy clusters found in the fatigued samples seems to be independent of the loading history. In monotonically fractured samples, no vacancy clusters have been found. If this difference holds as a general rule, positron lifetime spectroscopy would be suitable to distinguish quantitatively monotonic front fatigue fracture without any further fractographic studies.
BibTeX:
	@article{Egger2004,
	  author = {Egger, W. and Kögel, G. and Sperr, P. and Triftshäuser, W. and Bär, J. and Rödling, S. and Gudladt, H.-J.},
	  title = {Measurements of defect structures of a cyclically deformed Al-Mg-Si alloy by positron annihilation techniques},
	  journal = {Materials Science and Engineering A},
	  year = {2004},
	  volume = {387-389},
	  number = {1-2 SPEC. ISS.},
	  pages = {317--320},
	  note = {13th International Conference on Strength of Materials (ICSMA 13), Budapest, HUNGARY, AUG, 2003},
	  url = {http://www.sciencedirect.com/science/article/pii/S0921509304004563},
	  doi = {http://doi.org/10.1016/j.msea.2004.02.070}
	}
	
Positron Annihilation Spectroscopy in Polymers
F. Faupel, J. Kanzow, K. Gunther-Schade, C. Nagel, P. Sperr and G. Kögel; Materials Science Forum 445-446 (2004) 219-223.
Abstract: During the last decades positron annihilation has become a very powerful tool for the investigation of polymers. In particular, positron annihilation lifetime spectroscopy (PALS) yields valuable information on free volume and other properties. The present invited paper gives examples from recent research of the Kiel group. Generally, the so-called standard model, developed by Tao and Eldrup, is used to determine the size of free volume holes from the ortho-positronium (o-Ps) lifetime tau(oPs). Despite its success, the model resorts to several assumptions, including a spherical hole shape. Although the deviations from spherical shape are significant for holes above the size of positronium, average hole sizes V(h), determined by the standard model from T,p, show a good correlation with diffusivities D of inert gas molecules when plotted as 1/V(h) vs logD, as predicted by the free volume approach. The correlation can further be improved by taking into account the cohesive energy density of the polymers. The o-Ps intensity I(o-Ps) is often taken as a measure of the hole density. However, I(o-Ps) is also affected by the Ps formation probability and drops during mechanical milling of polymers due to formation of free radicals by chain scission, for instance. I(o-Ps), is also seen to change during phase separation in polymer blends. This can be explored to detect both, the binodal and the spinodal decomposition, already at the initial stage which is not easily accessible by other techniques. PALS was also used to study thermosets. Here we show in-situ results on the cross-linking of an epoxy resin. Finally, we demonstrate the benefits of the positron beam technique which allows investigations of polymer thin films and surfaces. For example, very recent results, obtained at the positron beam in Munich, on the structure and dynamics of epoxy films as function of film thickness will be presented.
BibTeX:
	@article{Faupel2004,
	  author = {Faupel, F. and Kanzow, J. and Gunther-Schade, K. and Nagel, C. and Sperr, P. and Kögel, G.},
	  title = {Positron Annihilation Spectroscopy in Polymers},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {219--223},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.219},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.219}
	}
	
Comparison of Vacancy Creation by Nuclear and Electronic Processes in Silicon Irradiated with Swift Kr and Bi Ions
P.M. Gordo, L. Liszkay, K. Havancsák, V.A. Skuratov, P. Sperr, W. Egger, C. Lopes Gil, A.P. de Lima and Z. Kajcsos; Materials Science Forum 445-446 (2004) 93-95.
Abstract: Silicon samples, irradiated with swift Kr (245 MeV) and Bi (710 MeV) ions at room temperature, were investigated using a continuous and a pulsed positron beam and conventional Doppler broadening and lifetime spectroscopy. In the fluence and depth ranges studied, creation of large voids and amorphization was not observed. The dominant defects were found to be divacancies, present from the near surface region all along the ion tracks. We found that the formation of divacancies from ion-induced vacancies as predicted by Monte-Carlo-calculations is higher in the case of the heavier Bi ion.
BibTeX:
	@article{Gordo2004,
	  author = {Gordo, P. M. and Liszkay, L. and Havancsák, K. and Skuratov, V. A. and Sperr, P. and Egger, W. and Lopes Gil, C. and de Lima, A. P. and Kajcsos, Z.},
	  title = {Comparison of Vacancy Creation by Nuclear and Electronic Processes in Silicon Irradiated with Swift Kr and Bi Ions},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {93--95},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.93},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.93}
	}
	
NEPOMUC - the New Positron Beam Facility at FRM II
C. Hugenschmidt, G. Kögel, R. Repper, K. Schreckenbach, P. Sperr, B. Straßer and W. Triftshäuser; Materials Science Forum 445-446 (2004) 480-482.
Abstract: An in-pile positron source is installed in the vicinity of the reactor core of the new research reactor FRM-II. This neutron induced positron source at Munich NEPOMUC is based on absorption of high-energy prompt gamma-rays from thermal neutron capture in (113)Cd. For this purpose, a cadmium cap is placed inside the tip of the inclined beam tube SR 11 in the heavy water moderator tank. Inside the cadmium cap a structure of platinum foils is placed in order to convert the high-energy gamma-radiation into positron-electron pairs. Due to the negative positron work function, moderation in annealed platinum leads to emission of monoenergetic positrons. After acceleration to several keV by electrical lenses the positron beam is magnetically guided in a solenoid field of 7.5 mT. The beam profile is determined with a micro-channel plate (MCP) detector at the end of the beam line. In addition, a NaI-scintillator detects the 511 keV gamma-radiation of the positrons which annihilate at the front side of the MCP in order to perform intensity measurements as a function of acceleration energy and magnetic guide field.
BibTeX:
	@article{Hugenschmidt2004,
	  author = {Hugenschmidt, C. and Kögel, G. and Repper, R. and Schreckenbach, K. and Sperr, P. and Straßer, B. and Triftshäuser, W.},
	  title = {NEPOMUC - the New Positron Beam Facility at FRM II},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {480--482},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.480},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.480}
	}
	
The neutron induced positron source at Munich - NEPOMUC
C. Hugenschmidt, G. Kögel, R. Repper, K. Schreckenbach, P. Sperr, B. Straßer and W. Triftshäuser; Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 221 (1-4) (2004) 160-164.
Abstract: Monoenergetic positron beams of high intensity are of major interest in atomic and particle physics as well as in material science. The intensity of conventional positron beams using beta(+) sources amounts typically to 10(6) positrons per second. Therefore, great efforts are undertaken to generate positrons by pair production using bright gamma-sources, i.e. bremsstrahlung in a target at linear accelerators, gamma-radiation from the nuclear fission or high-energy prompt gamma-rays after thermal neutron capture. The NEutron induced POsitron source at MUniCh, NEPOMUC, is installed at the new research reactor FRM II. The world's highest positron intensity of up to 10(10) positrons per second is expected at this novel user dedicated positron beam facility. Since the final license for reactor operation was granted in May 2003 positrons will presently be available at FRM II. In this work, a brief overview of positron beams is given and the status of the positron project NEPOMUC as well as the linked experiments is reported.
BibTeX:
	@article{Hugenschmidt2004a,
	  author = {Hugenschmidt, C. and Kögel, G. and Repper, R. and Schreckenbach, K. and Sperr, P. and Straßer, B. and Triftshäuser, W.},
	  title = {The neutron induced positron source at Munich - NEPOMUC},
	  journal = {Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms},
	  year = {2004},
	  volume = {221},
	  number = {1-4},
	  pages = {160--164},
	  note = {12th International Workshop on Positron and Positronium Physics (POSITRON03), Aarhus Univ, Inst Phys & Astron, Sandbjerg, DENMARK, JUL 19-21, 2003},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168583X04003842},
	  doi = {http://doi.org/10.1016/j.nimb.2004.03.048}
	}
	
Investigation of Fatigue Cracks in an Al-Based Alloy by Means of Pulsed Positron (Micro-)Beams
G. Kögel, W. Egger, S. Rödling and H.-J. Gudladt; Materials Science Forum 445-446 (2004) 126-128.
Abstract: Positron lifetime studies have been undertaken with the Munich pulsed beam and the scanning positron microscope to investigate areas close to fatigue cracks of a precipitation-hardened AlMgSi (6013) alloy. These cracks were initiated by cyclic loading under full automatic stress and crack length control. Close to the surface of fatigue cracks, always three lifetimes of about 245 ps, 300 - 500 ps and 1200 - 2500 ps are observed which are attributed to annihilations at dislocations, vacancy clusters and voids. These defects are believed to indicate remnants of dislocation interactions within the cyclic plastic zone in front of the fatigue crack tip.
BibTeX:
	@article{Koegel2004,
	  author = {Kögel, G. and Egger, W. and Rödling, S. and Gudladt, H.-J.},
	  title = {Investigation of Fatigue Cracks in an Al-Based Alloy by Means of Pulsed Positron (Micro-)Beams},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {126--128},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.126},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.126}
	}
	
Vacancy defects in epitaxial InN: Identification and electrical properties
A. Laakso, J. Oila, A. Kemppinen, K. Saarinen, W. Egger, L. Liszkay, P. Sperr, H. Lu and W.J. Schaff; Journal of Crystal Growth 269 (1) (2004) 41-49.
Abstract: We have used a low-energy positron beam to identify and quantify the dominant vacancy defects in InN layers grown on Al2O3 by molecular beam epitaxy. By applying both continuous and pulsed positron beams, we can show that In vacancies are formed during the crystal growth. Their concentration decreases from similar to 5 x 10(18); to below loll cm(-3) with increasing layer thickness (120-800 nm). The In vacancy concentration correlates with the free electron concentration and decreases with increasing electron Hall mobility. These results suggest that In vacancies act as both compensating defects and electron scattering centers in InN films.
BibTeX:
	@article{Laakso2004,
	  author = {Laakso, A. and Oila, J. and Kemppinen, A. and Saarinen, K. and Egger, W. and Liszkay, L. and Sperr, P. and Lu, H. and Schaff, W. J.},
	  title = {Vacancy defects in epitaxial InN: Identification and electrical properties},
	  journal = {Journal of Crystal Growth},
	  year = {2004},
	  volume = {269},
	  number = {1},
	  pages = {41--49},
	  note = {1st International Workshop on Indium Nitride, Fremantle, AUSTRALIA, NOV 16-20, 2003},
	  url = {http://www.sciencedirect.com/science/article/pii/S0022024804001800},
	  doi = {http://doi.org/10.1016/j.jcrysgro.2004.02.032}
	}
	
Positron Beam Splitter at the High Intensity Positron Beam in Munich
L. Liszkay, G. Kögel, P. Sperr, W. Egger, C. Hugenschmidt and W. Triftshäuser; Materials Science Forum 445-446 (2004) 492-494.
Abstract: The high intensity positron source at the new research reactor FRM-II in Munich will be used to serve three instruments simultaneously: a scanning positron microscope, a pulsed positron beam and a system for positron induced Auger electron spectroscopy. In order to split the continuous beam into three parts with variable intensity, a beam splitter is being built. The device consists of an electrostatic optical system embedded in the constant magnetic guiding field of the beam line. In the first stage an electrostatic quadrupole is used to change the cross section of the beam into an ellipse. The resulting beam is then split into three parts by two pairs of deflection plates. The central part is transported to the microscope without deflection. The beam intensities can be varied by mechanical movement of the plates. At the exit port of the beam splitter the guiding magnetic field is split into three beamlines with the help of a magnetic shield made of mumetal.
BibTeX:
	@article{Liszkay2004,
	  author = {Liszkay, L. and Kögel, G. and Sperr, P. and Egger, W. and Hugenschmidt, C. and Triftshäuser, W.},
	  title = {Positron Beam Splitter at the High Intensity Positron Beam in Munich},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {492--494},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.492},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.492}
	}
	
Influence of layer thickness on the formation of in vacancies in InN grown by molecular beam epitaxy
J. Oila, A. Kemppinen, A. Laakso, K. Saarinen, W. Egger, L. Liszkay, P. Sperr, H. Lu and W.J. Schaff; Applied Physics Letters 84 (9) (2004) 1486-1488.
Abstract: We have used a low-energy positron beam to identify In vacancies in InN layers grown on Al2O3 by molecular beam epitaxy. Their concentration decreases from similar to5x10(18) to below 10(16) cm(-3) with increasing layer thickness (120-800 nm). The decrease in the vacancy concentration coincides with the increase in the electron Hall mobility, suggesting that In vacancies act as electron scattering centers.
BibTeX:
	@article{Oila2004,
	  author = {Oila, J. and Kemppinen, A. and Laakso, A. and Saarinen, K. and Egger, W. and Liszkay, L. and Sperr, P. and Lu, H. and Schaff, W. J.},
	  title = {Influence of layer thickness on the formation of in vacancies in InN grown by molecular beam epitaxy},
	  journal = {Applied Physics Letters},
	  year = {2004},
	  volume = {84},
	  number = {9},
	  pages = {1486-1488},
	  url = {http://scitation.aip.org/content/aip/journal/apl/84/9/10.1063/1.1651327},
	  doi = {http://doi.org/10.1063/1.1651327}
	}
	
Defect structural characterization of organic polymer layers
O. Osiele, D. Britton, M. Härting, P. Sperr, M. Topic, S. Shaheen and H. Branz; Journal of Non-Crystalline Solids 338-340 (1 SPEC. ISS.) (2004) 612-616.
Abstract: In this paper, we study the effect of blending the conducting polymers, P3HT with the fullerene complex PCBM on its structural and defect characteristics. The films were deposited on glass substrates by spin casting, and were characterized with positron annihilation, microscopy and other techniques with regard to thickness, and structural homogeneity. The unblended polymers have positron annihilation characteristics similar to most non-polar polymers, exhibiting a relatively broad electron momentum distribution, a long-lived ( > 1 ns) positron state corresponding to the formation of orthopositronium, and strong-localization of the positron. Blending with PCBM causes the sample electron momentum distribution to narrow, and results in a single state with a lifetime of around 370 ps in both polymer mixtures. We postulate that this state corresponds to annihilation with low-momentum electrons in the fullerene cage.
BibTeX:
	@article{Osiele2004,
	  author = {Osiele, O.M. and Britton, D.T. and Härting, M. and Sperr, P. and Topic, M. and Shaheen, S.E. and Branz, H.M.},
	  title = {Defect structural characterization of organic polymer layers},
	  journal = {Journal of Non-Crystalline Solids},
	  year = {2004},
	  volume = {338-340},
	  number = {1 SPEC. ISS.},
	  pages = {612--616},
	  note = {20th International Conference on Amorphous and Microcrystalline Semiconductors, Campos do Jordao, BRAZIL, AUG 25-29, 2003},
	  url = {http://www.sciencedirect.com/science/article/pii/S0022309304002224},
	  doi = {http://doi.org/10.1016/j.jnoncrysol.2004.03.053}
	}
	
Positron Annihilation Characteristics of Polymer Films for Photovoltaic Applications
O. Osiele, D. Britton, M. Härting, P. Sperr, M. Topic, S. Shaheen and H. Branz; Materials Science Forum 445-446 (2004) 337-339.
Abstract: In this paper we study the effect of combining the conducting polymers, P3HT and OC(1)C(10)-PPV with the fullerene complex PCBM, on its positron annihilation characteristics. The films were deposited on glass substrates by spin casting, and were also characterised with scanning electron microscopy with regard to thickness, and structural homogeneity. Both pure polymers have positron annihilation characteristics similar to most non-polar polymers, exhibiting a relatively broad electron momentum distribution, a long-lived (>1 ns) positron state corresponding to the formation of ortho-positronium, and strong-localisation of the positron. Mixing with PCBM causes the sampled electron momentum distribution to narrow, and results in a single annihilation state with a lifetime of around 370 ps in both polymer mixtures. We postulate that this state corresponds to annihilation with the low-momentum electrons in the fullerene cage.
BibTeX:
	@article{Osiele2004a,
	  author = {Osiele, O.M. and Britton, D.T. and Härting, M. and Sperr, P. and Topic, M. and Shaheen, S.E. and Branz, H.M.},
	  title = {Positron Annihilation Characteristics of Polymer Films for Photovoltaic Applications},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {337--339},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.337},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.337}
	}
	
Positron annihilation Investigations of defects in copper alloys selected for nuclear fusion technology
V. Sluge?, J. Kuriplach, P. Ballo, P. Domonkos, G. Kögel, P. Sperr, W. Egger, W. Triftshäuser, V.M. Domankova, P. Kovac, I. Vavra, S. Stancek, M. Petriska and A. Zeman; Fusion Engineering and Design 70 (2) (2004) 141-153.
Abstract: A positron annihilation spectroscopy (PAS) based on positron lifetime measurements, using the pulsed low energy positron system (PLEPS), is used to investigate defects create by hydrogen implantation and thermal treat of copper alloys. These alloys are designated for the use in the international thermonuclear experimental reactor (ITER). The results show that the changes in the microstructure of selected copper alloys (CuCrZr, CuAl25) depend strongly on the preparing technology of alloys as well as on the implantation dose experimentally simulating the neutron treatment. The full recovering of the structure after isochronal annealing in vacuum is observed in all implanted specimens at the temperature of 450 °C. Using the PLEPS technique, for the first time, depth profiling of the positron lifetime spectra in the near-surface region (20-500 nm) of hydrogen implanted copper alloys was performed and compared with the TRIM calculations and transmission electron microscopy (TEM) studies. Possible annihilation channels in CuCrZr and CuAl25 materials are discussed in details together with corresponding annihilation characteristics determined theoretically and using computer simulations. We discuss the results of positron lifetime measurements of the irradiated and non-irradiated CuZrCr and CuAl25 specimens. We identified the most probable types of positron trapping sites. Finally, the results are discussed in terms of microstructural changes of the studied materials upon irradiation and subsequent heat treatment.
BibTeX:
	@article{Slugen2004,
	  author = {Sluge?, V. and Kuriplach, J. and Ballo, P. and Domonkos, P. and Kögel, G. and Sperr, P. and Egger, W. and Triftshäuser, W. and Domankova, V. M. and Kovac, P. and Vavra, I. and Stancek, S. and Petriska, M. and Zeman, A.},
	  title = {Positron annihilation Investigations of defects in copper alloys selected for nuclear fusion technology},
	  journal = {Fusion Engineering and Design},
	  year = {2004},
	  volume = {70},
	  number = {2},
	  pages = {141--153},
	  url = {http://www.sciencedirect.com/science/article/pii/S0920379603004691},
	  doi = {http://doi.org/10.1016/j.fusengdes.2003.10.002}
	}
	
Investigation of Defects in Copper Alloys Selected for Nuclear Fusion Technology
V. Sluge?, G. Kögel, J. Kuriplach, P. Ballo, P. Sperr, W. Egger, W. Triftshäuser, P. Domonkos, M. Petriska and A. Zeman; Materials Science Forum 445-446 (2004) 183-185.
Abstract: Positron lifetime measurements using a Pulsed Low Energy Positron System (PLEPS) were applied to the investigation of defects in hydrogen implanted and subsequently thermally treated copper alloys which are designated for the use in the International Thermonuclear Experimental Reactor (ITER). PLEPS results showed that the changes in the microstructure of selected copper-alloys (CuCrZr, CuAl25) depend strongly on the preparing technology of alloys and on the implantation dose. The full recovery of the structure after isochronal annealing in vacuum in a region of 100-600 °C was observed in all implanted specimens at a level of about 450 °C. With the PLEPS technique, for the first time, depth profiling of the near-surface region (20-500 nm) of hydrogen implanted copper alloys was performed and compared with the transmission electron microscopy (TEM) studies.
BibTeX:
	@article{Slugen2004a,
	  author = {Sluge?, V. and Kögel, G. and Kuriplach, J. and Ballo, P. and Sperr, P. and Egger, W. and Triftshäuser, W. and Domonkos, P. and Petriska, M. and Zeman, A.},
	  title = {Investigation of Defects in Copper Alloys Selected for Nuclear Fusion Technology},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {183--185},
	  note = {ICPA 13},
	  url = {https://www.scientific.net/MSF.445-446.183},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.183}
	}
	
Positron Microprobes
W. Triftshäuser; Materials Science Forum 445-446 (2004) 452-456.
Abstract: The history of the concept and the attempts for the realisation of a positron microprobe as well as the present state of development will be reviewed and evaluated. The principle and the design of a scanning positron microscope (SPM), built and operated at our university in Munich, will be presented. A pulsed positron beam, with a variable energy from 0.5 to 20 keV and a final spot diameter of currently 2 mum, can be electronically scanned over an area of 0.6 mm x 0.6 mm. In this way, microscopic regions of 2 mum x 2 mum x 0.1 mum can be evaluated. This beam is formed after a double-stage moderation of positrons emitted from a radioactive isotope acting as primary positron source. In the first stage, the beam spot of about 3 nun is reduced to a diameter of less than 20 mum. After the second moderation stage, this diameter is then further reduced to 2 mum. This is possible, due to the special properties of the magnetic lenses used in the design. In order to enable positron lifetime measurements, the positron beam is pulsed (time interval 20 ns). This feature makes this SPM the first and up to now only system worldwide. Included in the system is a conventional scanning electron microprobe for surface analysis and for the selection of interesting regions. Three-dimensional positron lifetime spectra of mechanically damaged and fatigued specimens will be presented. The real potential power of the SPM will be achieved when the intense reactor-based positron source at the new Munich research reactor FRM-II can be used, which will be possible now in the very near future.
BibTeX:
	@article{Triftshaeuser2004,
	  author = {Triftshäuser, W.},
	  title = {Positron Microprobes},
	  journal = {Materials Science Forum},
	  year = {2004},
	  volume = {445-446},
	  pages = {452--456},
	  note = {ICPA 13},
	  url = {http://www.scientific.net/MSF.445-446.452},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.445-446.452}
	}
	

2003

Median implantation depth and implantation profile of 3-18 keV positrons in amorphous polymers
J. Algers, P. Sperr, W. Egger, G. Kögel and F. Maurer; Physical Review B - Condensed Matter and Materials Physics 67 (12) (2003) 1254041-1254047.
Abstract: Most applications of positron beams require knowledge of the implantation characteristics for an appropriate interpretation of the experimental data. In this work, the median implantation depth as a function of implantation energy, z1/2(E), of 3-18 keV positrons and their implantation profile P(z, E) in a total of 13 thin films of atactic polystyrene, poly(styrene-co-acrylonitrile), and polymethylmethacrylate spin coated onto a silicon substrate were determined from positron lifetime measurements using a pulsed, low-energy positron beam. z1/2(E) and P(z,E) were determined from the measurement of the ortho-positronium yield obtained from the intensity I3 of the long lifetime, z1/2(E) was parametrized with the commonly used power-law fit z1/2(E) = (A/?)En, with ? and E in units of g cm-3 and keV, respectively, yielding A = 2.81(± 0.2) ?g cm-2 and n = 1.71(± 0.05). Excellent agreement between amorphous polymer and literature data on A1 and Cu suggests that the median implantation depth of positrons for low- to medium-Z materials in the studied energy range is independent of structure and only a function of mass density. Fitting of the Makhovian implantation profile to the experimental data suggested that the value of the parameter m varies between 1.7 and 2.3, systematically increasing with z at constant implantation energy, but is independent of the implantation energy. Using an equation proposed by Baker et al., the experimental data of 12 of the 13 studied polymer films could be described with a slightly better agreement than the Makhovian equation.
BibTeX:
	@article{Algers2003,
	  author = {Algers, J. and Sperr, P. and Egger, W. and Kögel, G. and Maurer, F.H.J.},
	  title = {Median implantation depth and implantation profile of 3-18 keV positrons in amorphous polymers},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {2003},
	  volume = {67},
	  number = {12},
	  pages = {1254041--1254047},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.67.125404},
	  doi = {http://doi.org/10.1103/PhysRevB.67.125404}
	}
	
