Papers by Keyword: Positron

Abstract: The matrix graphite of fuel elements (FEs) with infiltration of 2LiF-BeF₂(FLiBe) at different pressures varying from 0.4 MPa to 1.0 MPa, has been studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and positron annihilation lifetime (PAL) measurement. The result of XRD reveals that diffraction patterns of FLiBe appear in matrix graphite infiltrated with FLiBe at a pressure of 0.8 MPa and 1.0 MPa. The surface morphology from SEM shows that FLiBe mainly distributes within macro-pores of matrix graphite. PAL measurement indicates that there are mainly two positron lifetime components in all specimens:τ₁~0.21 ns and τ_{2 ­}~0.47 ns, ascribed to annihilation of positrons in bulk and trapped-positrons at surface, respectively. The average positron lifetime decreases with infiltration pressure, due to the decrease in annihilation fraction of positrons with surface after infiltration of FLiBe into macro-pores.

Abstract: Taking liquid water as an example, we have considered energetics of the positronium formation. It is shown that quasi-free Ps as well as the Ps localized in a bubble cannot decay into hydrated electron and positron. The most probable value of the positron work function in water, V₀⁺, is 1.5…2 eV. Ps formation from the hydrated electron and positron is energetically possible. By the end of thermalization major fraction of positrons escapes the blob and hydrates outside. The low-mobile е⁺_aq has no time to diffuse back and form Ps with intrablob e-. These е⁺_aq mostly annihilate as “free positrons”. Positrons, which are thermalized within the blob, recombine with the quasi-free intrablob electrons and form Ps.

Abstract: By simulation with the relativistic PIC code EPOCH, it is found that a hollow target ismore beneficial than a plane one for the collimation and compression of the positrons. Irradiating anultra-intense laser (4×10²³ W/cm²) on a hollow target, 8.74×10¹⁵ positrons are generated. Due tothe focusing effect of the transverse electric field formed in the hollow wall, the divergence angle isaround 21° , while the maximum density of the beam is 4.77×10²¹ cm^-3. When the laser intensity is doubled, both the yield (5.31 ×10¹⁶) and the maximum density (1.57 ×10²² cm^-3) are greatly increased while the beam divergence gets even smaller (15.7 °).

Abstract: In order to enhance the surface anti-fouling property of separation membrane, TiO2/polysulfone (PSF) membrane was prepared by the phase inversion method, and the surface was treated by oxygen plasma to obtain a composite membrane with photo catalysis activity. To evaluate the effect of oxygen plasma on the surface modification, positron annihilation γ-ray spectroscopy coupled with a positron beam was utilized to probe the surface feature, while the catalysis evaluation system was employed to study the anti-fouling activity of the membrane. The results indicate that with increasing plasma treatment time, the membrane exhibits higher S parameter in the lower energies, while the photo catalysis activity is strengthened. It can be concluded that plasma technique can improve the photo catalysis activity of the composite membrane.

Abstract: We observed for the first time the thermally stable point positron-sensitive center of a vacancy type in n–FZ–Si (P) material irradiated at RT by ~ 0.9-MeV electrons. The center that emerges after isochronal annealing at T_anneal.≈ 260 – 280 oC is found to be similar to the vacancy-group-V-atom complex revealed in the same Si material irradiated by 15-MeV protons; the detecting of the centers by the positron trapping is finalized at T_anneal.≥ 520 oC. The annihilation gamma-quanta to be emitted from the positron trap gives rise to a characteristic positron lifetime τ₂ (I₂ ~ 38–19 %) ≤ 276 – 294 ps which is somewhat longer than the one predicted for unrelaxed single vacancy τ_V.≈ 254 – 261 ps. Our data suggested a configuration of the complex V_opPV_op, wherein the atom of phosphorus is tied to a split open vacancy volume 2V_op. It is argued that V_op volume detected by the positron trapping may be formed by extended semi-vacancy, V_s-ext , or by the relaxed inwards vacancy, V_inw , thus resulting in a distorted V_s-extPV_s-ext or V_inwPV_inw configurations.

