Papers by Keyword: Positron

Abstract: The theoretical description of positron and positronium scattering by atoms within the context of the coupled pseuodostate approximation is briefly considered.

Abstract: A single crystal Al (111) sample cleaned by repeated cycles of ion bombardment and annealing was irradiated by a subnanosecond high density positron beam [1] and the resulting positronium lifetime spectra were measured using single shot positron annihilation lifetime spectroscopy (SSPALS) [2]. We observed the amount of positronium emitted dependence on the incident positron beam density, which indicates the formation of positronium molecules (Ps2) at the Al (111) surface [3]. The Ps2 formation probability appears to be extremely sensitive to surface contamination and further experiments under improved vacuum conditions are planned to clarify this effect.

Abstract: Positron and Positronium chemistry has been pursued and advanced by many scientists and engineers in both fundamental understanding of Positronium atom and its applications to chemical and polymeric systems during the last decade. This paper presents our recent results from collaborative investigations of positron annihilation in polymeric membranes. Future perspectives of applying Positronium chemistry to membrane science and technology and other related disciplines of nanotechnology, chemical engineering, materials science, energy research, molecules with positrons, biological and medical sciences appear to be promising.

Abstract: Angular correlation of annihilation radiation technique (ACAR) has been used for studying a microstructure of the vacancy-group-V-impurity complexes (DV) formed by irradiation with 60Co γ – rays at Tirr. ≈ 280K in oxygen-lean n-Ge doped with group-V-impurity atoms As, Sb, and Bi. The probability of annihilation of positrons with the core electrons of DV complexes to be reconstructed from ACAR spectra has been analyzed on the basis of Chapman-Kolmogorov formalism; the Coulomb repulsion is proved to regulate the penetration of a positron into Ge4+ and D5+ ion cores. In passing from AsV to SbV and BiV complexes the ion cores D5+ are found to contribute more effectively to the probability of the positron annihilation in the core region. These data correlate well with the augmentation of the entropy of ionization (4S ~ 2,9 ÷ 4,2K) observed by means of capacitance transient techniques with the use of Au-Ge Schottky barriers in the same row of a similar vacancy-impurity complexes. The results obtained by ACAR spectroscopy suggest the full-vacancy configuration of DV pair with relaxation of atoms inward towards the vacancy.

Abstract: Positron annihilation radiation Doppler broadening spectroscopy was used to study defects in semi-insulating (SI) silicon carbide (SiC) substrates grown by high-temperature chemical vapour deposition (HTCVD). The Doppler broadening measurements show (i) that the measured samples contain vacancy clusters (ii) that the positron trapping to the clusters is increased in annealing (iii) that the chemical environment of the defects in the un-annealed samples is different from those of the annealed samples.

Abstract: A probing of the atomic environment of positron in Cz-Si single crystal heat-treated at T=600C and T=450C has been performed by one-dimensional angular correlation of annihilation radiation (ACAR). It has been established that positrons get trapped by the oxygen-related complexes. The penetration of positrons into the core region of surrounding atoms results in emission of the elementally specific high-momentum annihilation radiation. The processes of expelling of positron from ion cores and its penetration into the core region are regulated by the potential barrier (to be considered as the Coulomb’s one as a first approximation). The characteristic electron-positron ion radius and the probabilities of correlated events of the highmomentum annihilation are due to the chemical nature of the ion cores of atoms involved in the composition of the oxygen-related complexes. The interpretation of the results is based on the notion of the positron localization in the field of negative effective charge resulted from comparatively high electron affinity of the oxygen impurity atom. The presence of a free volume (perhaps, a vacancy) as well as the carbon atom in the microstructure of the oxygen-related positron-sensitive thermal defects is briefly discussed.

Abstract: Positron lifetime spectroscopy was used to study defects in semi-insulating (SI) silicon carbide (SiC) substrates grown by high-temperature chemical vapor deposition (HTCVD). The measured positron lifetime spectra can be decomposed into two components, of which the longer corresponds to vacancy clusters. We have carried out atomic superposition calculations to estimate the size of these clusters.