Fatigue and fracture-induced defect structures of metals investigated by positron microscopy
W. Egger, G. Kögel, P. Sperr, W. Triftshäuser, J. Bär, S. Rödling and H.-J. Gudladt; Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques 94 (6) (2003) 687-693.
Abstract: Defect distributions close to cracks from monotonic and fatigue fractures have been investigated for the first time by positron lifetime studies. The positron beams probe either the depth profile of defects at sub-?m resolution or the lateral distribution of defects at a few ?m spatial resolution. Both in pure Cu and in the precipitation-hardened alloy Al 6013, in addition to dislocations, large clusters of up to 30 vacancies were detected close to a fatigue crack exclusively, whereas at monotonic fractures only the annihilation characteristics of dislocations were observed. If this characteristic difference holds as a general rule, then positron lifetime microscopy will provide a simple, quantitative method to distinguish a posteriori monotonic fractures from fatigue fractures without any further fractographic studies.
BibTeX:
	@article{Egger2003,
	  author = {Egger, W. and Kögel, G. and Sperr, P. and Triftshäuser, W. and Bär, J. and Rödling, S. and Gudladt, H.-J.},
	  title = {Fatigue and fracture-induced defect structures of metals investigated by positron microscopy},
	  journal = {Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques},
	  year = {2003},
	  volume = {94},
	  number = {6},
	  pages = {687--693},
	  url = {http://www.hanser-elibrary.com/doi/abs/10.3139/146.030687},
	  doi = {http://doi.org/10.3139/146.030687}
	}
	
Positron source based on neutron capture
C. Hugenschmidt, G. Kögel, R. Repper, K. Schreckenbach, P. Sperr and W. Triftshäuser; Radiation Physics and Chemistry 68 (3-4) (2003) 669-671.
Abstract: A positron beam based on absorption of high-energy prompt ?-rays from thermal neutron capture in 113Cd was installed at a neutron guide of the High Flux Reactor at the ILL in Grenoble. The positron source consists of platinum foils acting as ?-e+e --converter and positron moderator. After acceleration to 3keV by electrical lenses the positrons were magnetically guided through the beamline. Measurements were performed for various source geometries. The beam diameter and moderation characteristics such as positron work function, moderation efficiency and degradation were determined as well. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II. © 2003 Elsevier Science Ltd. All rights reserved.
BibTeX:
	@article{Hugenschmidt2003,
	  author = {Hugenschmidt, C. and Kögel, G. and Repper, R. and Schreckenbach, K. and Sperr, P. and Triftshäuser, W.},
	  title = {Positron source based on neutron capture},
	  journal = {Radiation Physics and Chemistry},
	  year = {2003},
	  volume = {68},
	  number = {3-4},
	  pages = {669--671},
	  url = {http://www.sciencedirect.com/science/article/pii/S0969806X0300197X},
	  doi = {http://doi.org/10.1016/S0969-806X(03)00197-X}
	}
	
Study of radiation defects in semiconductors by means of positron annihilation
R. Krause-Rehberg, V. Bondarenko, F. Redmann, F. Borner, H. Feick, E. Weber, C. Da Via, W. Egger, G. Kögel, P. Sperr and W. Triftshäuser; In: Proceedings of the 3rd international Symposium on Material Chemistry in Nuclear Environment (Materials Chemistry MC’02) 36 (2003) 114-117 .
Abstract: In a nuclear environment, a strong degradation of important properties is observed for many materials which are otherwise very reliable. This is especially valid for silicon, the most important semiconductor. In the presented paper, two examples for the study of lattice defects in silicon by means of positron annihilation will be given. Firstly, the degradation of silicon detectors used for the particle detection in high-luminosity collider experiments starts to limit the lifetime of the whole experiment. An annealing experiment on n-irradiated Si will be presented. Beside the destructive effect of high-radiation conditions, such radiation-induced defects can have a beneficial result. This will be demonstrated for the creation of new gettering zones by high-energy self-implantation of silicon.
BibTeX:
	@inproceedings{Krause-Rehberg2003,
	  author = {Krause-Rehberg, R. and Bondarenko, V. and Redmann, F. and Borner, F. and Feick, H. and Weber, E. and Da Via, C. and Egger, W. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Study of radiation defects in semiconductors by means of positron annihilation},
	  booktitle = {Proceedings of the 3rd international Symposium on Material Chemistry in Nuclear Environment (Materials Chemistry MC’02)},
	  year = {2003},
	  volume = {36},
	  number = {50},
	  pages = {114--117},
	  url = {https://inis.iaea.org/search/search.aspx?orig_q=RN:36116010}
	}
	

2002

Vacancy defects in as-polished and in high-fluence H+-implanted 6H-SiC detected by slow positron annihilation spectroscopy
M.-F. Barthe, P. Desgardin, L. Henry, C. Corbel, D. Britton, G. Kögel, P. Sperr, W. Triftshäuser, P. Vicente and L. Dicioccio; Materials Science Forum 389-393 (1) (2002) 493-496.
Abstract: The vacancy defects in near surface region are investigated in n-type 6H-SiC after polishing or low-energy-proton implantation using slow-positron-beam-based positron annihilation spectroscopy. Measuring the momentum distribution of annihilating electron-positron pairs by Doppler broadening spectroscopy we detect an about 100 nm thick vacancy-defects layer under the surface of mechanically polished wafers. No damaged layer is detected after mechano-chemical finishing. Measuring positron lifetime as a function of temperature, we show that neutral and negatively-charged vacancy clusters exist in the track region of low-energy proton-implanted 6H-SiC(H). Depending on annealing, they give rise to positron lifetimes of 257 ± 2 ps, 281 ± 4 ps and 345 ± 2 ps, respectively. By comparison with theory, the 257 ps and 280 ps are attributed to (VC-VSi) 2 and (VC-VSi)3 clusters, respectively. The (VC-VSi)3 cluster has likely an ionization level near the middle of the bandgap. © 2002 Trans Tech Publications.
BibTeX:
	@article{Barthe2002,
	  author = {Barthe, M.-F. and Desgardin, P. and Henry, L. and Corbel, C. and Britton, D.T. and Kögel, G. and Sperr, P. and Triftshäuser, W. and Vicente, P. and Dicioccio, L.},
	  title = {Vacancy defects in as-polished and in high-fluence H+-implanted 6H-SiC detected by slow positron annihilation spectroscopy},
	  journal = {Materials Science Forum},
	  year = {2002},
	  volume = {389-393},
	  number = {1},
	  pages = {493--496},
	  url = {https://www.scientific.net/MSF.389-393.493},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.389-393.493}
	}
	
Shallow traps and positron dynamics in epitaxial silicon carbide
D. Britton, M.-F. Barthe, C. Corbel, P. Desgardin, W. Egger, P. Sperr, G. Kögel and W. Triftshäuser; Applied Surface Science 194 (1-4) (2002) 122-126.
Abstract: We have used slow positron beam-based positron lifetime spectroscopy to study positron diffusion in thick epitaxial n-type 6H-SiC layers. The layers are considerably thicker than the maximum positron penetration depth, and can therefore, be treated as homogeneous semi-infinite bulk material in an analysis including the time-dependent diffusion of a single group of probe particles. Temperature-dependent measurements show a reduction in positron diffusion at low temperatures, which has been interpreted by an increase in trapping to negatively charged defect states. © 2002 Published by Elsevier Science B.V.
BibTeX:
	@article{Britton2002,
	  author = {Britton, D.T. and Barthe, M.-F. and Corbel, C. and Desgardin, P. and Egger, W. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Shallow traps and positron dynamics in epitaxial silicon carbide},
	  journal = {Applied Surface Science},
	  year = {2002},
	  volume = {194},
	  number = {1-4},
	  pages = {122--126},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433202001101},
	  doi = {http://doi.org/10.1016/S0169-4332(02)00110-1}
	}
	
Positron states in hydrogenated amorphous silicon
D. Britton, M. Härting, A. Hempel, G. Kögel, P. Sperr, W. Triftshäuser, M. Hempel and D. Knoesen; Journal of Non-Crystalline Solids 299-302 (Part 1) (2002) 249-253.
Abstract: Low-hydrogen-concentration a-Si:H grown by HW-CVD forms a continuous random network with no detectable free volume in the form of microvoids, and no evidence of a microcrystalline phase. Over a wide temperature range we observe a single positron state in the amorphous network with a temperature-independent lifetime of 322 ps. From the temperature dependence of the positron diffusion we show that this is a localized state and present direct observation of hopping diffusion of positrons. On annealing up to 400 °C the amorphous network is seen to relax and the first stages of crystallization occur. There is also evidence of vacancy clustering to form a low concentration of microvoids. The structural relaxation has a very low-activation energy, around 0.1 eV, and is interpreted in terms of a reconfiguration of the fundamental defect identifed by positron annihilation. © 2002 Published by Elsevier Science B.V.
BibTeX:
	@article{Britton2002a,
	  author = {Britton, D.T. and Härting, M. and Hempel, A. and Kögel, G. and Sperr, P. and Triftshäuser, W. and Hempel, M. and Knoesen, D.},
	  title = {Positron states in hydrogenated amorphous silicon},
	  journal = {Journal of Non-Crystalline Solids},
	  year = {2002},
	  volume = {299-302},
	  number = {Part 1},
	  pages = {249--253},
	  url = {http://www.sciencedirect.com/science/article/pii/S0022309301011656},
	  doi = {http://doi.org/10.1016/S0022-3093(01)01165-6}
	}
	
Vacancy clusters close to a fatigue crack observed with the München scanning positron microscope
W. Egger, G. Kögel, P. Sperr, W. Triftshäuser, S. Rödling, J. Bär and H.-J. Gudladt; Applied Surface Science 194 (1-4) (2002) 214-217.
Abstract: The accumulation of plastic strain by low cycle fatigue has been expected for a long time to induce vacancy clusters, particularly in the most damaged region, i.e. close to a fatigue crack. For the first time, the München scanning positron microscope (SPM) enables the detection of the expected vacancy clusters by positron lifetime studies of a fatigue crack with micrometer resolution. A fatigue crack with a length of about 8 mm was created in a single edge-notched specimen of cold-rolled technical copper. The fatigue crack propagation test was performed under K-controlled conditions with a constant stress intensity factor of 9 MPa m1/2. Lifetime images from a region of about 200 × 400 ?m2 around the crack tip were obtained at about 5 ?m spatial resolution for 5, 8 and 16 keV positron implantation energies. Independent of position, a positron lifetime of about 190 ps is observed, indicating annihilation of positrons at dislocations. Within about 40 ?m from the crack path, however, a second lifetime in the range of 360-420 ps is observed at all positron implantation energies and with an intensity up to 25%. Therefore in this region there must be large vacancy clusters with a trapping rate comparable to one of the dislocations. © 2002 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Egger2002,
	  author = {Egger, W. and Kögel, G. and Sperr, P. and Triftshäuser, W. and Rödling, S. and Bär, J. and Gudladt, H.-J.},
	  title = {Vacancy clusters close to a fatigue crack observed with the München scanning positron microscope},
	  journal = {Applied Surface Science},
	  year = {2002},
	  volume = {194},
	  number = {1-4},
	  pages = {214--217},
	  url = {http://www.sciencedirect.com/science/article/pii/S016943320200106X},
	  doi = {http://doi.org/10.1016/S0169-4332(02)00106-X}
	}
	
The In-Pile Positron Source at the Munich Research Reactor FRM-II
C. Hugenschmidt, K. Schreckenbach, R. Repper and G. Kögel; In: Annual Meeting on Nuclear Technology 2002 Proceedings (2002) 519-522 .
BibTeX:
	@inproceedings{Hugenschmidt:1062,
	  author = {Hugenschmidt, C. and Schreckenbach, K. and Repper, R. and Kögel, G.},
	  title = {The In-Pile Positron Source at the Munich Research Reactor FRM-II},
	  booktitle = {Annual Meeting on Nuclear Technology 2002 Proceedings},
	  year = {2002},
	  pages = {519--522}
	}
	
Intense positron source at the Munich research reactor FRM-II
C. Hugenschmidt, G. Kögel, R. Repper, K. Schreckenbach, P. Sperr, B. Straßer and W. Triftshäuser; Applied Physics A: Materials Science and Processing 74 (SUPPL.I) (2002) S295-S297.
Abstract: The principle and the design of the in-pile positron source at the new Munich research reactor FRM-II are presented. Absorption of high-energy prompt ?-rays from thermal neutron capture in 113Cd generates positrons by pair production. For this purpose, a cadmium cap is placed inside the tip of the inclined beam tube SR11 in the neutron field of the reactor, where an undisturbed thermal neutron flux up to 2 × 1014 n cm-2 s-1 is expected. At this position the flux ratio of thermal to fast neutrons will be better than 104. Monte Carlo calculations showed that a mean capture rate in cadmium between 4.5 and 6.0 × 1013 n cm-2 s-1 can be expected. Inside the cadmium cap a structure of platinum foils is placed for converting ?-radiation into positron-electron pairs. The heated foils also act as positron moderators to generate monoenergetic positrons. After acceleration to 5 keV a positron beam is formed by electric lenses and guided by magnetic fields. In the primary positron beam an intensity up to 1010 slow positrons per second is expected.
BibTeX:
	@article{Hugenschmidt2002,
	  author = {Hugenschmidt, C. and Kögel, G. and Repper, R. and Schreckenbach, K. and Sperr, P. and Straßer, B. and Triftshäuser, W.},
	  title = {Intense positron source at the Munich research reactor FRM-II},
	  journal = {Applied Physics A: Materials Science and Processing},
	  year = {2002},
	  volume = {74},
	  number = {SUPPL.I},
	  pages = {S295-S297},
	  url = {http://link.springer.com/article/10.1007%2Fs003390201398},
	  doi = {http://doi.org/10.1007/s003390201398}
	}
	
First platinum moderated positron beam based on neutron capture
C. Hugenschmidt, G. Kögel, R. Repper, K. Schreckenbach, P. Sperr and W. Triftshäuser; Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 198 (3-4) (2002) 220-229.
Abstract: A positron beam based on absorption of high energy prompt ?-rays from thermal neutron capture in 113Cd was installed at a neutron guide of the high flux reactor at the ILL in Grenoble. Measurements were performed for various source geometries, dependent on converter mass, moderator surface and extraction voltages. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II. The positron source consists of platinum foils acting as ?-e+e--converter and positron moderator. Due to the negative positron work function moderation in heated platinum leads to emission of monoenergetic positrons. The positron work function of polycrystalline platinum was determined to 1.95(5) eV. After acceleration to several keV by four electrical lenses the beam was magnetically guided in a solenoid field of 7.5 mT leading to a NaI-detector in order to detect the 511 keV ?-radiation of the annihilating positrons. The positron beam with a diameter of less than 20 mm yielded an intensity of 3.1 × 104 moderated positrons per second. The total moderation efficiency of the positron source was about ? = 1.06(16) × 10-4. Within the first 20 h of operation a degradation of the moderation efficiency of 30% was observed. An annealing procedure at 873 K in air recovers the platinum moderator. © 2002 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Hugenschmidt2002a,
	  author = {Hugenschmidt, C. and Kögel, G. and Repper, R. and Schreckenbach, K. and Sperr, P. and Triftshäuser, W.},
	  title = {First platinum moderated positron beam based on neutron capture},
	  journal = {Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms},
	  year = {2002},
	  volume = {198},
	  number = {3-4},
	  pages = {220--229},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168583X02015276},
	  doi = {http://doi.org/10.1016/S0168-583X(02)01527-6}
	}
	
Monoenergetic positron beam at the reactor based positron source at FRM-II
C. Hugenschmidt, G. Kögel, R. Repper, K. Schreckenbach, P. Sperr, B. Straßer and W. Triftshäuser; Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 192 (1-2) (2002) 97-101.
Abstract: The principle of the in-pile positron source at the Munich research reactor FRM-II is based on absorption of high energy prompt ?-rays from thermal neutron capture in 113Cd. For this purpose, a cadmium cap is placed inside the tip of the inclined beam tube SR-11 in the moderator tank of the reactor, where an undisturbed thermal neutron flux up to 2 × 1014 n cm-2s-1 is expected. Inside the cadmium cap a structure of platinum foils is placed for converting high energy ?-radiation into positron-electron pairs. Due to the negative positron work function, moderation in annealed platinum leads to emission of monoenergetic positrons. Therefore, platinum will also be used as moderator, since its moderation property seems to yield long-term stability under reactor conditions and it is much easier to handle than tungsten. Model calculations were performed with SIMION-7.0w to optimise geometry and potential of Pt-foils and electrical lenses. It could be shown that the potentials between the Pt-foils must be chosen in the range of 1-10 V to extract moderated positrons. After successive acceleration to 5 keV by four electrical lenses the beam is magnetically guided in a solenoid field of 7.5 mT resulting in a beam diameter of about 25 mm. An intensity of about 1010 slow positrons per second is expected in the primary positron beam. Outside of the reactor shield a W(1 0 0) single crystal remoderation stage will lead to an improvement of the positron beam brilliance before the positrons are guided to the experimental facilities. © 2002 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Hugenschmidt2002b,
	  author = {Hugenschmidt, C. and Kögel, G. and Repper, R. and Schreckenbach, K. and Sperr, P. and Straßer, B. and Triftshäuser, W.},
	  title = {Monoenergetic positron beam at the reactor based positron source at FRM-II},
	  journal = {Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms},
	  year = {2002},
	  volume = {192},
	  number = {1-2},
	  pages = {97--101},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168583X02007887},
	  doi = {http://doi.org/10.1016/S0168-583X(02)00788-7}
	}
	
Radiation-induced defects in 4H- And 6H-SiC epilayers studied by positron annihilation and deep-level transient spectroscopy
A. Kawasuso, M. Weidner, F. Redmann, T. Frank, R. Krause-Rehberg, G. Pensl, P. Sperr, W. Triftshäuser and H. Itoh; Materials Science Forum 389-393 (1) (2002) 489-492.
Abstract: Vacancy defects in high-quality 4H and 6H SiC epilayers induced by electron irradiation have been characterized using positron annihilation and deep level transient spectroscopy (DLTS). Vacancy defects were annealed in two steps below 700°C and above 1200°C irrespective to polytype. From the correlation between positron annihilation and DLTS data using the same wafers, it was confirmed that complexes including silicon vacancies are the origin of the E1/2 levels in 6H SiC and the Z1/2 level in 4H SiC. © 2002 Trans Tech Publications.
BibTeX:
	@article{Kawasuso2002,
	  author = {Kawasuso, A. and Weidner, M. and Redmann, F. and Frank, T. and Krause-Rehberg, R. and Pensl, G. and Sperr, P. and Triftshäuser, W. and Itoh, H.},
	  title = {Radiation-induced defects in 4H- And 6H-SiC epilayers studied by positron annihilation and deep-level transient spectroscopy},
	  journal = {Materials Science Forum},
	  year = {2002},
	  volume = {389-393},
	  number = {1},
	  pages = {489--492},
	  url = {https://www.scientific.net/MSF.389-393.489},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.389-393.489}
	}
	
Microscopes/microprobes
G. Kögel; Applied Surface Science 194 (1-4) (2002) 200-209.
Abstract: Our new pulsed positron microbeam is not only a microprobe for the local analysis of defect characteristics but also a microscope providing a complete lifetime image of defect distributions at micron resolution. Both aspects are discussed with examples from recent applications to basic problems of materials research. For fundamental reasons an image of defects at nanometer resolution by the present generation of microbeams is impossible. Therefore, a more advanced dual microbeam system will be proposed, where the defects are stained with positrons. Then a scanned electron beam with nanometer spot size is sensitive to the stained defects. © 2002 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Koegel2002,
	  author = {Kögel, G.},
	  title = {Microscopes/microprobes},
	  journal = {Applied Surface Science},
	  year = {2002},
	  volume = {194},
	  number = {1-4},
	  pages = {200--209},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433202001022},
	  doi = {http://doi.org/10.1016/S0169-4332(02)00102-2}
	}
	
Improved defect profiling with slow positrons
R. Krause-Rehberg, F. Börner, F. Redmann, W. Egger, G. Kögel, P. Sperr and W. Triftshäuser; Applied Surface Science 194 (1-4) (2002) 210-213.
Abstract: Monoenergetic positrons are widely used to study defects in near-surface regions and buried interfaces of solids. Depth information is usually obtained by varying the positron implantation energy. However, at energies larger than 10 keV the stopping profile becomes much broader than the positron diffusion length. The study shows that optimum depth resolution can be obtained by stepwise removal of the surface and measurement with the smallest possible positron implantation depth. The removal from the surface can be done by ion sputtering or chemical etching. Furthermore, excellent defect depth profiles can be obtained when a sample is wedge-shaped polished (wedge angle about 1 °). A line scan using a scanning positron microbeam along the wedge with a small positron implantation depth gives then the defect profile with optimum depth resolution. © 2002 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Krause-Rehberg2002,
	  author = {Krause-Rehberg, R. and Börner, F. and Redmann, F. and Egger, W. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Improved defect profiling with slow positrons},
	  journal = {Applied Surface Science},
	  year = {2002},
	  volume = {194},
	  number = {1-4},
	  pages = {210--213},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433202001046},
	  doi = {http://doi.org/10.1016/S0169-4332(02)00104-6}
	}
	
Investigation of defect distributions in neutron-irradiated and thermally treated reactor steels by positron annihilation
V. Slugen, G. Kögel, P. Sperr and W. Triftshäuser; Applied Surface Science 194 (1-4) (2002) 150-154.
Abstract: Positron annihilation spectroscopy (PAS) based on positron lifetime (PL) measurements using the pulsed low-energy positron system (PLEPS) was applied for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in the micro structure of the RPV-steel properties caused by neutron-irradiation and post-irradiation heat treatment can be well detected. From the lifetime measurements in the near-surface region (20-550nm) the defect density in the Russian types of RPV-steels was calculated using the diffusion trapping model. The post-irradiation heat treatment studies performed on non-irradiated specimens are also presented. © 2002 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Slugen2002,
	  author = {Slugen, V. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Investigation of defect distributions in neutron-irradiated and thermally treated reactor steels by positron annihilation},
	  journal = {Applied Surface Science},
	  year = {2002},
	  volume = {194},
	  number = {1-4},
	  pages = {150--154},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433202001162},
	  doi = {http://doi.org/10.1016/S0169-4332(02)00116-2}
	}
	