Abstract: The free volume of the thermally stable vacancy center in n-FZ-Si:P has been probed by positrons. The defects were produced with 15 MeV protons, and then the irradiated material was subjected to the isochronal annealing. The positron lifetime has been determined over the temperature range ~ 30 K – 300 K; the samples-satellites have been characterized by Hall effect measurements. The microstructure of the center involves, at least, one atom of phosphorus and it manifests itself as a deep donor. The center is singly negatively charged and the cascade phonon-assisted trapping of positron proceeds over the length characteristic of the point defect, l₀ ~3.62 a. u. Obeying ~ T ^–3 law, the positron trapping cross section ranges 3∙10^–12 cm² (66K) to 2.5∙10^–14 cm² (266 K). The positron lifetimes ranging from ~240 ps to ~280 ps suggest that the atomic relaxation is directed inward towards the free volume of the deep donor involving, at least, two vacancies.

Abstract: Different compositions of surfactant systems give rise to a rich variety of structures of aggregates. At higher concentrations of surfactant in water, the surfactant molecules aggregate to form lyotropic liquid crystals [1]. In the present work we have prepared two surfactant systems consisting of (i) 20% of cetyl-trimethyl-ammonium-bromide (CTAB) in water, and (ii) 30% of tetra-decyl-trimethyl-ammonium-bromide (TTAB) in water. Both the systems exhibit various lyotropic liquid crystal structures when an increasing amount of co-surfactant is added as third component [2, 3]. These liquid crystalline structures are very sensitive to the solution conditions such as co-surfactant concentration, temperature, ionic strength, counter ion polarizability etc. In this study, positron life time spectroscopy and conductivity measurement have been employed to locate various phases exhibited by the lyotropic liquid crystals. In addition to delineating various phase boundaries of the systems, positron annihilation technique has also yielded new findings.

Abstract: The Low Energy Positron Toroidal Accumulator (LEPTA) at JINR proposed for generation of positronium in flight can be used for positron annihilation spectroscopy (PAS) [1]. The positron injector of the LEPTA facility can generate continuous a slow positron beam with the intensity up to 1∙10⁷s^-1 at the energy in the range of a few eV to 100 keV and width of the spectrum 1 – 2 eV. The injector is based on radioactive ²²Na isotope. The solid neon is used as a moderator to generate monochromatic beam. The parameters of the positron beam allow scanning the condensed matter in depth up to 10 microns with resolutions less than 10 nanometers and investigating layered structures for microelectronics and properties of a surface.

Abstract: At present time the Low Energy Positron Toroidal Accumulator (LEPTA) at JINR is under commissioning with circulating positron beam. The LEPTA facility is a small positron storage ring equipped with the electron cooling system and positron injector. The maximum positron energy is of 10 keV. The storage ring is aimed for generation of direct fluxes of ortho-positronium (o-Ps), produced in recombination of the positron beam circulating in the ring with single pass cooling electron beam. The project has few goals: annihilation spectroscopy (PAS) to monitoring defects of nanometer sizes in materials as a function of depth managed by the positron energy in a rage of a few eV to 100 keV.

Abstract: In the present work hydrothermally grown ZnO single crystals were electrochemically charged with hydrogen. The influence of hydrogen on ZnO microstructure was investigated by positron annihilation spectroscopy (PAS) combined with X-ray diffraction (XRD) using synchrotron radiation. Hydrogen concentration in the samples was determined by nuclear reaction analysis (NRA). It was found that a high concentration of hydrogen can be introduced into ZnO by electrochemical loading. At low concentrations, absorbed hydrogen causes elastic volume expansion of ZnO crystal. At higher concentration, hydrogen-induced stresses exceed the yield stress in ZnO and plastic deformation of the crystal takes place leading to formation of a defected subsurface layer in the crystals.