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1
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A Magnetically Guided Slow Positron Beam for Defect Studies

100%
Anwand W., Kissener H.-R., Brauer G.
Acta Physica Polonica A
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1995
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vol. 88
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issue 1
7-11
EN
The design and construction of a compact, magnetically guided slow positron beam is discussed. The system uses a 30 mCi ^{22}Νa source. It consists of three main parts: (i) the source chamber with a tungsten foil transmis­sion moderator and the extraction optics, (ii) the beam line with magnetic beam guidance, a bent tube for the separation of the fast positrons and the accelerator stage, (iii) the target chamber with the sample holder and the detector electronics. The energy of the incident positrons can be varied from 30 eV up to 50 keV. Furthermore source geometries, pre-acceleration, main acceleration sections and various magnetic induction profiles have been considered, such as (i) rectangular, conical and bent Wehnelt electrodes, (iii) pre-accelerator voltage shared over several electrodes, (iii) weak, strong, constant and z-dependent B-profiles, (iv) geometric options in the main accelerator region, (v) purely electrostatic and combined electric/magnetic fields. The beam is mainly designed for defect profile studies in ultra high vacuum conditions.
2
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Characterization of Sn Films on Silicon by Slow Positron Implantation Spectroscopy

100%
Nancheva N., Docheva P., Anwand W., Brauer G.
Acta Physica Polonica A
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2001
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vol. 99
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issue 3-4
435-440
EN
Sn films grown on silicon substrate by d.c. magnetron sputtering have been investigated by slow positron implantation spectroscopy. As the substrate bias is one of the most important factors affecting the structure of a sputtered film, films grown at various substrate bias (+80 V, 0 V, -80 V) are compared and their properties are discussed.
3
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Magnetron Sputtered SnO_{x} Films on Tin Probed by Slow Positron Implantation Spectroscopy

88%
Nancheva N., Docheva P., Anwand W., Brauer G., Coleman P.
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|
vol. 95
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issue 4
623-626
EN
SnO_{x} films grown on tin substrates via d.c. magnetron sputtering at different bias were studied by slow positron implantation spectroscopy. The change of substrate bias from -40 V to -140 V and its influence on the films is shown and discussed.
4
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A Study of Positron Properties in Quartz Crystals and Synthetic Silica Glass

88%
Anwand W., Brauer G., Hasegawa M., Dersch O., Rauch F.
Acta Physica Polonica A
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2001
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vol. 99
|
issue 3-4
321-328
EN
The monoenergetic positron beamline "SPONSOR" at Rossendorf has been used to investigate the positron behaviour in a naturally grown Brasilian quartz, two synthetic quartz crystals of different origin, and synthetic silica glass. The measurements allow us to obtain the positron diffusion length of free positrons and Bloch para-positronium, if formed, in these materials. In addition, hydrothermal treatment of a synthetic quartz has been used to introduce hydrogen into the crystal up to a certain depth. The presence of hydrogen is found to influence the formation of para-positronium. The depth distribution of hydrogen has been measured independently by nuclear reaction analysis, and will be discussed in comparison with the results deduced from the positron studies.
5
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Structural Changes in Flash Lamp Annealed Amorphous Si Layers Probed by Slow Positron Implantation Spectroscopy

76%
Anwand W., Schumann T., Brauer G., Skorupa W., Xiong S., Wu C., Gebel T.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1273-1278
EN
Flash lamp annealing was applied to the modification of thin amorphous Si layers on SiO_2 and glass. Slow positron implantation spectroscopy was used for the characterisation of the microstructure before and after flash lamp annealing. Changes in the structure down to a depth of some micrometres below the surface observed with slow positron implantation spectroscopy will be presented and discussed.
6
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Defects Studies of ZnO Single Crystals Prepared by Various Techniques

76%
Lukáč F., Čížek J., Procházka I., Melikhova O., Anwand W., Brauer G.
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EN
The aim of the present work was a comparison of defects in ZnO crystals grown by various techniques available nowadays, namely hydrothermal growth, pressurized melt, Bridgman method growth and vapor phase growth. Positron annihilation spectroscopy was employed as a principal tool for characterization of defects in ZnO crystals grown by above mentioned various techniques. ZnO crystals can be divided into two groups: (i) hydrothermal grown crystals, which exhibit positron lifetime of 179-182 ps and (ii) ZnO crystals grown by the other techniques (pressurized melt, Bridgman method, vapor phase growth) which are characterized by the lower lifetimes falling in the range of 160-173 ps. Comparison of experimental data with ab initio theoretical calculations revealed that HT grown ZnO crystals contains Zn vacancies associated with hydrogen atom in a bond-centered site. On the other hand, ZnO crystals prepared by other techniques contain most probably stacking faults created by stresses induced by temperature gradients in the melt.
7
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Hydrogen Interaction with Vacancies in Electron Irradiated Niobium