Positron annihilation studies of neutron irradiated and thermally treated reactor pressure vessel steels
V. Slugen, G. Kögel, P. Sperr and W. Triftshäuser; Journal of Nuclear Materials 302 (2-3) (2002) 89-95.
Abstract: Positron annihilation lifetime measurements using the pulsed low energy positron system (PLEPS) were applied for the first time for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in the microstructure of the RPV-steel properties caused by neutron irradiation and post-irradiation thermal treatment can be detected. The samples originated from the Russian 15Kh2MFA and Sv10KhMFT steels, commercially used at WWER-440 reactors, were irradiated near the core at NPP Bohunice (Slovakia) to neutron fluences in the range from 7.8 × 1023 to 2.5 × 1024 m-2. © 2002 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Slugen2002a,
	  author = {Slugen, V. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Positron annihilation studies of neutron irradiated and thermally treated reactor pressure vessel steels},
	  journal = {Journal of Nuclear Materials},
	  year = {2002},
	  volume = {302},
	  number = {2-3},
	  pages = {89--95},
	  url = {http://www.sciencedirect.com/science/article/pii/S0022311502008152},
	  doi = {http://doi.org/10.1016/S0022-3115(02)00815-2}
	}
	

2001

Negatively charged vacancy defects in 6H-SiC after low-energy proton implantation and annealing
M.-F. Barthe, D. Britton, C. Corbel, A. Hempel, L. Henry, P. Desgardin, W. Bauer-Kugelmann, G. Kögel, P. Sperr and W. Triftshäuser; Physica B: Condensed Matter 308-310 (2001) 668-670.
Abstract: We have used pulsed-slow-positron-beam-based positron lifetime spectroscopy to investigate the nature of acceptors and charge states of vacancy-type defects in low-energy proton-implanted 6H-SiC(H). We can infer from the temperature dependence of the lifetime spectra that neutral and negatively charged vacancy clusters exist in the track region. Depending on annealing, they give rise to positron lifetimes of 257 ±2, 281 ±4 and 345 ±2 ps, respectively. The 281 ps cluster has likely an ionization level near the middle of the band gap. By comparison with theory, the 257 and 280 ps are identified as (VC-VSi)2 and (VC-VSi)3 clusters, respectively. In addition, other acceptors of ionic type act as strong trapping centers at low temperature (T < 150 K). Neutral monovacancy-like complexes are also detected with a lifetime of 160 ±2 after 900°C annealing. © 2001 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Barthe2001,
	  author = {Barthe, M.-F. and Britton, D.T. and Corbel, C. and Hempel, A. and Henry, L. and Desgardin, P. and Bauer-Kugelmann, W. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Negatively charged vacancy defects in 6H-SiC after low-energy proton implantation and annealing},
	  journal = {Physica B: Condensed Matter},
	  year = {2001},
	  volume = {308-310},
	  pages = {668--670},
	  url = {http://www.sciencedirect.com/science/article/pii/S0921452601007864},
	  doi = {http://doi.org/10.1016/S0921-4526(01)00786-4}
	}
	
Latest version of the Munich pulsed low energy positron system
W. Bauer-Kugelmann, P. Sperr, G. Kögel and W. Triftshäuser; Materials Science Forum 363-365 (2001) 529-531.
Abstract: Further improvements of the Munich pulsed low energy positron system have been performed. A new chopper, configured as a double plate deflection system with an external resonator and a new buncher working like a classical double gap buncher, are implemented. The complete rf-power electronic was redesigned and operates now at an overall master-frequency of 50 MHz for all bunching and chopping components. A new target station with an enlarged Faraday cage is installed. The sample temperature is variable between 30 K and 600 K. Up to ten samples can be stored in a magazine and transferred under vacuum conditions to the measuring position. With a primary source of 30 mCi 22Na a count rate of up to 4 kHz can be achieved with a peak-to-background ratio of 3000:1. This ratio can be further improved by the use of a Wien filter. A beam diameter of about 2 mm was determined. The total time resolution (pulsing plus detector system) is 250 ps (FWHM).
BibTeX:
	@article{Bauer-Kugelmann2001,
	  author = {Bauer-Kugelmann, W. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Latest version of the Munich pulsed low energy positron system},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {529--531},
	  url = {https://www.scientific.net/MSF.363-365.529},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.529}
	}
	
Evidence for negatively charged vacancy defects in 6H-SiC after low-energy proton implantation
D. Britton, M.-F. Barthe, C. Corbel, A. Hempel, L. Henry, P. Desgardin, W. Bauer-Kugelmann, G. Kögel, P. Sperr and W. Triftshäuser; Applied Physics Letters 78 (9) (2001) 1234-1236.
Abstract: We have used pulsed-slow-positron-beam-based positron lifetime spectroscopy to investigate the nature of acceptors and charge states of vacancy-type defects in low-energy proton-implanted 6H-SiC(H). We can infer from the temperature dependence of the lifetime spectra that neutral and negatively charged vacancy clusters exist in the track region. Depending on annealing, they give rise to positron lifetimes of 257±2, 281±4, and 345±2 ps, respectively. The 281 ps cluster likely has an ionization level near the middle of the band gap. By comparison with theory, the 257 and 280 ps are identified as (VC-VSi)2 and (VC-VSi)3 clusters, respectively. In addition, other acceptors of ionic type act as strong trapping centers at low temperature (T<150 K). Neutral monovacancy-like complexes are also detected with a lifetime of 160±2 after 900 °C annealing. © 2001 American Institute of Physics.
BibTeX:
	@article{Britton2001,
	  author = {Britton, D.T. and Barthe, M.-F. and Corbel, C. and Hempel, A. and Henry, L. and Desgardin, P. and Bauer-Kugelmann, W. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Evidence for negatively charged vacancy defects in 6H-SiC after low-energy proton implantation},
	  journal = {Applied Physics Letters},
	  year = {2001},
	  volume = {78},
	  number = {9},
	  pages = {1234--1236},
	  url = {http://aip.scitation.org/doi/10.1063/1.1350961},
	  doi = {http://doi.org/10.1063/1.1350961}
	}
	
Pulsed positron beam study of as-grown defects in epitaxial SiC
D. Britton, D. Gxawu, A. Hempel, M.-F. Barthe, L. Henry, P. Desgardin, C. Corbel, W. Bauer-Kugelmann, P. Sperr, G. Kögel and W. Triftshäuser; Materials Science Forum 363-365 (2001) 460-462.
Abstract: We have used slow positron beam based positron lifetime spectroscopy to study positron diffusion in a thick epitaxial n-type 6H-SiC. The layer is considerably thicker than the maximum positron penetration depth, and can therefore be treated as homogeneous semi-infinite bulk material in an analysis including the time-dependent diffusion of a single group of probe particles. Temperature dependent measurements show a reduction in the positron diffusivity below 100K, which can be interpreted by an increase in trapping to shallow defect states. Above this temperature, the behaviour of the diffusivity is consistent with the expected T1/2 dependence due to acoustic phonon scattering.
BibTeX:
	@article{Britton2001a,
	  author = {Britton, D.T. and Gxawu, D. and Hempel, A. and Barthe, M.-F. and Henry, L. and Desgardin, P. and Corbel, C. and Bauer-Kugelmann, W. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Pulsed positron beam study of as-grown defects in epitaxial SiC},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {460--462},
	  url = {https://www.scientific.net/MSF.363-365.460},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.460}
	}
	
Defect characterisation of PECVD-grown diamond
D. Britton, A. Hempel, M. Hempel, M. Härting, W. Bauer-Kugelmann and W. Triftshäuser; Materials Science Forum 363-365 (2001) 511-513.
Abstract: We present the results of a combined study of the defect structure and residual stress in a diamond layer, grown by PECVD on a polycrystalline copper substrate with a titanium interlayer. For the defect studies, both continuous and pulsed positron beam techniques were applied. X-ray diffraction techniques were used for both the stress determination in the diamond layer and for a phase analysis of the complete composite structure. The layer was found to contain a significant fraction of vacancy clusters and single vacancy type defects, situated within the individual grains. The presence of the larger defects can be correlated to a compressive stress in the layer.
BibTeX:
	@article{Britton2001b,
	  author = {Britton, D.T. and Hempel, A. and Hempel, M. and Härting, M. and Bauer-Kugelmann, W. and Triftshäuser, W.},
	  title = {Defect characterisation of PECVD-grown diamond},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {511--513},
	  url = {https://www.scientific.net/MSF.363-365.511},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.511}
	}
	
Hopping transport of positrons in hydrogenated amorphous silicon
D. Britton, A. Hempel and W. Triftshäuser; Physical Review Letters 87 (21) (2001) 2174011-2174014.
Abstract: Positron diffusion was investigated in hydrogenated amorphous silicon using positron beam timing spectroscopy. The measurement of the rate of emission of annihilation photons as a function of the time interval between annihilation and incoming pulse of positrons, allowed the measurement of positron lifetime spectrum at different depths. The results indicated that the dominant positron state in a particular covalent random network was a localized state located at hydrogen terminated dangling bond defects. Hopping diffusion of positrons was observed and the migration enthalpy for positrons in that state was found to be 17.7(3) meV.
BibTeX:
	@article{Britton2001c,
	  author = {Britton, D.T. and Hempel, A. and Triftshäuser, W.},
	  title = {Hopping transport of positrons in hydrogenated amorphous silicon},
	  journal = {Physical Review Letters},
	  year = {2001},
	  volume = {87},
	  number = {21},
	  pages = {2174011--2174014},
	  url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.87.217401},
	  doi = {http://doi.org/10.1103/PhysRevLett.87.217401}
	}
	
Annealing and recrystallization of hydrogenated amorphous silicon
D. Britton, A. Hempel, M. Härting, G. Kögel, P. Sperr, W. Triftshäuser, C. Arendse and D. Knoesen; Physical Review B 64 (7) (2001) 075403.
Abstract: Using a combination of positron annihilation and x-ray-diffraction techniques, we have shown that low hydrogen concentration hot wire chemical vapor deposition grown a-Si:H forms a continuous random network with no detectable free volume in the form of microvoids, and no evidence of a microcrystalline phase. On annealing up to 400 degreesC, the amorphous network is seen to relax and the first stages of recrystallization occur. There is also evidence of vacancy clustering to form a low concentration of microvoids. The structural relaxation has a very low activation energy, around 0.1 eV, and is probably caused by a reconfiguration of hydrogen-terminated dangling, bond defects. The formation of microvoids and therecrystallization can both be interpreted by the migration of unterminated dangling-bond defects.
BibTeX:
	@article{Britton2001d,
	  author = {Britton, D.T. and Hempel, A. and Härting, M. and Kögel, G. and Sperr, P. and Triftshäuser, W. and Arendse, C. and Knoesen, D.},
	  title = {Annealing and recrystallization of hydrogenated amorphous silicon},
	  journal = {Physical Review B},
	  year = {2001},
	  volume = {64},
	  number = {7},
	  pages = {075403},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.64.075403},
	  doi = {http://doi.org/10.1103/PhysRevB.64.075403}
	}
	
Lifetime measurements with a scanning positron microscope
A. David, G. Kögel, P. Sperr and W. Triftshäuser; Physical Review Letters 87 (6) (2001) 674021-674024.
Abstract: The visible and invisible damage to the defect structure of a GaAs wafer was identified by first lifetime measurements using the scanning positron microscope. A pulsed positron beam formed by cooling positrons from a radioactive source was scanned over an area of 0.6×0.6 mm 2. A scanning electron microprobe was also included in the system for surface analysis.
BibTeX:
	@article{David2001,
	  author = {David, A. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Lifetime measurements with a scanning positron microscope},
	  journal = {Physical Review Letters},
	  year = {2001},
	  volume = {87},
	  number = {6},
	  pages = {674021--674024},
	  url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.87.067402},
	  doi = {http://doi.org/10.1103/PhysRevLett.87.067402}
	}
	
Investigation of stainless steel films sputtered on glass
J. De Baerdemaeker, T. Van Hoecke, S. Van Petegem, D. Segers, W. Bauer-Kugelmann, P. Sperr and G. Terwagne; Materials Science Forum 363-365 (2001) 496-498.
Abstract: The influence of the sputter atmosphere on glass supported magnetron sputtered stainless steel films was investigated using Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler-Broadening Annihilation Radiation (DBAR) as well as Rutherford Backscattering (RBS), Integral Low Energy Electron Mössbauer Spectroscopy (ILEEMS), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM).
BibTeX:
	@article{DeBaerdemaeker2001,
	  author = {De Baerdemaeker, J. and Van Hoecke, T. and Van Petegem, S. and Segers, D. and Bauer-Kugelmann, W. and Sperr, P. and Terwagne, G.},
	  title = {Investigation of stainless steel films sputtered on glass},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {496--498},
	  url = {https://www.scientific.net/MSF.363-365.496},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.496}
	}
	
Identification of defect sites in FeAl by Doppler spectroscopy of core electrons
W. Egger, G. Bischof, V. Gröger and G. Krexner; Materials Science Forum 363-365 (2001) 82-84.
Abstract: Doppler spectroscopy of core electrons is used to identify defect sites in the intermetallic compound FeAl. Two-dimensional annihilation spectra were recorded both for elemental Al and Fe and for the intermetallic compound FeAl. Experimental data are compared with calculations based on density functional theory using the generalized gradient approximation (GGA) for the positron annihilation. In accordance with earlier work vacancies are found to be located on the Fe sublattice. The results show that the accuracy achieved, both experimentally and theoretically, is sufficient to distinguish clearly between vacancies on different sublattices.
BibTeX:
	@article{Egger2001,
	  author = {Egger, W. and Bischof, G. and Gröger, V. and Krexner, G.},
	  title = {Identification of defect sites in FeAl by Doppler spectroscopy of core electrons},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {82--84},
	  url = {https://www.scientific.net/MSF.363-365.82},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.82}
	}
	
Annealing effects in hydrogenated amorphous silicon layers
A. Hempel, A. Dabrowski, M. Härting, M. Hempel, D. Knoesen, W. Bauer-Kugelmann, G. Kögel, W. Triftshäuser and D. Britton; Materials Science Forum 363-365 (2001) 463-465.
Abstract: The annealing behaviour of defect structures in hydrogenated amorphous silicon, produced by hot wire chemical vapour deposition (HWCVD) has been studied by pulsed and conventional positron beam techniques and X-ray diffraction. Positron lifetime measurements show a dominant component corresponding to small vacancy clusters. Doppler Broadening measurements indicate that the size and concentration of defects varies with annealing temperatures up to 400°C. This behaviour is accompanied by a change from the amorphous to a partly crystalline structure, which can be observed by X-ray diffraction studies.
BibTeX:
	@article{Hempel2001,
	  author = {Hempel, A. and Dabrowski, A. and Härting, M. and Hempel, M. and Knoesen, D. and Bauer-Kugelmann, W. and Kögel, G. and Triftshäuser, W. and Britton, D.T.},
	  title = {Annealing effects in hydrogenated amorphous silicon layers},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {463--465},
	  url = {https://www.scientific.net/MSF.363-365.463},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.463}
	}
	
Defect studies of diamond hard coatings
A. Hempel, M. Hempel, M. Härting, D. Britton, W. Bauer-Kugelmann and W. Triftshäuser; Radiation Effects and Defects in Solids 156 (1) (2001) 215-220.
Abstract: We present the results of a combined study of the defect structure and residual stress in a diamond layer, grown by PECVD on a polycrystalline copper substrate with a titanium interlayer. For the defect studies, electron spectroscopies based on electron-positron annihilation were applied. X-ray diffraction techniques were used for both the stress determination in the diamond layer and for a phase analysis of the complete composite structure. The layer was found to contain a significant fraction of vacancy clusters and single vacancy type defects, which are probably situated within the individual grains. The presence of the larger defects may be related to a compressive stress in the layer.
BibTeX:
	@article{Hempel2001a,
	  author = {Hempel, A. and Hempel, M. and Härting, M. and Britton, D.T. and Bauer-Kugelmann, W. and Triftshäuser, W.},
	  title = {Defect studies of diamond hard coatings},
	  journal = {Radiation Effects and Defects in Solids},
	  year = {2001},
	  volume = {156},
	  number = {1},
	  pages = {215--220},
	  url = {http://www.tandfonline.com/doi/abs/10.1080/10420150108216896},
	  doi = {http://doi.org/10.1080/10420150108216896}
	}
	
Intense positron source at the Munich research reactor FRM-II
C. Hugenschmidt, G. Kögel, K. Schreckenbach, P. Sperr, B. Straßer and W. Triftshäuser; Materials Science Forum 363-365 (2001) 425-429.
Abstract: Principle and design of the intense positron source facility at the new Munich research reactor FRM-II is presented. Absorption of high energy prompt ? rays from thermal neutron capture in 113Cd generates positrons by pair production. For this purpose, a cadmium cap will be placed inside a beam tube in the neutron field of the reactor, where an undisturbed thermal neutron flux up to 2·1014 n·cm-2s-1 is expected. At this position the flux ratio of thermal to fast neutron will be better than 104. Monte Carlo calculation showed that a mean capture rate in cadmium between 4.5 and 6.0·1013 n·cm-2s-1 can be expected. Absorption of ? rays would lead to a heat impact less than 4 Wcm-2. Inside the cadmium cap a structure of platinum is placed for converting the ? radiation into positron-electron pairs. As converting material platinum is used, since the cross section for pair production is even higher than in tungsten. The heated platinum foils also act as moderators. The positron beam with a primary energy of about 5 keV is formed by electric lenses and magnetic fields. A remoderation stage leads to an improvement of the positron beam brilliance, where an intensity of about 109 slow positrons per second can be expected.
BibTeX:
	@article{Hugenschmidt2001,
	  author = {Hugenschmidt, C. and Kögel, G. and Schreckenbach, K. and Sperr, P. and Straßer, B. and Triftshäuser, W.},
	  title = {Intense positron source at the Munich research reactor FRM-II},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {425--429},
	  url = {https://www.scientific.net/MSF.363-365.425},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.425}
	}
	
Annealing behavior of vacancies and Z1/2 levels in electron-irradiated 4H-SiC studied by positron annihilation and deep-level transient spectroscopy
A. Kawasuso, F. Redmann, R. Krause-Rehberg, M. Weidner, T. Frank, G. Pensl, P. Sperr, W. Triftshäuser and H. Itoh; Applied Physics Letters 79 (24) (2001) 3950-3952.
Abstract: Annealing behavior of vacancies and the Z1/2 levels in n-type 4H-SiC epilayers after 2 MeV electron irradiation has been studied using positron annihilation and deep-level transient spectroscopy. Isochronal annealing studies indicate that silicon vacancy-related defects are primarily responsible for positron trapping. The Z1/2 levels are the predominant deep centers after irradiation and subsequent annealing at 1200 °C. Both the positron-trapping rate at vacancies and the Z1/2 concentration decrease in a similar manner while annealing from 1200 to 1500 °C. It is thus proposed that the Z1/2 levels originate from silicon vacancy-related defects. © 2001 American Institute of Physics.
BibTeX:
	@article{Kawasuso2001,
	  author = {Kawasuso, A. and Redmann, F. and Krause-Rehberg, R. and Weidner, M. and Frank, T. and Pensl, G. and Sperr, P. and Triftshäuser, W. and Itoh, H.},
	  title = {Annealing behavior of vacancies and Z1/2 levels in electron-irradiated 4H-SiC studied by positron annihilation and deep-level transient spectroscopy},
	  journal = {Applied Physics Letters},
	  year = {2001},
	  volume = {79},
	  number = {24},
	  pages = {3950--3952},
	  url = {http://aip.scitation.org/doi/10.1063/1.1426259},
	  doi = {http://doi.org/10.1063/1.1426259}
	}
	
Vacancies and deep levels in electron-irradiated 6H SiC epilayers studied by positron annihilation and deep level transient spectroscopy
A. Kawasuso, F. Redmann, R. Krause-Rehberg, T. Frank, M. Weidner, G. Pensl, P. Sperr and H. Itoh; Journal of Applied Physics 90 (7) (2001) 3377-3382.
Abstract: The annealing behavior of defects in n-type 6H SiC epilayers irradiated with 2 MeV electrons have been studied using positron annihilation and deep level transient spectroscopy. Vacancy-type defects are annealed at 500-700°C and 1200-1400°C. From the analysis of Doppler broadening spectra (core electron momentum distribution), the latter annealing process is attributed to the disappearance of complexes related to silicon vacancies and not to nearest neighbor divacancies. Among the observed deep levels, the E1/E2 levels show similar annealing behavior to that of positron annihilation centers above 1000°C. It is thus proposed that the E1/E2 levels originate from complexes containing silicon vacancies. © 2001 American Institute of Physics.
BibTeX:
	@article{Kawasuso2001a,
	  author = {Kawasuso, A. and Redmann, F. and Krause-Rehberg, R. and Frank, T. and Weidner, M. and Pensl, G. and Sperr, P. and Itoh, H.},
	  title = {Vacancies and deep levels in electron-irradiated 6H SiC epilayers studied by positron annihilation and deep level transient spectroscopy},
	  journal = {Journal of Applied Physics},
	  year = {2001},
	  volume = {90},
	  number = {7},
	  pages = {3377--3382},
	  url = {http://aip.scitation.org/doi/10.1063/1.1402144},
	  doi = {http://doi.org/10.1063/1.1402144}
	}
	
Annealing Process of Defects in Epitaxial SiC Induced by He and Electron Irradiation: Positron Annihilation Study
A. Kawasuso, F. Redmann, R. Krause-Rehberg, P. Sperr, T. Frank, M. Weidner, G. Pensl and H. Itoh; Materials Science Forum 353-356 (2001) 537-540.
Abstract: Annealing processes of vacancy-type defects in epitaxial 6H SiC after 2 MeV electron irradiation and multiple He implantation have been investigated using positron annihilation spectroscopy. Vacancy-type defects are found to disappear in two annealing stages: at 500-800°C and 1200-1500°C. Silicon vacancies are the major positron trapping centers after electron irradiation. Two annealing stages after electron irradiation are attributed to the disappearance of isolated silicon vacancies and complexes associated with silicon vacancies, respectively. In He-irradiated SiC, divacancies are also generated in addition to silicon vacancies.
BibTeX:
	@article{Kawasuso2001b,
	  author = {Kawasuso, Atsuo and Redmann, F. and Krause-Rehberg, R. and Sperr, Peter and Frank, Thomas and Weidner, Michael and Pensl, Gerhard and Itoh, Hisayoshi},
	  title = {Annealing Process of Defects in Epitaxial SiC Induced by He and Electron Irradiation: Positron Annihilation Study},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {353-356},
	  pages = {537--540},
	  url = {https://www.scientific.net/MSF.353-356.537},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.353-356.537}
	}
	
