Evolution of pores in mesoporous silica films: Porogen loading effect

Mesoporous silica films were synthesized via a sol-gel process under an acidic condition. Various amounts of triblock copolymer F38 were loaded to precursor sols as the pore generator. The evolution of the pores generated by porogen decomposition was investigated as a function of F38 loading by positron annihilation gamma-ray energy spectroscopy and positron annihilation lifetime spectroscopy based on slow positron beams. The threshold of pore percolation is found to be around 10 wt% of F38 loading by positron annihilation gamma-ray energy spectroscopy. Positron annihilation lifetime spectroscopy in the films show that the pore size increases from 1 nm to 3 nm with increasing F38 loading from 5 wt% to 30 wt%.     

Study of the effect of annealing on defects in Fe-Mn-Si-Cr-Ni-C alloy by slow positron beam

FeMnSi shape memory alloys (SMAs) have received much attention as one-way SMAs due to their cost-effectiveness. Variable-energy (0-30 keV) positron beam studies have been carried out on a Fe-Mn-Si-Cr-Ni-C alloy with different degrees of deformation. Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by deformation. The variation of the nature and the concentration of defects are studied as a function of isochronal annealing temperature. These results are correlated with the data measured with the positron annihilation lifetime spectroscopy (PALS). The positron annihilation results are compared to XRD and optical microscopy (OM).     

Doppler broadening of positron annihilation photons in metals

The Doppler broadening of positron annihilation photons was measured in 17 metals. A model which considers the positron lifetime spectra in metals to be composed of terms for annihilation with conduction and core electrons and surface centers of low electron momentum is used to correlate calculated core annihilation rates with the Doppler lineshape. Ta metal was doped with defects with high energy implantations of¹⁴N⁺⁴ ions at variable doses. Differences in the Doppler linewidths were ascribed as being principally a reflection of the probabilty of annihilation with core electrons relative to annihilation with conduction electrons.     

Momentum distribution in benzene: A comparison of compton scattering and positron annihilation

Probabilities in momentum space for the benzene molecule are calculated for two cases: An ordinary benzene molecule, and benzene with a bound positron which is coalesced with one of the electrons. The former probability is related to the Compton scattering profile; the latter, to angular correlation measurements made in positron annihilation experiments. In this work we make two comparisons on the basis of quantum mechanical calculations; a) between Compton scattering and positron annihilation results, and b) among the several possible positron annihilation results associated with different symmetries for the positronic molecular orbital (PMO). The Compton scattering results are found to be similar to the positron annihilation results for the more symmetrical PMOs; and all these are quite different from the positron annihilation results for the less symmetrical PMOs. A suggestion for a crucial experiment is made.     

Damage depth-profiling of Au+ and O+-irradiated amorphous PEEK by monoenergetic positron beams

+ and O⁺ ions was exposed to positron beams to measure the positron annihilation Doppler broadening as a function of the positron energy. As in the previous case of O⁺-irradiated semicrystalline PEEK, the annihilation lines recorded at relatively low positron energies became broader with increasing irradiation dose. The thickness of the damaged layer estimated from the positron data was compared with the mean depth of the implanted ions calculated by the TRIM code. For the Au⁺-irradiated samples, some discrepancy was observed between the two quantities. Received: 6 March 1996/Accepted: 19 November 1996     

Applications of slow positrons to cancer research: Search for selectivity of positron annihilation to skin cancer

Slow positrons and positron annihilation spectroscopy (PAS) have been applied to medical research in searching for positron annihilation selectivity to cancer cells. We report the results of positron lifetime and Doppler broadening energy spectroscopies in human skin samples with and without cancer as a function of positron incident energy (up to 8 μm depth) and found that the positronium annihilates at a significantly lower rate and forms at a lower probability in the samples having either basal cell carcinoma (BCC) or squamous cell carcinoma (SCC) than in the normal skin. The significant selectivity of positron annihilation to skin cancer may open a new research area of developing positron annihilation spectroscopy as a novel medical tool to detect cancer formation externally and non-invasively at the early stages.     

Mesoporosity in silica films studied by a slow positron beam and ellipsometry

Porous silica films were synthesized via a sol–gel method using a nonionic amphiphilic triblock copolymer F127 as the structural template. Mesoporosities of the prepared silica films were investigated by Doppler broadening of positron annihilation radiation (DBAR) spectroscopy, positron annihilation gamma-ray energy spectroscopy based on a slow positron beam, and ellipsometry. For the mesoporous silica films, the variation of positron annihilation line shape parameter reveals that the porosity of the silica films increases with loading more F127, which is also confirmed by a decrease of refractive index n. Little variation in positron 3γ-annihilation fraction is found for the silica films prepared with F127 loading less than 15 wt%, whereas a remarkable increment is seen for the films with higher loading. This indicates the pore percolation in porous silica films occurs around a loading of F127 with 15 wt%.     

Tunable monoenergy positron annihilation spectroscopy of polyethylene glycol thin films

Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol(PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.     

