Papers by Keyword: Positronium

Abstract: The annihilation behavior of positronium in N and P type porous silicon is investigated via positron annihilation lifetime (PAL) and positronium time-of-flight (Ps-TOF) measurements. The result shows the N type sample has smaller positronium annihilation fraction and more interconnected pores. The microstructure of Rapid Thermal Oxidation (RTO) treated N type porous silicon also studied using PAL spectroscopy, and the relationship between its photoluminescence properties and porous structure was discussed.

Abstract: Ortho-positronium lifetime in alkanes and alcohols was measured in a broad range of temperatures. The results were compared with predictions of the bubble model. The bubble radius can be determined from the Tao-Eldrup equation as well as calculated using the model. The depth of potential well and surface tension are the main factors ruling the Ps bubble size. The best fit of the bubble model to the experimental data is obtained assuming the rectangular potential well to be 1 eV deep (equivalent to an infinite one broadened by 0.166 nm) and the microscopic surface tension increased about 3 times comparing to the macroscopic one.

Abstract: We have analyzed the positron annihilation Angular Correlation data in the second stage graphite-potassium intercalation C₂₄K with the theoretical formula extended from "topological quasi-positronium model" and have proposed one of the possible origins of the anisotropic narrow component in alkali-metal graphite intercalations compounds (AGICs).

Abstract: The distribution of electron trap depths in hydrocarbons seems to be continuous in the range from 0.6 eV to 2.2 eV. However, it seems that among the deepest traps in n-heptane also the discrete ones appear as well. The influence of the admixture of other alkane on Ps intensity in alkanes was examined. As far as the admixture of an alkane with a shorter chain is concerned, the depth of the traps and their amount remain almost the same like in the case of the pure solvent. When it comes to essentially non-miscible solid heptane and nonane, a total lack of electron accumulation effects was observed. It is an exception among the mixed alkanes investigated here and needs a careful study.

Abstract: Variations of ortho-positronium (o-Ps) lifetime and gas permeability of the Aquivion® E8705 membrane were studied as functions of temperature under vacuum and relative humidity at room temperature. When the temperature was varied between 0 and 100 °C in vacuum, the hole volume of Aquivion® E8705, deduced from the ortho-positronium lifetime, gradually increased. However, when the relative humidity was changed at room temperature, the hole volume was essentially unchanged. Good linear correlations between the logarithm of permeabilities of O₂ and H₂ and reciprocal hole volume at different temperatures indicates the importance role of free volume in gas permeation in dry Aquivion® E8705. However, for hydrated Aquivion® E8705 the permeability less depends on hole volume.

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: The effect of chromia additive on defects in yttria stabilized zirconia (YSZ) nanopowders was investigated in this work. It was found that positrons are trapped at vacancy-like misfit defect at grain boundaries and at larger defects situated at triple points. Moreover, a long-lived ortho-positronium contribution was found in YSZ nanopowder without chromia. Addition of chromia prolongs the lifetime of positrons trapped at vacancy-like misfit defects which indicates segregation of Cr ions at grain interfaces and interaction of Cr with vacancy-like misfit defets. Moreover addition of chromia completely suppresses formation of positronium.

Abstract: The subnanoscopic structures of polyrotaxanes, prepared from α-cyclodextrins, poly(ethylene glycol), and bulky adamantane end groups, were examined by means of the positron annihilation lifetime technique, in consideration of the free-volume hole, quantified from the long-lived ortho-positronium (o-Ps) lifetimes. The influence of the chemical structure on the temperature dependence of the o-Ps lifetimes are discussed.

Abstract: Polypropylene (PP) derivatives, prepared with different α-olefin comonomers of hexane and hexenol, were investigated using the positron annihilation lifetime technique. The free-volume hole sizes for the present polymers were quantified from the long-lived ortho-positronium (o-Ps) lifetimes. In contrast to the case of nonpolar, introduction of the polar hexenol into PP reduced the o-Ps lifetimes and the hole sizes derived from them. This indicates that the interchain interaction due to the hydrogen bonds in the amorphous region is much stronger than the enhancement of the segmental motion due to the butyl long branch. A comparison of the o-Ps lifetimes with the corresponding tensile strength suggested significant contribution of the subnanostructure in the amorphous region to the mechanical property.

Abstract: A review of data about positron annihilation lifetime in normal alkanes is presented. It was found that positronium in rigid phase locates in the interlamellar gap, while in the rotator one in free volumes inside lamellae. Interesting effects seem to be: identity of o-Ps lifetime in rotator and liquid phase in long chain alkanes; appearance of “premature” rotator phase in nonadecane with low tricosane admixture, long-term relaxation after the changes of temperature or pressure in some binary mixtures, formation of porous structure at solidification of argon saturated alkane under high pressure.