Vacancies in He-implanted 4H and 6H SiC epilayers studied by positron annihilation
A. Kawasuso, M. Weidner, F. Redmann, T. Frank, P. Sperr, R. Krause-Rehberg, W. Triftshäuser and G. Pensl; Physica B: Condensed Matter 308-310 (2001) 660-663.
Abstract: Defects in epitaxially grown 4H and 6H SiC induced by He-implantation have been studied by positron annihilation and deep level transient spectroscopy. Two major annealing processes of vacancy-type defects appeared at 500-800°C and above 1000°C irrespective of polytype and conduction type. In n-type samples, the latter process is dominated by two different types of defects. In n-type 6H SiC, Z1/2 levels emerged after annealing at 800°C. The Z1/2 levels disappeared around 1100°C with an appearance of E1/2 levels. The E1/2 levels are eventually annealed at 1500-1700°C. Similar annealing behavior was observed for the corresponding levels in n-type 4H SiC, i.e., RD1/2 and Z1/2 levels. The overall annealing behavior of vacancy-type defects by positron annihilation and the deep levels are in good agreement above 800°C suggesting that the above deep levels are related to the vacancy-type defects. © 2001 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Kawasuso2001c,
	  author = {Kawasuso, A. and Weidner, M. and Redmann, F. and Frank, T. and Sperr, P. and Krause-Rehberg, R. and Triftshäuser, W. and Pensl, G.},
	  title = {Vacancies in He-implanted 4H and 6H SiC epilayers studied by positron annihilation},
	  journal = {Physica B: Condensed Matter},
	  year = {2001},
	  volume = {308-310},
	  pages = {660--663},
	  url = {http://www.sciencedirect.com/science/article/pii/S0921452601007839},
	  doi = {http://doi.org/10.1016/S0921-4526(01)00783-9}
	}
	
Positron microscopy
G. Kögel; Materials Science Forum 363-365 (2001) 409-414.
Abstract: Positron Microscopy, particularly by means of a scanned positron microbeam, offers an universal route to control positrons before and, in principle, after the implantation into condensed matter. Besides spatial resolution also the measurements of defect concentrations and specific trapping rates are enabled. The current technique and some results will be reviewed. Instrumental requirements will be discussed for studies of various defect configurations as well as for the more advanced techniques of multi-positron pulses and of a release of positrons from surfaces and other traps.
BibTeX:
	@article{Koegel2001,
	  author = {Kögel, G.},
	  title = {Positron microscopy},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {409--414},
	  url = {https://www.scientific.net/MSF.363-365.409},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.409}
	}
	
Identification of getter defects in high-energy self-implanted silicon at Rp/2
R. Krause-Rehberg, F. Börner, F. Redmann, J. Gebauer, R. Kögler, R. Kliemann, W. Skorupa, W. Egger, G. Kögel and W. Triftshäuser; Physica B: Condensed Matter 308-310 (2001) 442-445.
Abstract: A strong gettering effect appears after high-energy Si self-implantation and subsequent annealing in two zones at the projected range of the silicon ions (Rp) and in a region at about Rp/2. The defects responsible for the impurity gettering at Rp/2 were studied by means of positron annihilation. It was found that diffusing Cu impurities were captured by small vacancy agglomerates. Monoenergetic positron beams with improved depth resolution were used to characterize the defects. Excellent depth resolution was obtained when samples were wedge-shaped polished and studied using the Munich Scanning Positron Microscope. © 2001 Elsevier Science B.V. All rights reserved.
BibTeX:
	@article{Krause-Rehberg2001,
	  author = {Krause-Rehberg, R. and Börner, F. and Redmann, F. and Gebauer, J. and Kögler, R. and Kliemann, R. and Skorupa, W. and Egger, W. and Kögel, G. and Triftshäuser, W.},
	  title = {Identification of getter defects in high-energy self-implanted silicon at Rp/2},
	  journal = {Physica B: Condensed Matter},
	  year = {2001},
	  volume = {308-310},
	  pages = {442--445},
	  url = {http://www.sciencedirect.com/science/article/pii/S0921452601007177},
	  doi = {http://doi.org/10.1016/S0921-4526(01)00717-7}
	}
	
Investigation of reactor steels
V. Slugen, J. Hascik, R. Gröne, P. Bartik, A. Zeman, G. Kögel, P. Sperr and W. Triftshäuser; Materials Science Forum 363-365 (2001) 47-51.
Abstract: Positron annihilation spectroscopy (PAS) techniques, Mössbauer spectroscopy (MS) and transmission electron microscopy (TEM) were applied in the investigation of thermally treated reactor pressure vessel steels (RPV). Results of these methods are discussed and correlated with hardness measurement. PAS and MS measurements have been performed also on the original RPV specimens which are used in frame of the "Extended surveillance specimen program" at the 3rd and 4th units of NPP Jaslovské Bohunice (Slovakia). PAS angular correlation (1D-ACAR) and lifetime spectra (PLEPS) showed that the degradation of the steel properties caused by neutron irradiation can be detected. General discussion about the contribution of PAS to micro structural study of RPV-steels is included as well.
BibTeX:
	@article{Slugen2001,
	  author = {Slugen, V. and Hascik, J. and Gröne, R. and Bartik, P. and Zeman, A. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Investigation of reactor steels},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {47--51},
	  url = {https://www.scientific.net/MSF.363-365.47},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.47}
	}
	
Pulsed positron beam in investigation of reactor steels
V. Sluge?, G. Kögel, P. Sperr and W. Triftshäuser; Journal of Electrical Engineering 52 (03-04) (2001) 88-91.
Abstract: The improved pulsed low-energy positron system (PLEPS) was used for positron lifetime spectroscopy in the investigation of irradiated nuclear reactor pressure vessel (RPV) steels microstructure. This system allows to study the microstructural changes in the region from 20 to 600 nm with small and very thin specimens by reducing the disturbing 60Co contribution to minimum. Such disturbance was the limiting factor for investigation of high-irradiated RPV specimens in the past. In the frame of the so-called "Extended surveillance specimens program'' started at the 3rd and 4th units of the nuclear power plant (NPP) Bohunice (Slovakia) in 1994, well-defined specimens were placed into irradiation channels and taken out after 1, 2 and 3 years in operated VVER-440 reactor, respectively. Samples from RPV base material (15Kh2MFA) and weld material (Sv10KhMFT) were measured before and after irradiation by neutron fluency in the range from 7.8x1023m-2 up to 2.3x1024m-2. Results from PLEPS measurements were correlated with those from other spectroscopic methods (Mössbauer spectroscopies and HV10) and discussed in detail.
BibTeX:
	@article{Slugen2001a,
	  author = {Sluge?, Vladimír and Kögel, Gottfried and Sperr, Peter and Triftshäuser, Werner},
	  title = {Pulsed positron beam in investigation of reactor steels},
	  journal = {Journal of Electrical Engineering},
	  year = {2001},
	  volume = {52},
	  number = {03-04},
	  pages = {88--91},
	  url = {http://iris.elf.stuba.sk/cgi-bin/jeeec?act=abs&no=03-04_101&ttl=6}
	}
	
'Smart cut' silicon by proton implantation: Lifetime studies with a pulsed positron beam
P. Sperr, G. Kögel, W. Bauer-Kugelmann, W. Triftshäuser and M. Fujinami; Materials Science Forum 363-365 (2001) 532-536.
Abstract: High dose implantation of hydrogen into silicon and subsequent annealing induce a splitting of silicon. The state of hydrogen and the implantation induced defects are of scientific and technological interest. Samples implanted with 1 · 1016 and 5 · 1016 H+/cm2 at an energy of 60 keV were investigated after different thermal annealing treatments with the pulsed positron beam as a function of the positron energy and the specimen temperature (80K to 500K). There is clear evidence of growth of defect clusters with increasing annealing temperature which correspond in depth with the mean projected range of the implanted hydrogen atoms. In addition to the large defect agglomerates, there are most likely shallow positron traps which are effective only when the specimens are measured at low temperature.
BibTeX:
	@article{Sperr2001,
	  author = {Sperr, P. and Kögel, G. and Bauer-Kugelmann, W. and Triftshäuser, W. and Fujinami, M.},
	  title = {'Smart cut' silicon by proton implantation: Lifetime studies with a pulsed positron beam},
	  journal = {Materials Science Forum},
	  year = {2001},
	  volume = {363-365},
	  pages = {532--536},
	  url = {https://www.scientific.net/MSF.363-365.532},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.363-365.532}
	}
	

2000

Structural characterization of hydrogenated a-Si using slow positron beam techniques
D. Britton, A. Hempel, D. Knoesen, W. Bauer-Kugelmann and W. Triftshäuser; Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 164 (2000) 1010-1015.
Abstract: Hydrogenated amorphous silicon (a-Si:H) grown by hot wire chemical vapour deposition is a promising candidate for robust inexpensive solar cells. However, prolonged exposure to light is known to lead to a reduction in efficiency of a-Si:H devices. The causes for this ageing effect are still unclear, but may be related to a structural relaxation or change in hydrogen content. In this work, results are presented for positron beam studies of the defect structure, using both lifetime and Doppler-broadening spectroscopy, of a-Si:H grown under different conditions.
BibTeX:
	@article{Britton2000,
	  author = {Britton, D.T. and Hempel, A. and Knoesen, D. and Bauer-Kugelmann, W. and Triftshäuser, W.},
	  title = {Structural characterization of hydrogenated a-Si using slow positron beam techniques},
	  journal = {Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms},
	  year = {2000},
	  volume = {164},
	  pages = {1010--1015},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168583X99010538},
	  doi = {http://doi.org/10.1016/S0168-583X(99)01053-8}
	}
	
Defect identification in GaAs grown at low temperatures by positron annihilation
J. Gebauer, F. Börner, R. Krause-Rehberg, T. Staab, W. Bauer-Kugelmann, G. Kögel, W. Triftshäuser, P. Specht, R. Lutz, E. Weber and M. Luysberg; Journal of Applied Physics 87 (12) (2000) 8368-8379.
Abstract: We use positron annihilation to study vacancy defects in GaAs grown at low temperatures (LT-GaAs). The vacancies in as-grown LT-GaAs can be identified to be Ga monovacancies, VGa, according to their positron lifetime and annihilation momentum distribution. The charge state of the vacancies is neutral. This is ascribed to the presence of positively charged As+ Ga antisite defects in vicinity to the vacancies. Theoretical calculations of the annihilation parameters show that this assignment is consistent with the data. The density of VGa is related to the growth stoichiometry in LT-GaAs, i.e., it increases with the As/Ga beam equivalent pressure (BEP) and saturates at 2×1018 cm-3 for a BEP?20 and a low growth temperature of 200°C. Annealing at 600°C removes VGa. Instead, larger vacancy agglomerates with a size of approximately four vacancies are found. It will be shown that these vacancy clusters are associated with the As precipitates formed during annealing. © 2000 American Institute of Physics.
BibTeX:
	@article{Gebauer2000,
	  author = {Gebauer, J. and Börner, F. and Krause-Rehberg, R. and Staab, T.E.M. and Bauer-Kugelmann, W. and Kögel, G. and Triftshäuser, W. and Specht, P. and Lutz, R.C. and Weber, E.R. and Luysberg, M.},
	  title = {Defect identification in GaAs grown at low temperatures by positron annihilation},
	  journal = {Journal of Applied Physics},
	  year = {2000},
	  volume = {87},
	  number = {12},
	  pages = {8368--8379},
	  url = {http://aip.scitation.org/doi/10.1063/1.373549},
	  doi = {http://doi.org/10.1063/1.373549}
	}
	
Positron annihilation in near-surface regions and layered structures
W. Triftshäuser; Vacuum 58 (1) (2000) 33-44.
Abstract: Positrons are very sensitive probes for vacancy-type defects of atomic dimensions, e.g. vacancies, vacancy agglomerates, dislocations or inner surfaces. It is well established that positrons can be trapped at these defects and, because of the locally reduced electron density, the lifetime of the positron localized at the defect increases. This lifetime has characteristic values for each defect type and therefore it is possible to separate out various atomic defect configurations and their relative abundance with very high sensitivity (approx. 1 ppm) and in a nondestructive way. With a pulsed positron beam (approx. 3 mm diameter) of variable energy, lifetime studies can be performed as a function of the positron energy and hence the penetrations depth. Results on silicon and silicon carbide subjected to different treatments will be discussed. For many applications in materials science a positron beam in the micrometer range is desirable. This leads to a scanning positron microscope consisting of a pulsed positron beam of micrometer dimension with a scanning facility. The design and the performance of this first system of its kind will be presented.
BibTeX:
	@article{Triftshaeuser2000,
	  author = {Triftshäuser, W.},
	  title = {Positron annihilation in near-surface regions and layered structures},
	  journal = {Vacuum},
	  year = {2000},
	  volume = {58},
	  number = {1},
	  pages = {33--44},
	  url = {http://www.sciencedirect.com/science/article/pii/S0042207X99001980},
	  doi = {http://doi.org/10.1016/S0042-207X(99)00198-0}
	}
	

1999

High intense positron beam at the new Munich research reactor FRM-II
C. Hugenschmidt, G. Kögel, K. Schreckenbach, P. Sperr, M. Springer, B. Straßer and W. Triftshäuser; Applied Surface Science 149 (1) (1999) 7-10.
Abstract: The Munich Intense POsitron Source (MIPOS) facility for producing a high intense positron beam at the new Munich research reactor (Forschungs-Reaktor Muenchen II) FRM-II is presented. Positrons are generated by pair production of high energy prompt ?-rays from neutron capture in cadmium: 113Cd(n,?)114Cd. A cadmium cap will be located inside a beamtube at an undisturbed thermal neutron flux of 2×1014 n cm-2 s-1. Model calculation showed that this would lead to a mean capture rate of 1.2×1013 n cm-2 s-1. Thermal load resulting from absorbed ?-rays is expected to be less than 4 W cm-2. Inside the cadmium cap a structure of platinum and a stack of tungsten foils is placed for converting the ?-rays into positron-electron pairs. Platinum is used as converting material, since the cross section for pair production is even higher (+11%) than in tungsten. The maximum of the energy spectrum of the positrons produced is about 800 keV. The tungsten foils also act as moderator. The positrons will be accelerated by electric lenses and guided by magnetic fields. Various arrangements are tested to improve the efficiency of the system. After remoderation of the positron beam an intensity of about 109 slow positrons per second is expected.
BibTeX:
	@article{Hugenschmidt1999,
	  author = {Hugenschmidt, C. and Kögel, G. and Schreckenbach, K. and Sperr, P. and Springer, M. and Straßer, B. and Triftshäuser, W.},
	  title = {High intense positron beam at the new Munich research reactor FRM-II},
	  journal = {Applied Surface Science},
	  year = {1999},
	  volume = {149},
	  number = {1},
	  pages = {7--10},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433299001634},
	  doi = {http://doi.org/10.1016/S0169-4332(99)00163-4}
	}
	
Characterization of RF-sputtered platinum films from industrial production plants using slow positrons
A. Osipowicz, M. Härting, M. Hempel, D. Britton, W. Bauer-Kugelmann and W. Triftshäuser; Applied Surface Science 149 (1) (1999) 198-203.
Abstract: Platinum films, used in thin film technology, produced by radiofrequency sputter deposition on alumina substrates under different conditions, have been studied by positron beam and other techniques, before and after production annealing. The defect structure in the layers has been characterized using both positron lifetime and Doppler broadening spectroscopy, and compared with X-ray studies of crystallinity and texture. The films are topographically irregular, with a grain size comparable to the thickness of the layer. All layers show pronounced crystallographic texture but this does not appear to be related to the sample processing.
BibTeX:
	@article{Osipowicz1999,
	  author = {Osipowicz, A. and Härting, M. and Hempel, M. and Britton, D.T. and Bauer-Kugelmann, W. and Triftshäuser, W.},
	  title = {Characterization of RF-sputtered platinum films from industrial production plants using slow positrons},
	  journal = {Applied Surface Science},
	  year = {1999},
	  volume = {149},
	  number = {1},
	  pages = {198--203},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433299002007},
	  doi = {http://doi.org/10.1016/S0169-4332(99)00200-7}
	}
	
A variable nanosecond delay for fast signals
P. Sperr and M. Maier; Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 431 (1) (1999) 234-235.
Abstract: A simple continuously variable nanosecond delay for fast signals has been devised. In this concept, the alignment of the fast channels may be performed manually or by a computer set control voltages. The performance of the design has been confirmed using conventional fast timing systems. Results from this application show that the simple delay generator is well applicable in fast timing measurements where a controllable continuous delay is needed.
BibTeX:
	@article{Sperr1999,
	  author = {Sperr, P. and Maier, M.R.},
	  title = {A variable nanosecond delay for fast signals},
	  journal = {Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment},
	  year = {1999},
	  volume = {431},
	  number = {1},
	  pages = {234--235},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168900299002405},
	  doi = {http://doi.org/10.1016/S0168-9002(99)00240-5}
	}
	

1998

Thermodynamics of liquid mixtures consisting of a very polar and a non-polar aromatic: (Benzonitrile + benzene, or toluene)
E. Wilhelm, W. Egger, M. Vencour, A. Roux, M. Polednicek and J.-P. Grolier; Journal of Chemical Thermodynamics 30 (12) (1998) 1509-1532.
Abstract: For the two binary liquid systems xC6H5CN + (1 - x)C6H6 and xC6H5CN + (1 - x) C6H5CH3, excess molar volumes VEm, excess molar enthalpies HEm, and excess molar heat capacities CEp,m at constant pressure have been measured as a function of mole fraction x. Values of VEm were determined at T = (298.15, 308.15, and 318.15) K, HEm was determined at T = (293.15, 298.15, and 303.15)K, and CEp,m was determined at T = 298.15K (all at atmospheric pressure). The instruments used were, respectively, a vibrating-tube densimeter (from Sodev), an LKB differential flow microcalorimeter equipped with two computer-controlled h.p.l.c. piston pumps (from Gilson), and a Picker flow microcalorimeter (from Setaram). In addition, isobaric heat capacities divided by volume C p,m/Vm of the pure liquids, as well as several selected mixtures, were measured with a programmable differential scanning calorimeter of the Calvet type (micro-d.s.c., from Setaram) between approximately T = 280K and T = 350K. Both systems show relatively small negative excess molar volumes, which become more negative with increasing temperature. The excess molar enthalpies are highly unusual in that for both systems an M-shaped composition dependence is observed (two maxima and one minimum). The M-shape is much more prenounced for (benzonitrile + toluene) than for (benzonitrile + benzene), and appears to vanish for the latter system below T = 298.15K. The results can be understood in terms of a simple theory of complex formation (Guggenheim-McGlashan). The excess molar heat capacity at constant pressure at T = 298.15K of (benzonitrile + toluene) is positive at all compositions, while that of (benzonitrile + benzene) is positive only for 0 < x < 0.071, and negative otherwise (sigmoidal shape). Combining the heat capacities obtained with the Picker calorimeter at T = 298.15 K with our results for Cp,m/Vm obtained with the micro-d.s.c. in conjunction with our density data, excess molar heat capacities have also been derived for (benzonitrile + toluene) at T = (288.15, 308.15, 318.15, and 328.15) K. For this system, the maximum of the curve CEp,m against x becomes somewhat smaller with increasing temperature and is shifted towards larger values of x, i.e. the curves become more symmetric. © 1998 Academic Press.
BibTeX:
	@article{Wilhelm1998,
	  author = {Wilhelm, E. and Egger, W. and Vencour, M. and Roux, A.H. and Polednicek, M. and Grolier, J.-P.E.},
	  title = {Thermodynamics of liquid mixtures consisting of a very polar and a non-polar aromatic: (Benzonitrile + benzene, or toluene)},
	  journal = {Journal of Chemical Thermodynamics},
	  year = {1998},
	  volume = {30},
	  number = {12},
	  pages = {1509--1532},
	  url = {http://www.sciencedirect.com/science/article/pii/S002196149890427X},
	  doi = {http://doi.org/10.1006/jcht.1998.0427}
	}
	
Low energy positron lifetime studies on open-volume defects of epitaxial high temperature superconductor thin film YBa2Cu3O 7-x
X. Zhou and W. Triftshäuser; He Jishu/Nuclear Techniques 21 (2) (1998) .
Abstract: By using a pulsed low energy positron lifetime system, the positron lifetime spectra were measured of the high-temperature superconducting epitaxial thin film YBa2Cu3O7-x samples prepared by the pulsed laser deposition (PLD), magnetron-sputtering method and evaporation method, respectively. It is found that besides shallow positron trapping centers there are a number of deep trapping centers which the bulk material lacks. At low temperature the deep trapping centers have a tendency to enlarge. The relationship between positron lifetime and the deposition conditions (substrate temperature and partial air pressure) of the PLD films indicates that the defect type is independent of the deposition conditions and the defect concentration increases with decreasing substrate temperature or rising partial air pressure.
BibTeX:
	@article{Zhou1998,
	  author = {Zhou, X. and Triftshäuser, W.},
	  title = {Low energy positron lifetime studies on open-volume defects of epitaxial high temperature superconductor thin film YBa2Cu3O 7-x},
	  journal = {He Jishu/Nuclear Techniques},
	  year = {1998},
	  volume = {21},
	  number = {2}
	}
	
Temperature dependence of positron lifetime in a laser-ablated YBa_2Cu_3O_7 - X film
X.Y. Zhou, B.Y. Li, G. Kögel and W. Triftshäuser; Chinese Journal of Low Temperature Physics 20 (5) (1998) 348-355.
BibTeX:
	@article{Zhou1998a,
	  author = {Zhou, X. Y. and Li, B. Y. and Kögel, G. and Triftshäuser, W.},
	  title = {Temperature dependence of positron lifetime in a laser-ablated YBa_2Cu_3O_7 - X film},
	  journal = {Chinese Journal of Low Temperature Physics},
	  year = {1998},
	  volume = {20},
	  number = {5},
	  pages = {348--355}
	}
	

1997

Diffusivity and surface transition rate of positrons in crystalline silicon as a function of dopant concentration
W. Bauer-Kugelmann, J. Duffy, J. Störmer, G. Kögel and W. Triftshäuser; Applied Surface Science 116 (1997) 231-235.
Abstract: Positron lifetime measurements have been performed on silicon as a function of dopant concentration with the pulsed low-energy positron system (PLEPS II). The lifetime spectra were analysed with a modified version of Positronfit. Values have been obtained for the lifetimes and the corresponding intensities as a function of implantation energy for each sample. Using a new approach for the solution of the diffusion-trapping-model, the mean lifetime and the surface lifetime intensity are used to calculate the diffusion coefficient D and surface transition rate v. Both parameters vary only smoothly in n-doped and weakly p-doped material. For p-type silicon in the concentration range from 1015 to 1021 cm-3 both values first decrease by two orders of magnitude, leading to a minimum in D and v at ca= 1018 cm-3. For higher acceptor concentrations the values for D and v increase again by one order of magnitude. We believe that the accumulation of positrons in so-called Debye-Hückel clouds around negatively charged acceptors in the crystal is the most important contribution.
BibTeX:
	@article{Bauer-Kugelmann1997,
	  author = {Bauer-Kugelmann, W. and Duffy, J.A. and Störmer, J. and Kögel, G. and Triftshäuser, W.},
	  title = {Diffusivity and surface transition rate of positrons in crystalline silicon as a function of dopant concentration},
	  journal = {Applied Surface Science},
	  year = {1997},
	  volume = {116},
	  pages = {231--235},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433296010604?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0169-4332(96)01060-4}
	}
	