64%
Čižek J., Procházka I., Brauer G., Anwand W., Gemma R., Nikitin E., Kirchheim R., Pundt A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1293-1299
EN
Defect studies of Nb irradiated with 10 MeV electrons were performed in the present work by means of positron annihilation spectroscopy. The lattice defects were characterized by positron lifetime spectroscopy. Moreover, defect depth profiles were studied by slow positron implantation spectroscopy. The experimental investigations were accompanied by first principles theoretical calculations of positron parameters. It was found that irradiation-induced vacancies in Nb specimens are surrounded by H, which causes a shortening of the lifetime of trapped positrons. The influence of a Pd and Cr over-layer on the H concentration in the Nb specimens was examined.
8
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Positronium Formation in Nanostructured Metals

64%
Čížek J., Melikhova O., Hruška P., Vlček M., Anwand W., Liedke M., Novotný M., Bulíř J.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1579-1584
EN
Positronium formation in Au films was studied using a magnetically guided continuous slow positron beam with variable energy. Black Au film with porous morphology was compared with conventional smooth Au film. In the smooth Au film positronium is formed on the film surface only. The black Au film exhibits porous sub-surface region containing micro-cavities interconnected with surface. Positronium is formed on inner surfaces of micro-cavities in the sub-surface region and travels through interconnected cavities towards the surface. 3-γ annihilation of ortho-positronium leaves a clear signature in two-dimensional coincidence Doppler broadening spectra. Measures of 3-γ annihilation contribution calculated from single γ-ray and coincidence Doppler broadening spectra were calculated and compared.
9
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Positronium Probing of Pores in Zirconia Nanopowders

64%
Prochazka I., Cizek J., Lukac F., Melikhova O., Hruska P., Anwand W., Liedke M., Konstantinova T., Danilenko I.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1564-1567
EN
In the present paper, conventional positron lifetime measurements on selected zirconia-based nanopowders are reported. The nanopowders were doped with various metal cations (Y³⁺, Eu³⁺, Gd³⁺, Lu³⁺ and Mg²⁺). Lifetime experiments were conducted in air and supplemented with mass density measurements. In a range of lifetimes, from a few ns to ≈70 ns, up to two individual lifetime components could be identified. Such observations confirmed positronium (Ps) formation with subsequent ortho-Ps pick-off annihilation as well as the occurrence of pores of different size. Pore sizes were estimated using a shape-free model of the correlation between pore size and ortho-Ps lifetime. The origins of pores are discussed on the basis of the ortho-Ps data in combination with the results of mass density measurements.
10
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Ultra Fine-Grained Metals Prepared by Severe Plastic Deformation: A Positron Annihilation Study

52%
Čížek J., Procházka I., Kužel R., Matĕj Z., Cherkaska V., Cieslar M., Smola B., Stulíková I., Brauer G., Anwand W., Islamgaliev R., Kulyasova O.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
745-752
EN
Recent investigations of ultra fine-grained metals (Cu, Fe, Ni) performed within a Prague-Rossendorf-Ufa collaboration will be reviewed. The specimens were prepared by severe plastic deformation: the high-pressure torsion and equal channel angular pressing. Positron annihilation spectroscopy was used as the main method including (i) the conventional lifetime and the Doppler broadening measurements with ^{22}Na and (ii) the slow-positron implantation spectroscopy with the Doppler broadening measurement. Other methods were also involved: transmission electron microscopy, X-ray diffraction, and microhardness. First, the mean grain size was determined and defects were identified in the as-deformed materials. Defects concentration and spatial distribution were studied in detail. Dislocations situated in distorted regions along grain boundaries, and a few-vacancy clusters distributed homogeneously inside dislocations-free grains, were observed in the ultra fine-grained Cu, Fe, and Ni. Subsequently, the thermal evolution of the ultra fine-grained structures during isochronal annealing was studied.
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