Hg1-xCdxTe晶体缺陷的正电子湮没寿命

利用正电子(e⁺)湮没寿命谱实验研究了Hg_1-xCd_xTe晶体样品的空位缺陷.碲溶剂法生长的样品，不论是n型导电还是p型导电都存在大量的Hg空位.经过合适的退火工艺，p型材料转为n型，同时对正电子的俘获效应减小，表现为正电子湮没平均寿命值减小14—17ps.若退火温度高于350℃，正电子湮没寿命值又增大，表明Hg空位浓度增加.得到HgCdTe中正电子的体寿命为τ_b=272ps.根据正电子湮没寿命和电参数的测量结果，得出 关键词：     

Electron—Positron annihilation in the narrow-band semiconductor Hg1−xCdxTe

Positron annihilation processes in mercury—cadmium—telluride semiconductor compounds have been studied. Calculations of the positron thermalization time, the bulk annihilation rate, and the positron capture rate for the simplest crystal lattice structural defects have been carried out. The annihilation properties of materials containing growth defects and defects induced by high-energy particles have been determined experimentally. The profiles of vacancy-type defects formed during ion implantation were determined using slow positrons. The positron annihilation data are compared with the results of electrophysical measurements.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 3–21, October, 1995.     

Positron annihilation in large polyatomic molecules. The role of vibrational Feshbach resonances and binding

We analyse the process of rapid positron annihilation in large polyatomic molecules due to positron capture into vibrational Feshbach resonances. Resonant annihilation occurs in molecules which can bind positrons, and we analyse positron binding to alkanes using zero-range potentials. Related questions of spectra of annihilation gamma quanta and molecular fragmentation following annihilation, are discussed briefly.     

Defects in semiconductors observed by positron annihilation

The origin of the effect of defects on positron annihilation in semiconductors has been studied. The electron-positron momentum densities in elemental semiconductors (Si and Ge), III-V compound semiconductors (GaAs, InP and GaSb), diamond and the proton irradiated Si were investigated by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The obtained results showed, as a whole, good agreement with the electron momentum distribution of the fully occupied Jones zone with a small exception for the fact that the low density channels are running along the three principal axes. This anisotropy was strong in elemental semiconductors, while it was weakened in compound semiconductors. This anisotropy and its dependence on the material were found to be generally understood by the incorporation of crystal symmetry. The anisotropy will be discussed by group theory in conjunction to the effect of defects on positron annihilation.     

Positron annihilation study of ordering processes in rare-earth cobalt permanent magnets

The ordering process in 1/5 and in 2/17 Sm-Co permanent magnets was followed by positron annihilation lifetime and Doppler broadening measurements. No correlation between magnetic hardness and positron annihilation parameters was found.     

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

Pseudo-potential caculations of positron annihilation rates in bubbles of helium contained in the metals Mg,Ag and Au

Abstract

A pseudo-potential technique has been applied to calculate the annihilation rates for positrons in bubbles of helium gas in gold and silver. The positron pseudo-wavefunction in each case was found to be in a surface state of the metal, the exact form of which displayed a slight dependance on the density of the helium gas. The annihilation rate of the positron was the sum of a nearly constant surface rate and a strongly density dependant annihilation rate with the helium electrons. There is a trend for annihilation with the helium to be greater in metals with low free electron densities, the helium in such metals resting closer to the metal surface thus increasing the overlap of the positron wavefunction with the helium.     

Wirkungsquerschnitte für Einquantenvernichtung von Positronen niedriger Energie am Jod

Positrons of 200 keV were focused on a slice of NaI. This acted as a scintillation spectrometer and measured the energy loss of the positrons before annihilation. Single-quanta, produced with a probability of only 2.6 · 10^-5, were registered by a second NaI-detector. Two-parameter analysis of the coincidences between these two detectors allowed the determination of six cross sections for single-quantum annihilation between positron energies of 80 and 180 keV. The cross section at 180 keV was 0.09 barn per iodine-atom and decreased with decreasing positron energy, due to the coulombic repulsion. Quantitative agreement was obtained with theoretical calculations within the limits of error.     

Investigation of nanocrystalline CoFe 2 O 4 by positron annihilation lifetime spectroscopy

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail.     

Positron annihilation studies of mesoporous silica films using a slow positron beam

Positron annihilation lifetime spectra were measured for mesoporous silica films, which were synthesized using triblock copolymer (EO₁₀₆PO₇₀EO₁₀₆) as a structure-directing agent. Different positron lifetime spectra for the deposited and calcined films indicated the formation of meso-structure after calcination, which was confirmed by Fourier transform infrared (FTIR) spectra and field emission-scanning electron microscopy (FE-SEM) observation. Open porosity or pore interconnectivity of a silica film might be evaluated by a two-dimensional positron annihilation lifetime spectrum of an uncapped film. Pore sizes and their distributions in the silica films were found to be affected by thermal treatments.     

Systematics of the rare-gas core contributions to the positron annihilation spectra of some simple and transition metals

Taking into account the core electron enhancement factor the rare-gas core contribution to the angular correlation positron annihilation spectra and to the total annihilation rate in some simple and transition metals are calculated. It is shown that the valence high-momentum part of the Gaussian fraction is not negligible in simple metals. In 3d and 4d metals, the rare-gas core part of the total annihilation rate amounts from 9 to 37%. The results are expected to help in the correct interpretation of the positron annihilation data.     

Positron annihilation in Ca++-doped NaCl single crystals

Positron lifetime measurements were made in Ga⁺⁺-doped NaCl crystals. A correlation between the dopant concentration and the positron annihilation parameters was found. The probability of the formation of positron bound states versus Ca⁺⁺ concentration could be approximated by a power equation.