Positron lifetimes and positron moderation of 4H-SiC subjected to various treatments
W. Bauer-Kugelmann, G. Kögel, P. Sperr and W. Triftshäuser; Materials Science Forum 255-257 (1997) 662-664.
Abstract: Epitaxial p-SiC layers on n-SiC substrates have been exposed to ion implantation, surface oxidation and post irradiation annealing prior to investigation by the Munich pulsed positron beam in order to obtain a better understanding of positron diffusion, defect trapping and positron re-emission. In the oxide layer, a fraction of up to 40% of the positrons annihilate with a very high lifetime of about 1600 ps, due to positronium formation. At intermediate energies all but the irradiated sample show a minimum in the mean lifetime which can be attributed to annihilation from the bulk. For higher positron implantation energies the mean lifetime is increasing again and the two component fit reveals a second lifetime of about 560 ps in this energy range. Therefore we conclude on defects close to the substrate surface which could be introduced in the nucleation period of epitaxy. In the Al-implanted layer we find trapping at defects with a positron lifetime of about 218 ps. After annealing these defects are almost completely removed. All the unoxidized samples show reemission of positrons at positron energies
BibTeX:
	@article{Bauer-Kugelmann1997a,
	  author = {Bauer-Kugelmann, W. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Positron lifetimes and positron moderation of 4H-SiC subjected to various treatments},
	  journal = {Materials Science Forum},
	  year = {1997},
	  volume = {255-257},
	  pages = {662--664},
	  url = {https://www.scientific.net/MSF.255-257.662},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.255-257.662}
	}
	
Progress report from the Munich Scanning Positron Microscope
A. David, G. Kögel, P. Sperr and W. Triftshäuser; Materials Science Forum 255-257 (1997) 741-743.
Abstract: A first prototype of a Scanning Positron Microscope (SPM) with a pulsed beam has been developed in Munich. Besides of the positron beam also an electron beam is provided which produces a conventional electron image of the specimen. Currently we are setting the SPM in operation. A number of problems have been encountered in this process, particularly with the beam control, with various deflection systems and with instabilities due to fluctuating magnetic fields. The technical modifications to overcome these problems and the improvements achieved in this way will be reported.
BibTeX:
	@article{David1997,
	  author = {David, A. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Progress report from the Munich Scanning Positron Microscope},
	  journal = {Materials Science Forum},
	  year = {1997},
	  volume = {255-257},
	  pages = {741--743},
	  url = {https://www.scientific.net/MSF.255-257.741},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.255-257.741}
	}
	
Investigation of band bending in silicon by slow positron lifetime measurements
J. Duffy, W. Bauer-Kugelmann, G. Kögel and W. Triftshäuser; Applied Surface Science 116 (1997) 241-246.
Abstract: A novel approach to the analysis of positron lifetime data in depth sensitive studies using a pulsed positron beam is applied to model the diffusion of positrons in silicon. By examining only the observable lifetime parameters, inhomogeneous effects can be studied without the need to solve the time dependent diffusion equation. In particular, we study the effect of band bending near the sample surface, which creates an internal electric field. We present our first results of this analysis on p- and n-type bulk silicon samples. No band-bending could be measured for samples covered with a natural oxide. After surface treatment with HF(aq), p-type samples exhibited band bending, but the results for n-type samples were not significantly affected. In only one case, a p-type Si(B) sample with acceptor concentration ca = 1020 cm-3 and with a H terminated surface, is evidence of Fermi level pinning observed.
BibTeX:
	@article{Duffy1997,
	  author = {Duffy, J.A. and Bauer-Kugelmann, W. and Kögel, G. and Triftshäuser, W.},
	  title = {Investigation of band bending in silicon by slow positron lifetime measurements},
	  journal = {Applied Surface Science},
	  year = {1997},
	  volume = {116},
	  pages = {241--246},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433296010628?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0169-4332(96)01062-8}
	}
	
Vacancy defects in low-temperature-grown GaAs observed by continuous and pulsed slow positrons
J. Gebauer, R. Krause-Rehberg, S. Eichler, W. Bauer-Kugelmann, G. Kögel, W. Trifthäuser, M. Luysberg, H. Sohn and E. Weber; Materials Science Forum 255-257 (1997) 204-208.
Abstract: A systematic investigation of GaAs grown at low temperatures (LT-GaAs) was carried out. The vacancy defects in the as-grown material were identified to be mainly Ga vacancies by comparing the core-(W) and valence-(S) annihilation parameters to that of Ga vacancies in highly Si-doped GaAs. The vacancy concentration increases up to 1018 cm-3. Isochronal annealing was done at various samples. The S parameter in the samples increases with annealing, suggesting the formation of new defects. By checking the correlation between S and W we estimated that the defects seen in annealed LT-GaAs are physically different from that in the as-grown state. The annealed samples showed nearly saturated trapping with a defect related positron lifetime of 345 ps, which can be attributed to vacancy clusters or the As precipitates found by correlated TEM measurements.
BibTeX:
	@article{Gebauer1997,
	  author = {Gebauer, J. and Krause-Rehberg, R. and Eichler, S. and Bauer-Kugelmann, W. and Kögel, G. and Trifthäuser, W. and Luysberg, M. and Sohn, H. and Weber, E.R.},
	  title = {Vacancy defects in low-temperature-grown GaAs observed by continuous and pulsed slow positrons},
	  journal = {Materials Science Forum},
	  year = {1997},
	  volume = {255-257},
	  pages = {204--208},
	  url = {https://www.scientific.net/MSF.255-257.204},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.255-257.204}
	}
	
Preparation of intense positron sources for beam applications
A. Kauffmann, P. Sperr, G. Kögel and W. Triftshäuser; Materials Science Forum 255-257 (1997) 680-682.
Abstract: Radioactive isotopes are most frequently used as primary positron sources for low energy positron beam systems. In order to obtain optimum positron beam conditions, small source diameters with high intensities are desirable. Two different types of sources were prepared and investigated. 58Co was produced in a nuclear reactor via the reaction 58Ni(n,p)58Co. The bulk material was chemically separated and the 58Co was electrochemically deposited on a 1 mm diameter gold surface. The other type of source was obtained from sodium-acetate, which is commercially available. The solution was deposited on various surfaces. The prepared sources were investigated regarding the influence of inactive carrier material, internal and capsule absorption as well as backing materials by direct measurements of the positron energy spectra using a PIN-diode. In order to avoid the problem of handling very high activities, which should be applicable in these preparations, inactive material was added to simulate the absorption processes in the source.
BibTeX:
	@article{Kauffmann1997,
	  author = {Kauffmann, A. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Preparation of intense positron sources for beam applications},
	  journal = {Materials Science Forum},
	  year = {1997},
	  volume = {255-257},
	  pages = {680--682},
	  url = {https://www.scientific.net/MSF.255-257.680},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.255-257.680}
	}
	
Positron diffusion in solids and the reconstruction of inhomogeneous defect distributions from lifetime measurements
G. Kögel; Applied Physics A: Materials Science and Processing 63 (3) (1997) 227-235.
Abstract: The time dependent diffusion trapping equations for positrons implanted into inhomogeneous solids are analyzed. This problem is of central importance in the study of polycrystalline materials and for the application of pulsed positron beams to defect studies in materials research. The main problem in previous investigations was the necessity to solve the time-dependent diffusion equation. It prevented analytical treatment in all but the simplest applications. For the first time this difficulty is eliminated by invoking a new concept, the observable local annihilation characteristics for local implantation of positrons into the thermalized ensemble. It will be shown that the local annihilation characteristics are governed by field equations which reduce to the well known quantities of the standard trapping model in the case of homogeneous defect distributions. Furthermore, inhomogeneous defect distributions are uniquely determined from the field equations provided the local annihilation characteristics are known. Analytical solutions are derived and applied successfully to recent experimental results for a selection of simple, but realistic problems. The formal procedure includes internal drift fields and could be extended to cover also the epithermal period of positron thermalization, if necessary.
BibTeX:
	@article{Koegel1997,
	  author = {Kögel, G.},
	  title = {Positron diffusion in solids and the reconstruction of inhomogeneous defect distributions from lifetime measurements},
	  journal = {Applied Physics A: Materials Science and Processing},
	  year = {1997},
	  volume = {63},
	  number = {3},
	  pages = {227--235},
	  url = {http://link.springer.com/article/10.1007%2FBF01567874},
	  doi = {http://doi.org/10.1007/BF01567874}
	}
	
The München scanning positron microscope
G. Kögel, W. Triftshäuser, P. Sperr, K. Uhlmann, D. Britton, P. Willutzki, R. Steindl, W. Junker, A. Zecca, R. Brusa, M. Duarte-Naia, G. Karwasz, J. Paridaens and A. Piazza; Applied Surface Science 116 (1997) 108-113.
Abstract: For the first time a scanning positron microscope (SPM) using a pulsed positron beam for the nondestructive, three-dimensionally resolved investigation of defect distributions has been constructed. A positron beam with variable energy from 0.5 to 30 keV, with a spot diameter of 1 ?m or below, can be scanned over an area of 0.6 × 0.6 mm2. Beam pulsing enables positron lifetime studies with a time resolution of 200 ps FWHM. The beam is formed by double stage stochastic cooling (moderation) of the positrons emitted from a radioactive isotope. Included in the system is a conventional scanning electron microprobe for easy alignment of the optical column and for surface analysis. The design of the entire SPM is determined by the special demands of positron physics, which are discussed briefly.
BibTeX:
	@article{Koegel1997a,
	  author = {Kögel, G. and Triftshäuser, W. and Sperr, P. and Uhlmann, K. and Britton, D.T. and Willutzki, P. and Steindl, R. and Junker, W. and Zecca, A. and Brusa, R.S. and Duarte-Naia, M.P. and Karwasz, G.P. and Paridaens, J. and Piazza, A.},
	  title = {The München scanning positron microscope},
	  journal = {Applied Surface Science},
	  year = {1997},
	  volume = {116},
	  pages = {108--113},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433296010380?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0169-4332(96)01038-0}
	}
	
Recent investigations on the reconstruction of defect profiles from data obtained by pulsed positron beams
G. Kögel; Materials Science Forum 255-257 (1997) 243-247.
Abstract: A general procedure is presented for the systematic determination of the microscopic defect structure from lifetime measurements with pulsed positron beams. The proposed procedure is based on exact relations with full regard of counting statistics. Initially, a deterministic search for characteristic features of the depth profile is performed in the correlation matrix of the experimental data. On this basis the entire problem is decomposed into a set of small subproblems which can be handled by well established iterative search methods.
BibTeX:
	@article{Koegel1997b,
	  author = {Kögel, G.},
	  title = {Recent investigations on the reconstruction of defect profiles from data obtained by pulsed positron beams},
	  journal = {Materials Science Forum},
	  year = {1997},
	  volume = {255-257},
	  pages = {243--247},
	  url = {https://www.scientific.net/MSF.255-257.243},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.255-257.243}
	}
	
Defect spectroscopy in diamond, a new model for positron trapping in insulators
M. Shi, W. Waeber and W. Triftshäuser; Applied Surface Science 116 (1997) 203-210.
Abstract: The purpose of this contribution is double. The first aim is an attempt to understand the positron's behavior in diamond, i.e. the positron annihilation rate in its delocalized states and the trapping process to vacancy-type defects. The second purpose is to qualify diamond membranes which have been produced for the application of field-assisted positron moderation. The results obtained from this comprehensive study of diamond single crystals and diamond membranes by using positron annihilation techniques are presented. The proposed mechanism for the trapping of positrons to vacancy-type defects, namely through positronium formation, gives a new picture for understanding the trapping process in insulators. It can be used to interpret the discrepancies in the earlier experimental results. The obtained results for diamond membranes indicate that they have sufficient quality to be used as the main body of field-assisted moderators.
BibTeX:
	@article{Shi1997,
	  author = {Shi, M. and Waeber, W.B. and Triftshäuser, W.},
	  title = {Defect spectroscopy in diamond, a new model for positron trapping in insulators},
	  journal = {Applied Surface Science},
	  year = {1997},
	  volume = {116},
	  pages = {203--210},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433296010550?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0169-4332(96)01055-0}
	}
	
Pulsing of low energy positron beams
P. Sperr, G. Kögel, P. Willutzki and W. Triftshäuser; Applied Surface Science 116 (1997) 78-81.
Abstract: In this contribution we report on the pulsing components and on the timing performance of our new positron beam facility (the first stage of the scanning positron microscope). Some details of the timing elements and the mechanical construction of the pulsing (bunching) elements are given. First experimental results of the timing measurements of the single stages of the pulsing elements are given.
BibTeX:
	@article{Sperr1997,
	  author = {Sperr, P. and Kögel, G. and Willutzki, P. and Triftshäuser, W.},
	  title = {Pulsing of low energy positron beams},
	  journal = {Applied Surface Science},
	  year = {1997},
	  volume = {116},
	  pages = {78--81},
	  url = {http://www.sciencedirect.com/science/article/pii/S016943329601032X?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0169-4332(96)01032-X}
	}
	
Present performance limits of pulsed positron beams
P. Sperr and G. Kögel; Materials Science Forum 255-257 (1997) 109-113.
Abstract: Pulsed positron beams can out perform conventional positron lifetime systems by orders of magnitude in both event rate and peak to background ratio. The actual performance of low energy pulsed positron beams, however, is is still far from its limits. The technical reasons are spotted and possible improvements are discussed with respect to over ten years of technical developments performed at Munich. Improvements like reduction of the pulse period to 50 MHz, saw-tooth prebuncher, pre-chopper and redesign of the sample chamber which were performed in our currently running system (version 2) are shown. It is argued, that the third version (now under construction) will finally accumulate 104 counts/s at a peak to background ratio of 105. We also show plans of a rf-accelerator for a pulsed variable energy positron beam with source and sample on ground potential.
BibTeX:
	@article{Sperr1997a,
	  author = {Sperr, P. and Kögel, G.},
	  title = {Present performance limits of pulsed positron beams},
	  journal = {Materials Science Forum},
	  year = {1997},
	  volume = {255-257},
	  pages = {109--113},
	  url = {https://www.scientific.net/MSF.255-257.109},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.255-257.109}
	}
	
Intense positron source at the Munich research reactor
M. Springer, G. Kögel, B. Straßer, W. Triftshäuser and K. Schreckenbach; Materials Science Forum 255-257 (1997) 695-697.
Abstract: An in-pile slow positron source was recently installed at the FRM research reactor of the Technical University of Munich. Neutron capture gamma rays of 113 Cd (n,?) 114 Cd are converted into e+ - e- pairs by absorption in tungsten foils. Moderated positrons are emitted from the foil surface, accelerated by electric lenses to a few keV and guided to a remoderator outside the reactor pool by a magnetic solenoid. In a first approach the beam at the entrance of the remoderator yielded an intensity of 2.108 slow positrons per sec. The remoderator stage is already constructed and presently tested with electrons. The source will provide an order of magnitude more intensity when it will be installed at the FRM-II high flux reactor, which is under construction.
BibTeX:
	@article{Springer1997,
	  author = {Springer, M. and Kögel, G. and Straßer, B. and Triftshäuser, W. and Schreckenbach, K.},
	  title = {Intense positron source at the Munich research reactor},
	  journal = {Materials Science Forum},
	  year = {1997},
	  volume = {255-257},
	  pages = {695--697},
	  url = {https://www.scientific.net/MSF.255-257.695},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.255-257.695}
	}
	
A high intense reactor based positron source
G. Triftshäuser, G. Kögel, W. Triftshäuser, M. Springer, B. Straßer and K. Schreckenbach; Applied Surface Science 116 (1997) 45-48.
Abstract: An intense source of monoenergetic, low energy positrons is presented. The positrons are produced by pair production from high energy ?-rays after the capture of thermal neutrons in cadmium. The pair production itself takes place in a stack of layers, each consisting of two concentric rings made of tungsten foils. The source is placed in a beam tube at the Research Reactor Munich at Garching. From first experimental data, we can extrapolate an intensity of 2 × 108 slow positrons per second and a spot size of less than 26 mm.
BibTeX:
	@article{Triftshaeuser1997,
	  author = {Triftshäuser, G. and Kögel, G. and Triftshäuser, W. and Springer, M. and Straßer, B. and Schreckenbach, K.},
	  title = {A high intense reactor based positron source},
	  journal = {Applied Surface Science},
	  year = {1997},
	  volume = {116},
	  pages = {45--48},
	  url = {http://www.sciencedirect.com/science/article/pii/S0169433296009725?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0169-4332(96)00972-5}
	}
	
A scanning positron microscope for defect analysis in materials science
W. Triftshäuser, G. Kögel, P. Sperr, D. Britton, K. Uhlmann and P. Willutzki; Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 130 (1-4) (1997) 264-269.
Abstract: The realisation of a scanning positron microscope will be presented and discussed. A positron beam with a variable energy from 0.5 to 30 keV, with a spot diameter of 1 ?m or below, can be scanned over an area of 0.6 × 0.6 mm2. This beam is formed after a double stage stochastic cooling (moderation) of positrons emitted from a radioactive isotope. In addition the positron beam will be pulsed in order to have a well-defined time base for positron lifetime measurements. In the system included is a conventional scanning electron microprobe for surface analysis. The design of the scanning positron microscope is dominated by the special demands of positron physics.
BibTeX:
	@article{Triftshaeuser1997a,
	  author = {Triftshäuser, W. and Kögel, G. and Sperr, P. and Britton, D.T. and Uhlmann, K. and Willutzki, P.},
	  title = {A scanning positron microscope for defect analysis in materials science},
	  journal = {Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms},
	  year = {1997},
	  volume = {130},
	  number = {1-4},
	  pages = {264--269},
	  url = {http://www.sciencedirect.com/science/article/pii/S0168583X97001742?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0168-583X(97)00174-2}
	}
	
The relationship between open volume defects and deposition conditions of superconducting thin-film YBa2Cu3O7-x
X. Zhou, X. Lu, H. Jiang, W. Bauer-Kugelmann, J. Duffy, G. Kögel and W. Triftshäuser; Journal of Physics: Condensed Matter 9 (6) (1997) L61-L66.
Abstract: The relationship between the open volume defects and the deposition conditions of superconducting thin-film YBa2Cu3O7-x was studied by the position lifetime technique. Using a low-energy pulsed positron system, positron lifetime as a function of implantation energy was measured on epitaxial superconducting thin-film YBa2Cu3O7-x deposited on yttrium stabilized cubic zirconia substrates (YSZ) with pulsed laser deposition in a partial pressure of air under different conditions. The results show that the type of open volume defect is independent of deposition conditions such as the substrate temperature, Ts, and the air pressure, pa. The defect concentration increases with decreasing Ts and increasing pa.
BibTeX:
	@article{Zhou1997,
	  author = {Zhou, X.Y. and Lu, X. and Jiang, H. and Bauer-Kugelmann, W. and Duffy, J.A. and Kögel, G. and Triftshäuser, W.},
	  title = {The relationship between open volume defects and deposition conditions of superconducting thin-film YBa2Cu3O7-x},
	  journal = {Journal of Physics: Condensed Matter},
	  year = {1997},
	  volume = {9},
	  number = {6},
	  pages = {L61-L66},
	  url = {http://iopscience.iop.org/article/10.1088/0953-8984/9/6/003/meta},
	  doi = {http://doi.org/10.1088/0953-8984/9/6/003}
	}
	
Positron trapping in a co-evaporation epitaxial superconducting thin film YBa2Cu3O7-x
X. Zhou, W. Bauer-Kugelmann, J. Störmer, G. Kögel and W. Triftshäuser; Physics Letters, Section A: General, Atomic and Solid State Physics 225 (1-3) (1997) 143-148.
Abstract: The positron lifetime as a function of implantation energy was measured on a co-evaporation epitaxial superconducting thin film YBa2Cu3O7-x by using a pulsed low energy positron beam system in Munich. Detailed analysis shows that (i) there are both shallow and deep trapping centers in the film; (ii) the defects concerning the long-lived lifetime, ?2, have an enlargement trend at lower temperature and may become one of the effective flux pinning centers. The variation of the mean lifetime with temperature indicates that almost all positrons escape from the shallow trapping centers at a temperature higher than the room temperature. At temperatures higher than 373 K, an annealing effect should be considered.
BibTeX:
	@article{Zhou1997a,
	  author = {Zhou, X.Y. and Bauer-Kugelmann, W. and Störmer, J. and Kögel, G. and Triftshäuser, W.},
	  title = {Positron trapping in a co-evaporation epitaxial superconducting thin film YBa2Cu3O7-x},
	  journal = {Physics Letters, Section A: General, Atomic and Solid State Physics},
	  year = {1997},
	  volume = {225},
	  number = {1-3},
	  pages = {143--148},
	  url = {http://www.sciencedirect.com/science/article/pii/S0375960196008560?via%3Dihub},
	  doi = {http://doi.org/10.1016/S0375-9601(96)00856-0}
	}
	

1996

Positron diffusion in solids and the reconstruction of inhomogeneous defect distributions from lifetime measurements
G. Kögel; Applied Physics A 63 (3) (1996) 227-235.
Abstract: The time dependent diffusion trapping equations for positrons implanted into inhomogeneous solids are analyzed. This problem is of central importance in the study of polycrystalline materials and for the application of pulsed positron beams to defect studies in materials research. The main problem in previous investigations was the necessity to solve the time-dependent diffusion equation. It prevented analytical treatment in all but the simplest applications. For the first time this difficulty is eliminated by invoking a new concept, the observable local annihilation characteristics for local implantation of positrons into the thermalized ensemble. It will be shown that the local annihilation characteristics are governed by field equations which reduce to the well known quantities of the standard trapping model in the case of homogeneous defect distributions. Furthermore, inhomogeneous defect distributions are uniquely determined from the field equations provided the local annihilation characteristics are known. Analytical solutions are derived and applied successfully to recent experimental results for a selection of simple, but realistic problems. The formal procedure includes internal drift fields and could be extended to cover also the epithermal period of positron thermalization, if necessary.
BibTeX:
	@article{Koegel1996,
	  author = {Kögel, G.},
	  title = {Positron diffusion in solids and the reconstruction of inhomogeneous defect distributions from lifetime measurements},
	  journal = {Applied Physics A},
	  year = {1996},
	  volume = {63},
	  number = {3},
	  pages = {227--235},
	  url = {http://link.springer.com/article/10.1007%2FBF01567874},
	  doi = {http://doi.org/10.1007/BF01567874}
	}
	
Vacancy-type defects in molecular beam epitaxy low temperature grown GaAs, a positron beam lifetime study
J. Störmer, W. Triftshäuser, N. Hozhabri and K. Alavi; Applied Physics Letters 69 (13) (1996) 1867-1869.
Abstract: Positron beam lifetime spectroscopy has been utilized to study the depth distribution of vacancy-type defects in molecular beam epitaxy GaAs grown at low temperature. Lifetime spectra were measured as a function of positron energy. From the analysis of the positron lifetime in as-grown and annealed low temperature grown GaAs, the concentrations of Ga monovacancies and voids are estimated. Our results show that in an as-grown sample the Ga monovacancy concentration is >3×1018 cm-3. It is also known that vacancy-cluster concentration in an annealed sample exceeds 1018 cm-3 with a nonuniform spatial distribution. © 1996 American Institute of Physics.
BibTeX:
	@article{Stoermer1996,
	  author = {Störmer, J. and Triftshäuser, W. and Hozhabri, N. and Alavi, K.},
	  title = {Vacancy-type defects in molecular beam epitaxy low temperature grown GaAs, a positron beam lifetime study},
	  journal = {Applied Physics Letters},
	  year = {1996},
	  volume = {69},
	  number = {13},
	  pages = {1867--1869},
	  url = {http://aip.scitation.org/doi/10.1063/1.117460},
	  doi = {http://doi.org/10.1063/1.117460}
	}
	
