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Use of Energy Summing for Selection of Coincidence Events in Positron-Lifetime Spectroscopy

100%
Bec̆vár̆ F., C̆iz̆ek J., Procházka I.
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vol. 95
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issue 4
448-454
EN
A BaF_{2} positron-lifetime spectrometer equipped with a coincidence system that represents a compromise between the traditional fast-fast and fast-slow arrangements is described. The main difference of the present configuration from both the fast-slow and fast-fast ones consists in use of a sum of the energy signals from the start and stop detectors to select the coincidence events. Quality of the spectrometer response function (≈ 150 ps FWHM for ^{22}Na) is very close to that observed for our fast-slow configuration with the equivalent detectors but throughput of the present apparatus is increased almost by a factor of two. Moreover, the electronic scheme becomes less complicated, which has also a positive impact on its cost.
2
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Defects in Zirconia Nanomaterials Doped with Rare-Earth Oxides

88%
Prochazka I., Cizek J., Melikhova O., Konstantinova T., Danilenko I.
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vol. 125
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issue 3
760-763
EN
Positron lifetime and coincidence Doppler broadening measurements on ZrO_2+3 mol.% RE_2O_3 (RE = Eu, Gd, Lu) nanopowders and ceramics obtained by sintering these nanopowders are reported. The initial nanopowders were prepared by a co-precipitation technique and exhibited a mean particle size of ≈ 15 nm. The nanopowders were calcined and pressure-compacted. All compacted nanopowders exhibited the prevailing tetragonal phase with at most 15% of the monoclinic admixture. Positrons in compacted nanopowders were found to annihilate almost exclusively at grain boundaries: (i) vacancy-like misfit defects along grain boundaries and (ii) larger defects situated at intersections of grain boundaries (triple points). In nanopowders, a small portion of positrons formed positronium in pores between crystallites. Sintering of nanopowders at 1500C caused a substantial grain growth and formation of ceramics. Sintering-induced grain growth led to a disappearance of the triple points and pores. The ceramics containing Eu and Gd dopants consist of mixture of the monoclinic and the tetragonal phase, while the ceramics with Lu dopant exhibits almost exclusively the tetragonal phase.
3
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Mismatch negativity in methamphetamine dependence: A pilot study

88%
Hosak L., Kremlacek J., Kuba M., Libiger J., Cizek J.
Acta Neurobiologiae Experimentalis
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2008
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vol. 68
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issue 1
97-102
EN
The objective of this study was to verify hypothesised changes in event related potentials (visual mismatch negativity, vMMN) in 17 subjects dependent on methamphetamine (MAMP) compared to age and gender matched 17 healthy volunteers. We found a significant correlation between vMMN and duration of methamphetamine abuse (Spearman correlation coefficient r=0.54 - 0.78; P<0.05). The positive correlation indicates drop of originally more negative response to deviant stimulus, what may indicate a pre-attentive processing enhancement in the first years of MAMP abuse with its decease later on. Accordingly, post-hoc analysis revealed significantly stronger vMMN in patients with length of MAMP abuse shorter than 5 years than in paired controls. There were no such differences in abusers with the length of abuse longer than 5 years. The results show that the visual processing on the pre-attentional level can be influenced by long-term MAMP abuse, what can be specifically assessed by vMMN.
4
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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.
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