Silicon carbide: A new positron moderator
J. Störmer, A. Goodyear, W. Anwand, G. Brauer, P. Coleman and W. Triftshäuser; Journal of Physics Condensed Matter 8 (7) (1996) L89-L94.
Abstract: Observation of copious positron re-emission from crystalline 6H-SiC, with no pretreatment and without the need for ultra-high-vacuum conditions, suggests that this material may form the basis of an important new moderator for the production of monoenergetic positrons. Its positron work function is measured to be -3.0 ± 0.2 eV. Its electrical characteristics point to SiC as a prime candidate for development as a field-assisted positron moderator, producing moderately intense slow-positron beams in laboratory-based systems and enabling a new generation of positron experimentation.
BibTeX:
	@article{Stoermer1996a,
	  author = {Störmer, J. and Goodyear, A. and Anwand, W. and Brauer, G. and Coleman, P.G. and Triftshäuser, W.},
	  title = {Silicon carbide: A new positron moderator},
	  journal = {Journal of Physics Condensed Matter},
	  year = {1996},
	  volume = {8},
	  number = {7},
	  pages = {L89-L94},
	  url = {http://iopscience.iop.org/article/10.1088/0953-8984/8/7/002/meta},
	  doi = {http://doi.org/10.1088/0953-8984/8/7/002}
	}
	
Positron annihilation in the epitaxial superconducting thin-film GdBa2Cu3O7-? studied by using a pulsed positron beam
X. Zhou, J. Störmer, R. Wang, J. Keimel, H. Li, G. Kögel and W. Triftshäuser; Physical Review B - Condensed Matter and Materials Physics 54 (2) (1996) 1398-1403.
Abstract: The positron lifetime as a function of implantation energy was measured on the epitaxial superconducting thin film GdBa2Cu3O7-? at different temperatures. The coexistence of both shallow and deep positron trapping centers was observed in the film. The shallow trapping centers include the screw dislocations and twin boundaries. The binding energy of the shallow trapping centers was estimated to be 56±12 meV. The deep trapping centers are assigned the cation vacancies, especially barium vacancies. On the surface of the sample there are macroscopic free volume holes in which positronium could be formed.
BibTeX:
	@article{Zhou1996,
	  author = {Zhou, X.Y. and Störmer, J. and Wang, R.L. and Keimel, J. and Li, H.C. and Kögel, G. and Triftshäuser, W.},
	  title = {Positron annihilation in the epitaxial superconducting thin-film GdBa2Cu3O7-? studied by using a pulsed positron beam},
	  journal = {Physical Review B - Condensed Matter and Materials Physics},
	  year = {1996},
	  volume = {54},
	  number = {2},
	  pages = {1398--1403},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.54.1398},
	  doi = {http://doi.org/10.1103/PhysRevB.54.1398}
	}
	

1995

Positron studies of polycrystalline TiC
G. Brauer, W. Anwand, E. Nicht, P. Coleman, A. Knights, H. Schut, G. Kögel and N. Wagner; Journal of Physics: Condensed Matter 7 (47) (1995) 9091-9099.
Abstract: The mean positron lifetime tau , positron diffusion length L, and the positron and electron work functions ( phi + and phi -) for polycrystalline TiC have been experimentally determined. The results were tau =160(2) ps, L+=138(27) nm and phi -=3.96(0.08) eV; phi + was shown to be almost certainly positive. These results strongly support the suggestion from recent first-principles electronic structure and positron state calculations that positions are trapped by and annihilate in metal vacancies in this material. XPS measurements indicate that the trapping sites may be predominantly in thin carbon-rich layers between grains, a picture which may also explain the long near-surface diffusion length.
BibTeX:
	@article{Brauer1995,
	  author = {Brauer, G. and Anwand, W. and Nicht, E. and Coleman, P.G. and Knights, A.P. and Schut, H. and Kögel, G. and Wagner, N.},
	  title = {Positron studies of polycrystalline TiC},
	  journal = {Journal of Physics: Condensed Matter},
	  year = {1995},
	  volume = {7},
	  number = {47},
	  pages = {9091--9099},
	  url = {http://iopscience.iop.org/article/10.1088/0953-8984/7/47/025/meta},
	  doi = {http://doi.org/10.1088/0953-8984/7/47/025}
	}
	
Magnetic positron optics
D. Britton, K. Uhlmann and G. Kögel; Applied Surface Science 85 (C) (1995) 158-164.
Abstract: Two different magnetic lenses, suitable for use as objective lenses in positron beams, which can be positioned outside the vacuum system, behind the target are presented. The first is a single-pole lens, optimised as a parallising objective for the reemission of slow positrons from a remoderator. The second is a side-gap lens designed as a probe-forming lens for variable incident energy up to 30 keV with no change in its optical properties. Additionally the effect of an additional solenoid field on the properties of the probe-forming lens has been investigated. © 1995.
BibTeX:
	@article{Britton1995,
	  author = {Britton, D.T. and Uhlmann, K. and Kögel, G.},
	  title = {Magnetic positron optics},
	  journal = {Applied Surface Science},
	  year = {1995},
	  volume = {85},
	  number = {C},
	  pages = {158--164},
	  url = {http://www.sciencedirect.com/science/article/pii/0169433294003262?via%3Dihub},
	  doi = {http://doi.org/10.1016/0169-4332(94)00326-2}
	}
	
Defect Profiling with Pulsed e+-Beams
G. Kögel; Materials Science Forum 175-178 (1995) 107-114.
BibTeX:
	@article{Koegel1995,
	  author = {Kögel, Gottfried},
	  title = {Defect Profiling with Pulsed e+-Beams},
	  journal = {Materials Science Forum},
	  year = {1995},
	  volume = {175-178},
	  pages = {107--114},
	  url = {https://www.scientific.net/MSF.175-178.107},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.175-178.107}
	}
	
A slow positron lifetime study of the annealing behaviour of an amorphous silicon layer grown by MBE
J. Störmer, P. Willutzki, D. Britton, G. Kögel, W. Triftshäuser, W. Kiunke, F. Wittmann and I. Eisele; Applied Physics A Materials Science & Processing 61 (1) (1995) 71-74.
Abstract: We have studied MBE grown amorphous silicon, which was recrystallized at different temperatures for one hour, with a pulsed positron beam. A positron lifetime of 538±10 ps in the as-grown state is attributed to microvoids containing at least 10 vacancies. An incompletely recrystallized sample annealed at 500°C shows an additional long lifetime from ortho-positronium (o-Ps) pick-off annihilation. The o-Ps component disappears for samples, recrystallized at 700°C and above, and the defect lifetime steadily decreases with higher annealing temperature until a value of 310 ps is reached for the layer annealed at 1200°C. This value is explained by positron trapping at dislocations or small vacancy defects stabilized by dislocations or impurities. © 1995 Springer-Verlag.
BibTeX:
	@article{Stoermer1995,
	  author = {Störmer, J. and Willutzki, P. and Britton, D.T. and Kögel, G. and Triftshäuser, W. and Kiunke, W. and Wittmann, F. and Eisele, I.},
	  title = {A slow positron lifetime study of the annealing behaviour of an amorphous silicon layer grown by MBE},
	  journal = {Applied Physics A Materials Science & Processing},
	  year = {1995},
	  volume = {61},
	  number = {1},
	  pages = {71--74},
	  url = {http://link.springer.com/article/10.1007%2FBF01538214},
	  doi = {http://doi.org/10.1007/BF01538214}
	}
	
A High Intense Positron Source at the Munich Research Reactor
G. Triftshäuser, G. Kögel, W. Triftshäuser, M. Springer, T. Hagner and K. Schreckenbach; Materials Science Forum 175-178 (1995) 221-224.
BibTeX:
	@article{Triftshaeuser1995,
	  author = {Triftshäuser, G. and Kögel, Gottfried and Triftshäuser, Werner and Springer, M. and Hagner, T. and Schreckenbach, K.},
	  title = {A High Intense Positron Source at the Munich Research Reactor},
	  journal = {Materials Science Forum},
	  year = {1995},
	  volume = {175-178},
	  pages = {221--224},
	  url = {https://www.scientific.net/MSF.175-178.221},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.175-178.221}
	}
	
A concept of a scanning positron microscope
K. Uhlmann, W. Triftshäuser, G. Kögel, P. Sperr, D. Britton, A. Zecca, R. Brusa and G. Karwasz; Fresenius' Journal of Analytical Chemistry 353 (5-8) (1995) 594-597.
Abstract: The concept of a scanning positron microscope which is under construction in München and Trento will be shown. A beam with a variable energy from 1 to 30 keV and a spot diameter of 1 ?m, which can be scanned over an area of (0.6×0.6) mm2, is formed after a double stage stochastic cooling of the positrons emitted from a radioactive source. Additionally, the beam will be pulsed to have a well-defined time base for positron lifetime measurements. The design of the microscope is dominated by special demands of positron physics. Therefore, the microscope contains electron optical elements which are well known but rarely used. These are the through the lens reflection remoderator and the optical column with a magnetic side gap lens as probe forming lens.
BibTeX:
	@article{Uhlmann1995,
	  author = {Uhlmann, K. and Triftshäuser, W. and Kögel, G. and Sperr, P. and Britton, D.T. and Zecca, A. and Brusa, R.S. and Karwasz, G.},
	  title = {A concept of a scanning positron microscope},
	  journal = {Fresenius' Journal of Analytical Chemistry},
	  year = {1995},
	  volume = {353},
	  number = {5-8},
	  pages = {594--597},
	  url = {http://link.springer.com/article/10.1007%2FBF00321331},
	  doi = {http://doi.org/10.1007/BF00321331}
	}
	
A new optical column for a scanning positron microscope
K. Uhlmann, D. Britton and G. Kögel; Measurement Science and Technology 6 (7) (1995) 932-938.
Abstract: An optical column for a scanning positron microprobe is described. A beam of variable energy from 1 to 30 keV and a spot diameter of 1 mu m, that can be scanned over an area of 0.6*0.6 mm2 is formed using electron optical elements that are already well known but rarely used in an optical column. The idea allows the construction of a sample chamber free of material and electric fields in a hemisphere above the sample with a radius of some 100 mm or more. The absence of diaphragms and the comfortable inner diameter of the lenses used makes it possible to work with a beam of comparatively large diameter and aperture. This is necessary because of the low intensity of the positron sources available, which makes a system with a very high transmission essential. Although the idea was created for a positron microprobe, other applications, such as with a retarding field analyser introduced into the sample chamber, are conceivable.
BibTeX:
	@article{Uhlmann1995a,
	  author = {Uhlmann, K. and Britton, D.T. and Kögel, G.},
	  title = {A new optical column for a scanning positron microscope},
	  journal = {Measurement Science and Technology},
	  year = {1995},
	  volume = {6},
	  number = {7},
	  pages = {932--938},
	  url = {http://iopscience.iop.org/article/10.1088/0957-0233/6/7/012/meta},
	  doi = {http://doi.org/10.1088/0957-0233/6/7/012}
	}
	
Investigations of positron lifetimes in InP with a pulsed positron beam
P. Willutzki, J. Störmer, D. Britton and W. Triftshäuser; Applied Physics A Materials Science & Processing 61 (3) (1995) 321-324.
Abstract: Indium Phosphide layers grown by gas source Molecular Beam Epitaxy, (MBE) have been studied by positron lifetime spectroscopy using the recently modified pulsed positron beam in Munich. The as-grown samples are known to be phosphorous rich and contain a high concentration of vacancy-type defects. On annealing, phosphorous precipitates are formed and the concentration of free volume defects increases. Positron lifetime spectroscopy has identified the grown in defects to be indium vacancies at a concentration around 1018cm-3. The dominant defects after annealing exhibit a positron lifetime characteristic of divacancies and are present at concentrations in excess of 5×1019cm-3. © 1995 Springer-Verlag.
BibTeX:
	@article{Willutzki1995,
	  author = {Willutzki, P. and Störmer, J. and Britton, D.T. and Triftshäuser, W.},
	  title = {Investigations of positron lifetimes in InP with a pulsed positron beam},
	  journal = {Applied Physics A Materials Science & Processing},
	  year = {1995},
	  volume = {61},
	  number = {3},
	  pages = {321--324},
	  url = {http://link.springer.com/article/10.1007%2FBF01538197},
	  doi = {http://doi.org/10.1007/BF01538197}
	}
	
An Upgraded Version of a Pulsed Positron Beam
P. Willutzki, J. Störmer, G. Kögel, P. Sperr, D. Britton, R. Steindl and W. Triftshäuser; Materials Science Forum 175-178 (1995) 237-240.
BibTeX:
	@article{Willutzki1995a,
	  author = {Willutzki, P. and Störmer, J. and Kögel, Gottfried and Sperr, Peter and Britton, D.T. and Steindl, R. and Triftshäuser, Werner},
	  title = {An Upgraded Version of a Pulsed Positron Beam},
	  journal = {Materials Science Forum},
	  year = {1995},
	  volume = {175-178},
	  pages = {237--240},
	  url = {https://www.scientific.net/MSF.175-178.237},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.175-178.237}
	}
	
A Pulsed Positron Microbeam
A. Zecca, R.S. Brusa, M.P. Duarte-Naia, G.P. Karwasz, J. Paridaens, A. Piazza, G. Kögel, P. Sperr, D.T. Britton, K. Uhlmann, P. Willutzki and W. Triftshäuser; Europhysics Letters (EPL) 29 (8) (1995) 617-622.
Abstract: he first pulsed microbeam for positrons in the keV energy range is described. The principle of operation and the construction details are given. The performance achieved is: 20 ?m (FWHM) spot diameter; 350 ps (FWHM) time resolution; 5000 e + /(s mCi) at 5 keV final energy.
BibTeX:
	@article{Zecca1995,
	  author = {Zecca, A. and Brusa, R. S. and Duarte-Naia, M. P. and Karwasz, G. P. and Paridaens, J. and Piazza, A. and Kögel, G. and Sperr, P. and Britton, D. T. and Uhlmann, K. and Willutzki, P. and Triftshäuser, W.},
	  title = {A Pulsed Positron Microbeam},
	  journal = {Europhysics Letters (EPL)},
	  year = {1995},
	  volume = {29},
	  number = {8},
	  pages = {617--622},
	  url = {http://stacks.iop.org/0295-5075/29/i=8/a=005},
	  doi = {http://doi.org/10.1209/0295-5075/29/8/005}
	}
	

1994

On the sensitivity of positrons to electric fields and defects in MBE-grown silicon structures
D. Britton, P. Willutzki, W. Triftshäuser, E. Hammerl, W. Hansch and I. Eisele; Applied Physics A Solids and Surfaces 58 (4) (1994) 389-393.
Abstract: The sensitivity of the positron to the internal electric fields in good quality thin (?100 nm) Molecular Beam Epitaxy (MBE)-grown layers is experimentally demonstrated. Both a thin intrinsic layer grown on a p-type substrate and a highly n-doped ? profile buried in intrinsic silicon form effective barriers to positron diffusion although no defects can be detected. We also extract, from a full treatment of the positron diffusion, a quantitative estimate of the concentration, below the detection limits of other methods, of large vacancy clusters in a thick (680 nm) film. © 1994 Springer-Verlag.
BibTeX:
	@article{Britton1994,
	  author = {Britton, D.T. and Willutzki, P. and Triftshäuser, W. and Hammerl, E. and Hansch, W. and Eisele, I.},
	  title = {On the sensitivity of positrons to electric fields and defects in MBE-grown silicon structures},
	  journal = {Applied Physics A Solids and Surfaces},
	  year = {1994},
	  volume = {58},
	  number = {4},
	  pages = {389--393},
	  url = {http://link.springer.com/article/10.1007%2FBF00323615},
	  doi = {http://doi.org/10.1007/BF00323615}
	}
	
On the electronic design and the performance of a ramp generator for a pulsed low energy positron beam
P. Sperr, P. Willutzki and M. Maier; Materials Science Forum 175-178 (pt 2) (1994) 993-996.
Abstract: In this work we report on the electronic design of a simple ramp generator for pulsed low energy positron beams. The generator is realized using ECL integrated circuits, fast comparators and special voltage programmable current sources. With the achieved sawtooth output signal of a frequency of 50MHz, up to 70% of a dc positron beam can be compressed into pulses of about 1.8ns (FWHM). The performance of the generator in a pulsed positron beam system is shown.
BibTeX:
	@article{Sperr1994,
	  author = {Sperr, P. and Willutzki, P. and Maier, M.R.},
	  title = {On the electronic design and the performance of a ramp generator for a pulsed low energy positron beam},
	  journal = {Materials Science Forum},
	  year = {1994},
	  volume = {175-178},
	  number = {pt 2},
	  pages = {993--996},
	  url = {https://www.scientific.net/MSF.175-178.993},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.175-178.993}
	}
	
Intense sources for positron research
W. Triftshäuser; Acta Physica Hungarica 75 (1-4) (1994) 61-69.
Abstract: Positron annihilation is a well-established method in solid state physics and material science. The positron being a very sensitive probe, can give very precise information on the momentum distribution of electrons in metals and alloys as well as on lattice defects in crystals. Starting with the energy distribution of positrons from a radioactive decay, the current development is directed more to monoenergetic positrons of variable energy and of high intensity. The impact of intense positron beams is straightforward: a decrease of the counting time. There are various possibilities and approaches to realize intense positron beams. The aim is to obtain a beam intensity in the order of ? 1010 positrons/s. Parallel to the instalment of intense positron beams, the development of positron microscopes is pursued. © 1994 Akadémiai Kiadó.
BibTeX:
	@article{Triftshaeuser1994,
	  author = {Triftshäuser, W.},
	  title = {Intense sources for positron research},
	  journal = {Acta Physica Hungarica},
	  year = {1994},
	  volume = {75},
	  number = {1-4},
	  pages = {61--69},
	  url = {http://link.springer.com/article/10.1007/BF03156559},
	  doi = {http://doi.org/10.1007/BF03156559}
	}
	
Through the lens reflection remoderator for positrons
K. Uhlmann, D. Britton and G. Kögel; Optik (Jena) 98 (1) (1994) 5-10.
Abstract: A magnetic remoderation system with both incident and reemitted positron beams normal to the moderator surface is presented. Incoming 5 keV positrons are focussed by a single pole lens to an aberration limited spot on the remoderator. Reemitted positrons are extracted along the same axis at a lower energy before being separated from the incoming beam by a toroidal deflector. The single pole lens used is very well suited to different types of positron beam measurements. Its major advantage being that the space around the target can be kept free of both material and electric fields, while maintaining a short focal length. For the outgoing beam, the lens has a parallising action, suggesting applications in reemission positron and positron annihilation Auger electron spectroscopy.
BibTeX:
	@article{Uhlmann1994,
	  author = {Uhlmann, K. and Britton, D.T. and Kögel, G.},
	  title = {Through the lens reflection remoderator for positrons},
	  journal = {Optik (Jena)},
	  year = {1994},
	  volume = {98},
	  number = {1},
	  pages = {5--10}
	}
	
An improved pulsed low-energy positron system
P. Willutzki, J. Störmer, G. Kögel, P. Sperr, D. Britton, R. Steindl and W. Triftshäuser; Measurement Science and Technology 5 (5) (1994) 548-554.
Abstract: A pulsed low-energy positron system for positron-lifetime spectroscopy has been up-graded and modified with regard to its three main components. A new moderator preparation chamber has been added, the radiofrequency pulsing concept has been modified using a pre-buncher and a pre-chopper in front of the existing chopper-buncher section. Furthermore, a new target station allowing measurements at variable temperatures has been incorporated. The lifetime spectra now reveal a strongly increased ratio of peak to background and of peak to satellite peak.
BibTeX:
	@article{Willutzki1994,
	  author = {Willutzki, P. and Störmer, J. and Kögel, G. and Sperr, P. and Britton, D.T. and Steindl, R. and Triftshäuser, W.},
	  title = {An improved pulsed low-energy positron system},
	  journal = {Measurement Science and Technology},
	  year = {1994},
	  volume = {5},
	  number = {5},
	  pages = {548--554},
	  url = {http://iopscience.iop.org/article/10.1088/0957-0233/5/5/013/meta},
	  doi = {http://doi.org/10.1088/0957-0233/5/5/013}
	}
	
Upgraded version of a pulsed positron beam
P. Willutzki, J. Störmer, G. Kögel, P. Sperr, D. Britton, R. Steindl and W. Triftshäuser; Materials Science Forum 175-178 (pt 1) (1994) 237-240.
Abstract: In this paper an overview on the modifications of the Munich pulsed low energy positron system is given. In particular the moderator section and the new prebuncher are described in detail. With a new preparation chamber for in situ annealing of the moderator and a new prebuncher the count rate has been increased by a factor of five. Due to the new master frequency of 50MHz (old system: 200MHz) the lifetime spectra have a length of 20ns. Long positron lifetimes can now be easily analysed. A faraday cage in front of the target station reduces strongly the perturbations of the spectra (high background, satellite peaks). Finally a new target station allows lifetime measurements at variable temperatures. The time resolution of the new system is 230ps (FWHM).
BibTeX:
	@article{Willutzki1994a,
	  author = {Willutzki, P. and Störmer, J. and Kögel, G. and Sperr, P. and Britton, D.T. and Steindl, R. and Triftshäuser, W.},
	  title = {Upgraded version of a pulsed positron beam},
	  journal = {Materials Science Forum},
	  year = {1994},
	  volume = {175-178},
	  number = {pt 1},
	  pages = {237--240},
	  url = {https://www.scientific.net/MSF.175-178.237},
	  doi = {http://doi.org/10.4028/www.scientific.net/MSF.175-178.237}
	}
	
Retarding field and timing measurements of positrons from a W[100] foil
P. Willutzki, J. Störmer, D.T. Britton, G. Kögel, P. Sperr, R. Steindl and W. Triftshäuser; AIP Conference Proceedings 303 (1) (1994) 542-549.
Abstract: Measurements with the new prebuncher for the existing pulsed positron beam (presented at ICPA?9) show the possibility of bunching a dc?beam with a time structure of 2.1ns (FWHM). The pulse width is very sensitive to the longitudinal energy distribution of the moderated positrons. A pulse width of 2.1ns corresponds to a longitudinal energy distribution of 0.35(5) eV. With a retarding grid configuration we measured the positron yield. Measurements with inhomogenic magnetic guiding fields clearly show the influence of elastically scattered events. Their fraction is very sensitive to the surface condition of the moderator. The nonscattered positrons are very well represented by a Gaussian distribution function. Its FWHM is in good overall agreement with the timing measurements.
BibTeX:
	@article{Willutzki1994b,
	  author = {Willutzki, P. and Störmer, J. and Britton, D. T. and Kögel, G. and Sperr, P. and Steindl, R. and Triftshäuser, W.},
	  title = {Retarding field and timing measurements of positrons from a W[100] foil},
	  journal = {AIP Conference Proceedings},
	  year = {1994},
	  volume = {303},
	  number = {1},
	  pages = {542--549},
	  url = {http://scitation.aip.org/content/aip/proceeding/aipcp/10.1063/1.45536},
	  doi = {http://doi.org/10.1063/1.45536}
	}
	

1993

Investigation of defect structures in deformed and helium-irradiated refractory metals by positron annihilation
G. Kögel, P. Sperr, J. Störmer and W. Triftshäuser; Journal of Physics: Condensed Matter 5 (23) (1993) 3987-4006.
Abstract: The defect structures in deformed and helium-irradiated refractory metals, vanadium, niobium and tantalum have been investigated by means of positron lifetime and Doppler broadening techniques. The evolution of the defects was studied by performing isochronal annealing programmes. In all three metals after deformation a single, well defined lifetime was observed which was 30 ps to 40 ps lower than that reported for vacancies. The recovery of the deformation-induced defects however, is remarkably different for niobium and tantalum as compared to vanadium. In helium-irradiated vanadium and niobium two distinctly different defect types were observed with characteristic lifetimes of about 190 ps and 375 ps, respectively. The longer lifetime is attributed to large helium bubbles. Both metals showed a similar behaviour in the annealing studies. All measured lifetime data are consistent with the simple trapping model. The influence of oxygen impurities and oxygen resorption during the isochronal annealing procedure is discussed.
BibTeX:
	@article{Koegel1993,
	  author = {Kögel, G. and Sperr, P. and Störmer, J. and Triftshäuser, W.},
	  title = {Investigation of defect structures in deformed and helium-irradiated refractory metals by positron annihilation},
	  journal = {Journal of Physics: Condensed Matter},
	  year = {1993},
	  volume = {5},
	  number = {23},
	  pages = {3987--4006},
	  url = {http://iopscience.iop.org/article/10.1088/0953-8984/5/23/026/meta},
	  doi = {http://doi.org/10.1088/0953-8984/5/23/026}
	}
	
A study of the Fermi surfaces of lithium and disordered lithium-magnesium alloys: Theory and experiment
S. Rajput, R. Prasad, R. Singru, W. Triftshäuser, A. Eckert, G. Kögel, S. Kaprzyk and A. Bansil; Journal of Physics: Condensed Matter 5 (35) (1993) 6419-6432.
Abstract: The authors present a theoretical and experimental study of the Fermi surfaces (FSs) of Li and the disordered alloys Li1-xMgx (0.6>or=x>0.0) in the BCC phase. Theoretical calculations employ the first-principles fully charge-self-consistent Korringa-Kohn-Rostoker coherent-potential-approximation scheme within the local-density approximation. The experiments involve the two-dimensional angular correlation of positron annihilation radiation measurements on four Li1-xMgx single-crystal specimens with x=0.0, 0.28, 0.40 and 0.60. Good overall agreement is found between experiment and theory with regard to the size and shape of the FS as x is increased from 0.0 to 0.6, although some discrepancies are noted. The question of the critical Mg concentration x=xc at which the FS first makes contact with the Brillouin zone boundary in Li1-xMg x alloys is considered in some detail.
BibTeX:
	@article{Rajput1993,
	  author = {Rajput, S.S. and Prasad, R. and Singru, R.M. and Triftshäuser, W. and Eckert, A. and Kögel, G. and Kaprzyk, S. and Bansil, A.},
	  title = {A study of the Fermi surfaces of lithium and disordered lithium-magnesium alloys: Theory and experiment},
	  journal = {Journal of Physics: Condensed Matter},
	  year = {1993},
	  volume = {5},
	  number = {35},
	  pages = {6419--6432},
	  url = {http://iopscience.iop.org/article/10.1088/0953-8984/5/35/007/meta},
	  doi = {http://doi.org/10.1088/0953-8984/5/35/007}
	}
	
Positron annihilation with vacancies in thin surface layer of As heavily doped Si
L. Zhi-Heng, D.-C. Wang and G. Kögel; Acta Physica Sinica (Overseas Edn) 2 (8) (1993) 577-582.
BibTeX:
	@article{Zhi-Heng1993,
	  author = {Zhi-Heng, Lu and Wang, Da-Chun and Kögel, G.},
	  title = {Positron annihilation with vacancies in thin surface layer of As heavily doped Si},
	  journal = {Acta Physica Sinica (Overseas Edn)},
	  year = {1993},
	  volume = {2},
	  number = {8},
	  pages = {577--582},
	  url = {http://iopscience.iop.org/article/10.1088/1004-423X/2/8/003/meta},
	  doi = {http://doi.org/10.1088/1004-423x/2/8/003}
	}
	

1992

Positron Annihilation at Clean Metallic Surfaces
G. Kögel; Materials Science Forum 105-110 (1992) 341-348.
BibTeX:
	@article{Koegel1992,
	  author = {Kögel, Gottfried},
	  title = {Positron Annihilation at Clean Metallic Surfaces},
	  journal = {Materials Science Forum},
	  year = {1992},
	  volume = {105-110},
	  pages = {341--348},
	  url = {https://www.scientific.net/MSF.105-110.341},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.105-110.341}
	}
	
Study of Defects Produced by Laser Surface Melting Using Positron Annihilation
C. Lopes Gil, M. Ferreira Marques, A.P.d. Lima, R. Vilar, A. de Deus and W. Kögel G.and Triftshäuser; Materials Science Forum 105-110 (1992) 1141-1144.
BibTeX:
	@article{LopesGil1992,
	  author = {Lopes Gil, C. and Ferreira Marques, M.F. and Lima, Adriano P. de and Vilar, Rui and de Deus, A.M. and Kögel, G.and Triftshäuser, W.},
	  title = {Study of Defects Produced by Laser Surface Melting Using Positron Annihilation},
	  journal = {Materials Science Forum},
	  year = {1992},
	  volume = {105-110},
	  pages = {1141--1144},
	  url = {https://www.scientific.net/MSF.105-110.1141},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.105-110.1141}
	}
	
Positron Lifetimes on Clean Metallic Surfaces
R. Steindl, G. Kögel, P. Sperr, P. Willutzki, D. Britton and W. Triftshäuser; Materials Science Forum 105-110 (1992) 1455-1458.
BibTeX:
	@article{Steindl1992,
	  author = {Steindl, R. and Kögel, G. and Sperr, P. and Willutzki, P. and Britton, D.T. and Triftshäuser, W.},
	  title = {Positron Lifetimes on Clean Metallic Surfaces},
	  journal = {Materials Science Forum},
	  year = {1992},
	  volume = {105-110},
	  pages = {1455--1458},
	  url = {https://www.scientific.net/MSF.105-110.1455},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.105-110.1455}
	}
	
The Electron Momentum Distribution of LiMg Alloys by Positron-Annihilation 2D-ACAR
W. Triftshäuser, A. Eckert, G. Kögel and P. Sperr; Materials Science Forum 105-110 (1992) 501-510.
BibTeX:
	@article{Triftshaeuser1992,
	  author = {Triftshäuser, W. and Eckert, A. and Kögel, G. and Sperr, P.},
	  title = {The Electron Momentum Distribution of LiMg Alloys by Positron-Annihilation 2D-ACAR},
	  journal = {Materials Science Forum},
	  year = {1992},
	  volume = {105-110},
	  pages = {501--510},
	  url = {https://www.scientific.net/MSF.105-110.501},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.105-110.501}
	}
	
A New Concept for a Pulsed Positron Source
P. Willutzki, P. Sperr, D. Britton, G. Kögel, R. Steindl and W. Triftshäuser; Materials Science Forum 105-110 (1992) 2009-2012.
BibTeX:
	@article{Willutzki1992,
	  author = {Willutzki, P. and Sperr, P. and Britton, D.T. and Kögel, Gottfried and Steindl, R. and Triftshäuser, Werner},
	  title = {A New Concept for a Pulsed Positron Source},
	  journal = {Materials Science Forum},
	  year = {1992},
	  volume = {105-110},
	  pages = {2009--2012},
	  url = {https://www.scientific.net/MSF.105-110.2009},
	  doi = {http://doi.org/10.4028/www.scientific.net/msf.105-110.2009}
	}
	

1990

Investigation of defect structures in deuteron irradiated and deformed stainless steel 316 by positron annihilation
C. Lopes Gil, A.P. De Lima, N.A.D. Campos, P. Sperr, G. Kögel and W. Triftshäuser; Radiation Effects and Defects in Solids 112 (4) (1990) 111-118.
Abstract: Deuteron-irradiated and deformed stainless steel specimens were investigated by positron lifetime and Doppler broadening measurements. The evolution of the defect structures was studied as a function of the isochronal annealing temperature and for various degrees of deformation. A different behaviour was observed for deformed and irradiated stainless steel specimens. Evidence for vacancy clusters was found in the deuteron-irradiated steel. These clusters disappear after annealing around 900 K.
BibTeX:
	@article{Gil1990,
	  author = {Lopes Gil, C. and De Lima, A. P. and Campos, N. Ayres De and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Investigation of defect structures in deuteron irradiated and deformed stainless steel 316 by positron annihilation},
	  journal = {Radiation Effects and Defects in Solids},
	  year = {1990},
	  volume = {112},
	  number = {4},
	  pages = {111--118},
	  url = {http://www.sciencedirect.com/science/article/pii/016890029190768L},
	  doi = {http://doi.org/10.1080/10420159008213037}
	}
	
The pulsed positron beam in Munich and a high intense positron source at Grenoble
W. Triftshäuser, G. Kögel, K. Schreckenbach and B. Krusche; Helvetica Physica Acta 63 (4) (1990) 378-384.
BibTeX:
	@article{Triftshaeuser1990,
	  author = {Triftshäuser, W. and Kögel, G. and Schreckenbach, K. and Krusche, B.},
	  title = {The pulsed positron beam in Munich and a high intense positron source at Grenoble},
	  journal = {Helvetica Physica Acta},
	  year = {1990},
	  volume = {63},
	  number = {4},
	  pages = {378--384},
	  url = {http://www.e-periodica.ch/digbib/view?var=true&pid=hpa-001:1990:63::1098#379},
	  doi = {http://doi.org/10.5169/seals-116227}
	}
	

1989

Neutron-irradiated reactor pressure vessel steels investigated by positron annihilation
C. Lopes Gil, A. De Lima, N. De Campos, J. Fernandes, G. Kögel, P. Sperr, W. Triftshäuser and D. Pachur; Journal of Nuclear Materials 161 (1) (1989) 1-12.
Abstract: The recovery of the defects produced m neutron-irradiated low alloy ferritic steels has been investigated by positron annihilation techniques during is ochronal annealing programs. A good correlation was found between the annealing stages observed with positrons and the changes of the mechanical properties. A dramatic change in the positron parameters was observed around 400 ° C in the A533B samples irradiated at 290° C with high dose. An interpretative model is proposed which presents good consistency with the results obtained for the non-irradiated and the irradiated steels. © 1989.
BibTeX:
	@article{Gil1989,
	  author = {Lopes Gil, C. and De Lima, A.P. and De Campos, N.A. and Fernandes, J.V. and Kögel, G. and Sperr, P. and Triftshäuser, W. and Pachur, D.},
	  title = {Neutron-irradiated reactor pressure vessel steels investigated by positron annihilation},
	  journal = {Journal of Nuclear Materials},
	  year = {1989},
	  volume = {161},
	  number = {1},
	  pages = {1--12},
	  url = {http://www.sciencedirect.com/science/article/pii/002231158990456X?via%3Dihub},
	  doi = {http://doi.org/10.1016/0022-3115(89)90456-X}
	}
	
On the investigation of amorphous hydrogenated carbon by positron annihilation
G. Kögel, D. Schödlbauer, W. Triftshäuser and J. Winter; Journal of Nuclear Materials 162-164 (C) (1989) 876-880.
Abstract: Positron annihilation studies are presented in order to reveal details in the microstructure of amorphous hydrogenated carbon (a-C : H) which cannot be resolved by transmission electron microscopy (TEM). The first study already shows that micropores with about 0.5 nm in diameter occupy at least 3% of the total volume in a-C : H. However, this result is consistent with quite contrary models for the microstructure of a-C : H. The further clarification by extended positron studies is discussed. © 1989.
BibTeX:
	@article{Koegel1989,
	  author = {Kögel, G. and Schödlbauer, D. and Triftshäuser, W. and Winter, J.},
	  title = {On the investigation of amorphous hydrogenated carbon by positron annihilation},
	  journal = {Journal of Nuclear Materials},
	  year = {1989},
	  volume = {162-164},
	  number = {C},
	  pages = {876--880},
	  url = {http://www.sciencedirect.com/science/article/pii/0022311589903784?via%3Dihub},
	  doi = {http://doi.org/10.1016/0022-3115(89)90378-4}
	}
	
Lifetime spectroscopy in sub-surface defect studies
G. Kögel; In: , L. Dorikens-Vanpraet, M. Dorikens and D. Segers (Eds.), Proceedings of the 8th International Conference on Positron Annihilation, ICPA-8 (1989) 52-55 .
BibTeX:
	@inproceedings{Koegel1989a,
	  author = {Kögel, G.},
	  title = {Lifetime spectroscopy in sub-surface defect studies},
	  booktitle = {Proceedings of the 8th International Conference on Positron Annihilation, ICPA-8},
	  year = {1989},
	  pages = {52--55},
	  editor = {Dorikens-Vanpraet, L. and Dorikens, M. and Segers, D.},
	  note = {Gent, Belgium, 29 August-3 September 1988}
	}
	
Study of nitrogen implantation layer and fatigued surface layer in titanium by pulsed monoenergetic positron beams
J.D. McGervey, G. Welsch, G. Kögel and D. Schödlbauer; In: , L. Dorikens-Vanpraet, M. Dorikens and D. Segers (Eds.), Proceedings of the 8th International Conference on Positron Annihilation, ICPA-8 (1989) 425-427 .
BibTeX:
	@inproceedings{McGervey1989,
	  author = {McGervey, J. D. and Welsch, G. and Kögel, G. and Schödlbauer, D.},
	  title = {Study of nitrogen implantation layer and fatigued surface layer in titanium by pulsed monoenergetic positron beams},
	  booktitle = {Proceedings of the 8th International Conference on Positron Annihilation, ICPA-8},
	  year = {1989},
	  pages = {425--427},
	  editor = {Dorikens-Vanpraet, L and Dorikens, M and Segers, D},
	  note = {Gent, Belgium, 29 August-3 September 1988}
	}
	
Helium precipitation and radiation damage in refractory metals
P. Sperr, G. Kögel, J. Störmer, W. Triftshäuser and R. Lasser; In: , L. Dorikens-Vanpraet, M. Dorikens and D. Segers (Eds.), Proceedings of the 8th International Conference on Positron Annihilation, ICPA-8 (1989) 437-439 .
BibTeX:
	@inproceedings{Sperr1989,
	  author = {Sperr, P. and Kögel, G. and Störmer, J. and Triftshäuser, W. and Lasser, R.},
	  title = {Helium precipitation and radiation damage in refractory metals},
	  booktitle = {Proceedings of the 8th International Conference on Positron Annihilation, ICPA-8},
	  year = {1989},
	  pages = {437--439},
	  editor = {Dorikens-Vanpraet, L and Dorikens, M and Segers, D},
	  note = {Gent, Belgium, 29 August-3 September 1988}
	}
	

1988

Positron Annihilation Studies in Neutron Irradiated Steels
C. Lopes Gil, A. de Lima, N. de Campos, J. Fernandes, G. Kögel, P. Sperr and W. Triftshäuser; In: E. Recknagel and J. Soares (Eds.), Nuclear Physics Applications on Materials Science , Vol. 144 , p. 441-442 , Springer Netherlands , 1988.
Abstract: Due to the sensitivity of the positrons to vacancy-type defects the positron annihilation techniques became of unique importance for the identification of the structural changes involved in the recovery stages of pure metals. The recent application of these techniques to materials with complex structure like steels has also contributed to the understanding of the microscopic processes involved in their recovery.
BibTeX:
	@incollection{Gil1988,
	  author = {Lopes Gil, C. and de Lima, A.P. and de Campos, N.Ayres and Fernandes, J.V. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Positron Annihilation Studies in Neutron Irradiated Steels},
	  booktitle = {Nuclear Physics Applications on Materials Science},
	  publisher = {Springer Netherlands},
	  year = {1988},
	  volume = {144},
	  pages = {441--442},
	  editor = {Recknagel, E. and Soares, J.C.},
	  url = {http://link.springer.com/chapter/10.1007%2F978-94-009-2800-8_38},
	  doi = {http://doi.org/10.1007/978-94-009-2800-8_38}
	}
	
Investigation of micropores in amorphous hydrogenated carbon by a pulsed positron beam
G. Kögel, D. Schödlbauer, W. Triftshäuser and J. Winter; Physical Review Letters 60 (15) (1988) 1550-1553.
Abstract: The positron lifetime (PLT) in a thin film of amorphous hydrogenated carbon (a-C:H) was studied by a pulsed positron beam. For the first time the PLT was measured in a layer which is as thin as 100 nm. PLTs of 400(5) ps for the surface state and 348(3) ps in the volume of the film were obtained. The diffusion length of the positrons was measured to be only 4(1) nm. From these results we conclude that in the specimen investigated at least 3% of the total volume is occupied by micropores about 0.5 nm in diameter which may make possible the diffusion of molecular hydrogen in a-C:H. © 1988 The American Physical Society.
BibTeX:
	@article{Koegel1988,
	  author = {Kögel, G. and Schödlbauer, D. and Triftshäuser, W. and Winter, J.},
	  title = {Investigation of micropores in amorphous hydrogenated carbon by a pulsed positron beam},
	  journal = {Physical Review Letters},
	  year = {1988},
	  volume = {60},
	  number = {15},
	  pages = {1550--1553},
	  url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.60.1550},
	  doi = {http://doi.org/10.1103/PhysRevLett.60.1550}
	}
	
A pulsing system for low energy positrons
D. Schödlbauer, P. Sperr, G. Kögel and W. Triftshäuser; Nuclear Inst. and Methods in Physics Research, B 34 (2) (1988) 258-268.
Abstract: A pulsing system for a low energy positron beam of variable energy was developed. The system consists of a beam chopper and a beam buncher. The performance of the system was tested in various positron lifetime measurements. A time resolution of 135 ps was achieved for the pulsing system at positron energies in the range between 0.5 keV and 28 keV. © 1988.
BibTeX:
	@article{Schoedlbauer1988,
	  author = {Schödlbauer, D. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {A pulsing system for low energy positrons},
	  journal = {Nuclear Inst. and Methods in Physics Research, B},
	  year = {1988},
	  volume = {34},
	  number = {2},
	  pages = {258--268},
	  url = {http://www.sciencedirect.com/science/article/pii/0168583X88907525?via%3Dihub},
	  doi = {http://doi.org/10.1016/0168-583X(88)90752-5}
	}
	

1987

Lifetime Measurements with a Pulsed Slow Positron Beam
D. Schödlbauer, G. Kögel, P. Sperr and W. Triftshäuser; physica status solidi (a) 102 (2) (1987) 549-554.
Abstract: The lifetimes of positrons annihilating in solids at the surface or close to the surface are measured by a pulsed positron beam of variable energy. A time resolution of 240 ps FWHM at a coincidence count rate of 70 s?1 is obtained. For stainless steel and nickel the lifetime associated to the annihilation in surface states is of the order of 500 ps. The lifetimes for the annihilation in trapping centers close to the surface are in complete agreement with the lifetimes observed in conventional lifetime measurements.
BibTeX:
	@article{Schoedlbauer1987,
	  author = {Schödlbauer, D. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Lifetime Measurements with a Pulsed Slow Positron Beam},
	  journal = {physica status solidi (a)},
	  year = {1987},
	  volume = {102},
	  number = {2},
	  pages = {549--554},
	  url = {http://onlinelibrary.wiley.com/doi/10.1002/pssa.2211020210/abstract},
	  doi = {http://doi.org/10.1002/pssa.2211020210}
	}
	
Timing measurements with barium flouride scintillators
P. Sperr; Nuclear Inst. and Methods in Physics Research, A 254 (3) (1987) 635-636.
Abstract: The timing properties of BaF2 were investigated using different reflector materials, coupling compounds, photomultiplier tubes and voltage divider circuits. © 1987.
BibTeX:
	@article{Sperr1987,
	  author = {Sperr, P.},
	  title = {Timing measurements with barium flouride scintillators},
	  journal = {Nuclear Inst. and Methods in Physics Research, A},
	  year = {1987},
	  volume = {254},
	  number = {3},
	  pages = {635--636},
	  url = {http://www.sciencedirect.com/science/article/pii/016890028790043X?via%3Dihub},
	  doi = {http://doi.org/10.1016/0168-9002(87)90043-X}
	}
	
Defects in amorphous alloys
W. Triftshäuser and G. Kögel; In: E. Lüscher, G. Fritsch and G. Jacucci (Eds.), Amorphous and Liquid Materials , Vol. 118 , p. 218-232 , Springer Netherlands , 1987.
Abstract: In all amorphous alloys investigated by positrons, intrinsic defects with a vacancy-like local structure have been identified. These empty spaces in M1 type alloys are larger than the Bernal holes and their concentration is so high (10** minus **3 to 10** minus **2) that these intrinsic defects must be considered as constituent parts of the amorphous structure. In the M2 type alloys the intrinsic defect concentration is comparatively low. In both types of amorphous alloys radiation-induced defects have been observed. After low temperature irradiation the defects behave like close Frenkel pairs.
BibTeX:
	@incollection{Triftshaeuser1987,
	  author = {Triftshäuser, W. and Kögel, G.},
	  title = {Defects in amorphous alloys},
	  booktitle = {Amorphous and Liquid Materials},
	  publisher = {Springer Netherlands},
	  year = {1987},
	  volume = {118},
	  number = {118},
	  pages = {218--232},
	  editor = {Lüscher, E. and Fritsch, G. and Jacucci, G.},
	  url = {http://link.springer.com/chapter/10.1007%2F978-94-009-3505-1_17},
	  doi = {http://doi.org/10.1007/978-94-009-3505-1_17}
	}
	

1986

Positron Annihilation
W. Triftshäuser; In: U. Gonser (Ed.), Microscopic Methods in Metals , Vol. 40 , Chapter 9 , p. 249-295 , Springer Berlin Heidelberg , 1986.
Abstract: It is intended to describe the use of positron annihilation spectroscopy in metals and alloys. In the sections following the introduction of the basic principles, applications of positron annihilation are presented to study defect and defect-free materials. The investigation of Fermi-surface properties in single crystals of metals and alloys is one of the very interesting topics in positron annihilation. Even more exciting and in some way unique is the positron method in the field of defects in metallic materials, such as vacancies in thermal equilibrium, quenched-in defects, dislocations, grain boundaries associated with phase transitions, amorphous structures, and radiation-induced defects and their agglomerates, together with implanted gases.
BibTeX:
	@incollection{Triftshaeuser1986,
	  author = {Triftshäuser, W.},
	  title = {Positron Annihilation},
	  booktitle = {Microscopic Methods in Metals},
	  publisher = {Springer Berlin Heidelberg},
	  year = {1986},
	  volume = {40},
	  pages = {249--295},
	  editor = {Gonser, Ulrich},
	  url = {http://link.springer.com/chapter/10.1007%2F978-3-642-46571-0_9},
	  doi = {http://doi.org/10.1007/978-3-642-46571-0_9}
	}
	

1985

Helium irradiated nickel investigated by positron annihilation
G. Kögel, F. Qin-Min, P. Sperr, W. Triftshäuser and B. Viswanathan; Journal of Nuclear Materials 127 (2-3) (1985) 125-131.
Abstract: Nickel specimens have been homogeneously implanted with helium ions at about 325 K to a total dose of 500 appm. Distinct defect structures are observed in the irradiated specimen during isochronal annealing. Positron lifetime and Doppler broadening techniques were applied to study the evolution of the defect configurations. The influence of helium on the positron annihilation characteristics is clearly visible. The nucleation of helium bubbles could be determined. At higher annealing temperatures two distinct positron bound states are found. For both states the temperature dependence of the annihilation characteristics is similar to that reported for voids. Clear differences in the annihilation parameters have been established for empty and helium-decorated vacancy clusters. © 1985.
BibTeX:
	@article{Koegel1985,
	  author = {Kögel, G. and Qin-Min, F. and Sperr, P. and Triftshäuser, W. and Viswanathan, B.},
	  title = {Helium irradiated nickel investigated by positron annihilation},
	  journal = {Journal of Nuclear Materials},
	  year = {1985},
	  volume = {127},
	  number = {2-3},
	  pages = {125--131},
	  url = {http://www.sciencedirect.com/science/article/pii/0022311585903459},
	  doi = {http://doi.org/10.1016/0022-3115(85)90345-9}
	}
	
Positron lifetime and Doppler-broadening studies of vacancy formation and phase transformation in uranium
G. Kögel, P. Sperr, W. Triftshäuser and S. Rothman; Journal of Nuclear Materials 131 (2-3) (1985) 148-157.
Abstract: The positron lifetime and the Doppler-broadening of the annihilation radiation have been investigated in high-purity uranium at temperatures between 290 K and 1400 K. We find a significant dependence of the bulk lifetime and the annihilation lineshape on crystal structure and on temperature. Trapping at vacancies is observed only in ?-uranium. The total trapping rate is surprisingly low and increases by only a factor of two from the ? ? ? transition temperature to the melting point. In ?-uranium the vacancy formation parameters as well as the specific trapping rate for positrons at vacancies most probably depend on temperature. The average formation enthalpy in the ?-phase is of the order of 0.3 eV. © 1985.
BibTeX:
	@article{Koegel1985a,
	  author = {Kögel, G. and Sperr, P. and Triftshäuser, W. and Rothman, S.J.},
	  title = {Positron lifetime and Doppler-broadening studies of vacancy formation and phase transformation in uranium},
	  journal = {Journal of Nuclear Materials},
	  year = {1985},
	  volume = {131},
	  number = {2-3},
	  pages = {148--157},
	  url = {http://www.sciencedirect.com/science/article/pii/0022311585904532?via%3Dihub},
	  doi = {http://doi.org/10.1016/0022-3115(85)90453-2}
	}
	
The reconstruction of defect distributions close to the surface from data obtained by slow positrons
G. Kögel; In: Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7 (1985) 965-967 .
BibTeX:
	@inproceedings{Koegel1985b,
	  author = {Kögel, G.},
	  title = {The reconstruction of defect distributions close to the surface from data obtained by slow positrons},
	  booktitle = {Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7},
	  year = {1985},
	  pages = {965--967}
	}
	
Positron Annihilation Studies of Neutron Irradiated Reactor Steels
C. Lopes Gil, P. Sperr, G. Kögel and W. Triftshäuser; In: Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7 (1985) 479-481 .
BibTeX:
	@inproceedings{LopesGil1985,
	  author = {Lopes Gil, C. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Positron Annihilation Studies of Neutron Irradiated Reactor Steels},
	  booktitle = {Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7},
	  year = {1985},
	  pages = {479--481}
	}
	
Study of Defects in Stainless Steel 316 by Positron Annihilation
C. Lopes Gil, P. Sperr, G. Kögel and W. Triftshäuser; In: Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7 (1985) 877-879 .
BibTeX:
	@inproceedings{LopesGil1985a,
	  author = {Lopes Gil, C. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Study of Defects in Stainless Steel 316 by Positron Annihilation},
	  booktitle = {Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7},
	  year = {1985},
	  pages = {877--879}
	}
	
A Pulsed Positron Beam for Lifetime Studies
D. Schödlbauer, P. Sperr, G. Kögel and W. Triftshäuser; In: Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7 (1985) 957-959 .
BibTeX:
	@inproceedings{Schoedlbauer1985,
	  author = {Schödlbauer, D. and Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {A Pulsed Positron Beam for Lifetime Studies},
	  booktitle = {Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7},
	  year = {1985},
	  pages = {957--959}
	}
	
Positron Lifetime and Doppler-Broadening studies of vacancy formation and phase transformation in uranium
P. Sperr, G. Kögel and W. Triftshäuser; In: Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7 (1985) 509-511 .
BibTeX:
	@inproceedings{Sperr1985,
	  author = {Sperr, P. and Kögel, G. and Triftshäuser, W.},
	  title = {Positron Lifetime and Doppler-Broadening studies of vacancy formation and phase transformation in uranium},
	  booktitle = {Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7},
	  year = {1985},
	  pages = {509--511}
	}
	
A Positron Annihilation Study of Ageing in Aluminum Alloy
W. Triftshäuser and G. Kögel; In: Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7 (1985) 874-876 .
BibTeX:
	@inproceedings{Triftshaeuser1985,
	  author = {Triftshäuser, W. and Kögel, G.},
	  title = {A Positron Annihilation Study of Ageing in Aluminum Alloy},
	  booktitle = {Proceedings of the 7th International Conference on Positron Annihilation, ICPA-7},
	  year = {1985},
	  pages = {874--876}
	}
	

1983

Helium implantation in metals investigated by monoenergetic positrons
G. Kögel and W. Triftshäuser; Radiation Effects 78 (1-4) (1983) 221-230.
Abstract: Helium-implanted metals and alloys were investigated with a positron beam system capable of producing positrons with energies ranging between 150 eV and 28 keV. Such a system is superior to the classical positron annihilation methods using high energy positrons from radioactive sources, since, for the first time, near-surface defects and defect agglomerates at high concentrations can be studied. Results are reported on crystalline nickel and amorphous nickel alloys irradiated at room temperature with helium up to doses of 1.8 × 1018 cm?2 at different energies. The Doppler broadening technique of the annihilation radiation is applied, and the lineshape parameter Ic is used to characterize the various defect configurations. In some favourable cases information about defect concentrations could be obtained.
BibTeX:
	@article{Koegel1983,
	  author = {Kögel, G. and Triftshäuser, W.},
	  title = {Helium implantation in metals investigated by monoenergetic positrons},
	  journal = {Radiation Effects},
	  year = {1983},
	  volume = {78},
	  number = {1-4},
	  pages = {221--230},
	  url = {http://www.tandfonline.com/doi/abs/10.1080/00337578308207373},
	  doi = {http://doi.org/10.1080/00337578308207373}
	}
	
Investigation of helium and deuteron irradiated stainless steel (and nickel) by positron annihilation
B. Viswanathan, W. Triftshäuser and G. Kögel; Radiation Effects 78 (1-4) (1983) 231–243.
Abstract: Positron annihilation studies have been carried out in austenitic stainless steel (SS 316), irradiated with high energy helium and deuteron ions from a cyclotron. Defect states have been deduced from simultaneous measurements of positron lifetime and Doppler lineshape in post-irradiation anneal up to 1473°K. Clear differences in the annealing characteristics are observed beyond 673°K between the helium- and deuteron-irradiated specimens. Evidence is presented for helium-vacancy interaction and nucleation of helium bubbles. Results on helium in nickel are also discussed for comparison. Helium agglomeration to microbubbles, which are well below the resolution of electron microscopy, is detected around 800°K. In the growth stage of bubbles at higher temperatures, two distinct positron bound states develop both in SS 316 and nickel. Helium retention in large bubbles is found to be stable even at 1473°K in SS 316.
BibTeX:
	@article{Viswanathan1983,
	  author = {Viswanathan, B. and Triftshäuser, W. and Kögel, G.},
	  title = {Investigation of helium and deuteron irradiated stainless steel (and nickel) by positron annihilation},
	  journal = {Radiation Effects},
	  year = {1983},
	  volume = {78},
	  number = {1-4},
	  pages = {231–243},
	  url = {http://www.tandfonline.com/doi/abs/10.1080/00337578308207374},
	  doi = {http://doi.org/10.1080/00337578308207374}
	}
	

1982

Helium implantation in metals investigated by monoenergetic positrons
W. Triftshäuser, G. Kögel and J. Bohdansky; Journal of Nuclear Materials 111-112 (C) (1982) 687-694.
Abstract: A new system has been developed to produce monoenergetic positrons of variable energy. The energy range available at present is between 150 eV and 28 keV. This positron beam can be used to investigate defects and defect structures produced below the surface in metals and alloys. The problems of extracting information about various defects in the near-surface region under the influence of the surface properties and the diffusion of the thermalized positrons will be discussed. Experimental results for neutron and helium-irradiated copper and nickel specimens as well as for amorphous alloys will be presented. The Doppler broadening technique of the annihilation radiation is applied for these measurements. © 1982.
BibTeX:
	@article{Triftshaeuser1982,
	  author = {Triftshäuser, W. and Kögel, G. and Bohdansky, J.},
	  title = {Helium implantation in metals investigated by monoenergetic positrons},
	  journal = {Journal of Nuclear Materials},
	  year = {1982},
	  volume = {111-112},
	  number = {C},
	  pages = {687--694},
	  url = {http://www.sciencedirect.com/science/article/pii/0022311582902902},
	  doi = {http://doi.org/10.1016/0022-3115(82)90290-2}
	}
	
Defect structures below the surface in metals investigated by monoenergetic positrons
W. Triftshäuser and G. Kögel; Physical Review Letters 48 (25) (1982) 1741-1744.
Abstract: A new compact system has been developed to produce monoenergetic positrons of variable energy. The energy range available at present is between 150 eV and 28 keV, so that depth profiles can be measured. This positron beam is being used to investigate defects and defect structures close to the surface in metals and alloys. The technique of Doppler broadening of the annihilation radiation is applied for these measurements. © 1982 The American Physical Society.
BibTeX:
	@article{Triftshaeuser1982a,
	  author = {Triftshäuser, W. and Kögel, G.},
	  title = {Defect structures below the surface in metals investigated by monoenergetic positrons},
	  journal = {Physical Review Letters},
	  year = {1982},
	  volume = {48},
	  number = {25},
	  pages = {1741--1744},
	  url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.48.1741},
	  doi = {http://doi.org/10.1103/PhysRevLett.48.1741}
	}
	
Defect structures close to the surface in metals investigated by slow positrons
W. Triftshäuser and G. Kögel; In: Point Defects and Defect Interactions in Metals, Yamada Conference Proceedings (1982) 15-18 .
BibTeX:
	@inproceedings{Triftshaeuser1982b,
	  author = {Triftshäuser, W. and Kögel, G.},
	  title = {Defect structures close to the surface in metals investigated by slow positrons},
	  booktitle = {Point Defects and Defect Interactions in Metals, Yamada Conference Proceedings},
	  year = {1982},
	  pages = {15--18}
	}
	
Investigation of helium irradiated stainless steel by positron annihilation.
B. Viswanathan, G. Kögel, P. Sperr and W. Triftshäuser; In: , P.G. Coleman, S.C. Sharma and L.M. Diana (Eds.), 6. International conference on positron annihilation; Arlington, TX (USA); 3 - 7 Apr 1982 , Positron Annihilation (1982) 520-522 , North-Holland; Amsterdam (Netherlands).
Abstract: Isochronal annealing experiments were performed on helium irradiated stainless steel specimens. From positron lifetime and Doppler broadening measurements characteristic defect configurations could be deduced. There is a strong indication that the nucleation process to helium bubbles in this material starts at 640 K.
BibTeX:
	@inproceedings{Viswanathan1982,
	  author = {Viswanathan, B. and Kögel, G. and Sperr, P. and Triftshäuser, W.},
	  title = {Investigation of helium irradiated stainless steel by positron annihilation.},
	  booktitle = {6. International conference on positron annihilation; Arlington, TX (USA); 3 - 7 Apr 1982},
	  journal = {Positron Annihilation},
	  publisher = {North-Holland; Amsterdam (Netherlands)},
	  year = {1982},
	  pages = {520--522},
	  editor = {Coleman, P. G. and Sharma, S. C. and Diana, L. M.}
	}
	

1980

Investigation of vacancy formation and phase transformations in uranium by positron annihilation
H. Matter, J. Winter and W. Triftshäuser; Journal of Nuclear Materials 88 (2-3) (1980) 273-278.
Abstract: Measurements of the positron annihilation coincidence count rate at the peak of the angular correlation curve (CCR) in uranium were performed as a function of temperature between 550 and 1350 K. The ? ? ? and ? ? ? phase transformations are reflected by discontinuities of the CCR. The low temperature slope of the curve, the change at the ? ? ? transition at 941 K, and the initial slope in the ? phase can be very well described by linear thermal expansion. The jump of the CCR at 1048 K is predominantly due to the vacancy trapping effect. Applying the trapping model to the data in the 7 phase, a lower limit of about 1 eV has been obtained for the monovacancy formation energy EF 1v. This result leaves only a very small value for the vacancy migration energy EM 1v. © 1980.
BibTeX:
	@article{Matter1980,
	  author = {Matter, H. and Winter, J. and Triftshäuser, W.},
	  title = {Investigation of vacancy formation and phase transformations in uranium by positron annihilation},
	  journal = {Journal of Nuclear Materials},
	  year = {1980},
	  volume = {88},
	  number = {2-3},
	  pages = {273--278},
	  url = {http://www.sciencedirect.com/science/article/pii/0022311580902834?via%3Dihub},
	  doi = {http://doi.org/10.1016/0022-3115(80)90283-4}
	}
	

1979

Phase transformations and vacancy formation energies of transition metals by positron annihilation
H. Matter, J. Winter and W. Triftshäuser; Applied Physics 20 (2) (1979) 135-140.
Abstract: The technique of the coincidence count rate at the peak of the angular correlation curve (CCR) in positron annihilation has been applied to the investigation of vacancy formation energies in thermal equilibrium in nickel, cobalt, and iron. The monovacancy formation energy E1v/F has been determined to (1.55±0.05) eV and (1.34±0.07) eV for nickel and cobalt, and (1.60±0.10) eV for ?-iron, and (1.40±0.15) eV for ?-iron, respectively. The structural phase transformations in cobalt (693 K) and iron (1183 K, 1663 K) are exhibited by discontinuities of the CCR. In the case of cobalt the CCR follows exactly the change of the thermal expansion at the transition temperature. The temperature dependence of the CCR in the prevacancy region is found to be proportional to the thermal expansion for all metals investigated. © 1979 Springer-Verlag.
BibTeX:
	@article{Matter1979,
	  author = {Matter, H. and Winter, J. and Triftshäuser, W.},
	  title = {Phase transformations and vacancy formation energies of transition metals by positron annihilation},
	  journal = {Applied Physics},
	  year = {1979},
	  volume = {20},
	  number = {2},
	  pages = {135--140},
	  url = {http://link.springer.com/article/10.1007%2FBF00885934},
	  doi = {http://doi.org/10.1007/BF00885934}
	}
	

1978

?+SR diffusion studies at LAMPF
R. Heffner, W. Gauster, D. Parkin, C. Huang, R. Hutson, M. Leon, M. Schillaci, M. Simmons and W. Triftshäuser; Hyperfine Interactions 4 (1-2) (1978) 838-843.
BibTeX:
	@article{Heffner1978,
	  author = {Heffner, R.H. and Gauster, W.B. and Parkin, D.M. and Huang, C.Y. and Hutson, R.L. and Leon, M. and Schillaci, M.E. and Simmons, M.L. and Triftshäuser, W.},
	  title = {?+SR diffusion studies at LAMPF},
	  journal = {Hyperfine Interactions},
	  year = {1978},
	  volume = {4},
	  number = {1-2},
	  pages = {838--843},
	  url = {http://link.springer.com/article/10.1007%2FBF01021947},
	  doi = {http://doi.org/10.1007/BF01021947}
	}
	
Interaction of hydrogen and vacancies in copper investigated by positron annihilation
B. Lengeler, S. Mantl and W. Triftshäuser; Journal of Physics F: Metal Physics 8 (8) (1978) 1691-1698.
Abstract: Doppler broadening measurements have been performed on Cu samples containing vacancies and/or hydrogen. Trapping of hydrogen by vacancies is indicated above about 150K. Vacancies occupied by hydrogen show a strongly reduced positron-trapping probability as compared to hydrogen-free vacancies. Detrapping of hydrogen is not observed below 450K. Therefore, the vacancy-hydrogen binding energy must be greater than 0.4 eV. The positron measurements also show that quenched-in vacancies become mobile at about 273K and form clusters. The close analogy to the annealing behaviour of electron-irradiated Cu confirms the interpretation of annealing stage III by single-vacancy migration.
BibTeX:
	@article{Lengeler1978,
	  author = {Lengeler, B. and Mantl, S. and Triftshäuser, W.},
	  title = {Interaction of hydrogen and vacancies in copper investigated by positron annihilation},
	  journal = {Journal of Physics F: Metal Physics},
	  year = {1978},
	  volume = {8},
	  number = {8},
	  pages = {1691--1698},
	  url = {http://iopscience.iop.org/article/10.1088/0305-4608/8/8/010/meta},
	  doi = {http://doi.org/10.1088/0305-4608/8/8/010}
	}
	
Defect specific temperature dependence of positron trapping
S. Mantl, W. Kesternich and W. Triftshäuser; Journal of Nuclear Materials 69-70 (C) (1978) 593-595.
BibTeX:
	@article{Mantl1978,
	  author = {Mantl, S. and Kesternich, W. and Triftshäuser, W.},
	  title = {Defect specific temperature dependence of positron trapping},
	  journal = {Journal of Nuclear Materials},
	  year = {1978},
	  volume = {69-70},
	  number = {C},
	  pages = {593--595},
	  url = {http://iopscience.iop.org/article/10.1088/0305-4608/8/8/010/meta},
	  doi = {http://doi.org/10.1016/0022-3115(78)90285-4}
	}
	
Defect annealing studies on metals by positron annihilation and electrical resitivity measurements
S. Mantl and W. Triftshäuser; Physical Review B 17 (4) (1978) 1645-1652.
Abstract: Doppler-broadening positron-annihilation measurements combined with electrical-resistivity measurements were performed on Cu and Al samples, irradiated by high-energy electrons at liquid-helium temperature. A defect-specific parameter R was determined from the shape of the 511-keV annihilation line. The R parameter is found to be independent of the defect concentration C and the positron trapping constant within the framework of the two-state trapping model. During isochronal annealing of electron-irradiated copper the line-shape parameter Iv increases in stage III, whereas the electrical resistivity decreases, indicating a reduction in the Frenkel-defect concentration. These two combined effects and the pronounced steps in the R parameter during stage-III annealing, which are found to be dose independent, signify that radiation-produced vacancies become mobile and coalesce into clusters before they anneal out. The positron trapping constant for vacancies in Cu was determined to be =(4.25×0.8)×1014 sec-1. Furthermore, the annealing behavior of copper and aluminum samples, deformed plastically at 77 K and irradiated at 4.2 K by electrons, respectively, is discussed. In both cases no indication for the formation of vacancy agglomerates is found during annealing. The R-parameter analysis is also applied to the data for electron-irradiated Mo, as reported by Eldrup et al. © 1978 The American Physical Society.
BibTeX:
	@article{Mantl1978a,
	  author = {Mantl, S. and Triftshäuser, W.},
	  title = {Defect annealing studies on metals by positron annihilation and electrical resitivity measurements},
	  journal = {Physical Review B},
	  year = {1978},
	  volume = {17},
	  number = {4},
	  pages = {1645--1652},
	  url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.17.1645},
	  doi = {http://doi.org/10.1103/PhysRevB.17.1645}
	}
	

1977

Measurement of the depolarization rate of positive muons in copper and aluminum
W. Gauster, R. Heffner, C. Huang, R. Hutson, M. Leon, D. Parkin, M. Schillaci, W. Triftshäuser and W. Wampler; Solid State Communications 24 (9) (1977) 619-622.
Abstract: Positive muon spin rotation experiments for polycrystalline Cu and Al from 19K to temperatures near the melting points are reported. At low temperatures, the depolarization associated with localization of the muons at octahedral interstitial sites is seen in Cu, while in A? only slight depolarization is observed below 250K. At high temperatures, no evidence for trapping of positive muons at vacancies in thermal equilibrium is found for either metal. It is concluded that the muons either diffuse too slowly to find vacancies or, if they do find vacancies, are bound too weakly to remain trapped. © 1977.
BibTeX:
	@article{Gauster1977,
	  author = {Gauster, W.B. and Heffner, R.H. and Huang, C.Y. and Hutson, R.L. and Leon, M. and Parkin, D.M. and Schillaci, M.E. and Triftshäuser, W. and Wampler, W.R.},
	  title = {Measurement of the depolarization rate of positive muons in copper and aluminum},
	  journal = {Solid State Communications},
	  year = {1977},
	  volume = {24},
	  number = {9},
	  pages = {619--622},
	  url = {http://www.sciencedirect.com/science/article/pii/0038109877903751},
	  doi = {http://doi.org/10.1016/0038-1098(77)90375-1}
	}
	
Annealing studies of voids in neutron-irradiated aluminium single crystals by positron annihilation
V.W. Lindberg, J.D. McGervey, R.W. Hendricks and W. Triftshäuser; Philosophical Magazine 36 (1) (1977) 117-128.
Abstract: Positron lifetimes and the angular correlation of annihilation radiation in neutron-irradiated high-purity aluminium single crystals have been measured, and companion small angle X-ray scattering experiments made to determine the radius of gyration and volume fraction of radiation-induced voids. The lifetime data are consistent with a model in which positrons are trapped both in voids and in other defects which are tentatively identified as vacancy clusters bound to transmutation-produced silicon. The lifetime in the latter defects increased from 230 to 430 psec as the voids annealed out (around 306°C), suggesting that the clusters grow in size as voids disappear. The lifetime in voids remained constant at (550 ± 30) psec. Angular correlation results are consistent with the lifetime results. A magnetic quenching experiment showed less than 1% positronium formation in the voids. The results support the Hodges and Stott model of a surface state for a positron in a void.
BibTeX:
	@article{Lindberg1977,
	  author = {Lindberg, V. W. and McGervey, J. D. and Hendricks, R. W. and Triftshäuser, W.},
	  title = {Annealing studies of voids in neutron-irradiated aluminium single crystals by positron annihilation},
	  journal = {Philosophical Magazine},
	  year = {1977},
	  volume = {36},
	  number = {1},
	  pages = {117--128},
	  url = {http://www.tandfonline.com/doi/abs/10.1080/00318087708244452},
	  doi = {http://doi.org/10.1080/00318087708244452}
	}