Positron annihilation in ionic crystals

Positron lifetime measurements were made on Ca²⁺ doped NaCl, Ba²⁺ doped NaCl and X-ray- and additively-coloured KCl crystals. In the NaCl(Ca) system a correlation which could be approximated by a power equation was found between the dopant concentration and the positron annihilation parameters. In the NaCl(Ba) system the decay parameters do not change significantly as expected and no significant difference in the decay parameters of the coloured and uncoloured crystals could be found. Paper C 4 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Positron annihilation study of the electronic structure of CeCu6 and LaCu6

Summary The results of a prelimianry study of the electronic structure of the heavy-fermion system CeCu₆ via the two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) technique were compared with those obtained for the isostructural reference non-f-electron material LaCu₆. The spectra relating to the 2 compounds displayed extremely similar anisotropic structures, with features related to the symmetry of the Brillouin zone. At present, these structures cannot however be attributed to Fermi-surface or positron wave function effects or to anisotropies of some full valence band. In the case of LaCu₆, data processing prior to the LCW procedure via new filtering techniques revealed a reasonably good agreement between the LCW data and LAPW Fermi-surface calculations while in the case of CeCu₆ it appeared difficult to relate the results, still of controversial interpretation, to any theoretical model.     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

Positron annihilation in alkali-halide crystals after electron irradiation

The effect of electron beam irradiation on the positron annihilation in KCl, KBr, NaCl single crystals was investigated. The narrowing of the angular correlation curves increases up to the saturation value with the absorbed radiation dose. The shape of the received narrow components differs from the narrow component measured in the additively colored samples. This phenomenon shows the considerable influence of the hole centres upon the positron annihilation in ionic crystals. Paper presented at 3rd Internat. Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Positron annihilation in silicon crystals with mechanically processed surfaces

The angular correlation curves of the annihilation photons and the probability of three-quantum annihilation in silicon single crystals with mechanically processed surfaces were measured. The narrowing of the angular correlation curves increases and the three-quantum yield decreases with the depth of the surface abrasion. This is tentatively interpreted in terms of formation of positronium states on the surface of the silicon crystals.     

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.     

Positron angular correlation measurements on single-crystal aluminum at various temperatures

We have tested the trapping model by comparing the temperature dependence of the peak and the tail of the angular correlation curve from single crystal aluminum. The integral over the tail decreased as the integral over the peak increased with temperature, in such a way that the same vacancy formation energy could be deduced from both integrals. Paper B6 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973). Case Western Reserve University, Cleveland, Ohio, USA     

Positron systematics in annealed metals

Abtract Deconvoluted Doppler-broadened annihilation energy spectra in annealed metals are fitted by a parabola and a Gaussian function whose width and intensity vary systematically with the atomic number.     

On positron annihilation in zinc

The purpose of this work is to understand Mogensen's and Petersen's positron annihilation curves for zinc. Mijnarends' approach is used as an auxiliary method of localizing inhomogeneities of the electronic density in momentum space, as defined in the paper. Evidence is found for a new effect consisting of a strong enhancement of the annihilation probability in the lenses obtained by the intersection of the Fermi surface with HMC surfaces. This effect, not the anisotropy of the Fermi surface, is the main reason for the anisotropy of the annihilation curves. Paper presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Positron decay in polymers: Molecular weight dependence in polystyrene

The decay time τ_l ofo-Ps by the electron pickoff process has been measured in polystyrene as a function of molecular weight. For samples below their glass transition temperature τ_l is essentially independent of . In the low molecular weight region, , τ_l increased with decreasing in line with earlier theoretical predictions. The overall behaviour reflects closely the dependence of free volume on . The effects of molecular motion on τ_l are examined and it is concluded that the decay time is insensitive to such motions in polystyrene below the melting point, in contrast to what is observed in polytetrafluoroethylene.     

Angular correlation of scattered annihilation photons, to test the possibility of hidden variables in quantum theory

Angular correlations of the annihilation photons, Compton scattered by plastic scintillators and detected by means of NaI (T1) crystals, have been measured in order to test the possibility of deviations of the experimental results from the predictions of the quantum theory. In fact, Jauch and Bohm, starting with different motivations, both arrive at the possibility of a lower correlation ratio between the two orthogonal polarization states of the two photons than predicted by quantum theory. This in turn should give a lower azimuthal anisotropy in the angular correlations. Our experimental results compared with the theoretical predictions, after correction for finite geometry by means of a Montecarlo method, do not confirm quantum theory and exclude the hypotheses of Jauch and of Bohm. We are continuing the experiment in order to test wether the breakdown in the polarization correlation depends on the distance (spatial and/or temporal) between the two correlated scattering events, as suggested by Jauch. Paper A 34 presented at 3 rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Positron beam studies on polyaniline and Ag-coated polyaniline

The effect of doping of the polyaniline emeraldine base (PEB), with Ni as well as Ni over layer coating has been investigated using variable low energy positron beam. Depth-resolved Doppler S-parameter measurements have been performed on undoped, Ni-doped polyaniline (PANI), and Ag (40 nm) film deposited PANI samples. Significant variation in S-parameter is observed for undoped and Ni-doped PANI. The size of the free volume hole has shifted to lower values upon doping with Ni as compared to that of undoped PANI, which is consistent with the conductivity measurements. For Ag-coated PANI systems, the S vs. E_p curves show distinct changes at the surface and interior regions. These results are discussed in the light of changes in free volume hole size distribution.     

Positron trapping and annihilation rates in some inorganic compounds

Summary The disappearance rates of positrons annihilated in some inorganic compounds were measured. The observed correlation between these rates is discussed on the basis of a two-state trapping model.

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Riassunto Sono state misurate le costanti di decadimento di positroni che annichilano in alcuni composti inorganici. Si pone in evidenza una correlazione tra queste costanti e la si discute sulla base di un modello a transizione tra due stati aventi differente densità elettronica.

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Резюме Измеряются постоянные распада позитронов, которые аннигилируют в некоторых неорганических соединениях. Обсуждается наблюденная корреляция между интенсивностями аннигиляции на основе модели перехода между двумя состояниями, имеющими различные электронные плотности.

     

Positron annihilation probing of crystallization effects in TAS-235 glass affected by Ga additions

Crystallization effects in Te₂₀As₃₀Se₅₀ glass known also as TAS-235 affected by Ga additions to Ga₂Te₂₀As₂₈Se₅₀ and Ga₅Te₂₀As₂₅Se₅₀ compositions are probed with positron annihilation spectroscopy in the measuring modes exploring positron lifetimes and Doppler broadening of annihilation line. Occurring of cubic-phase Ga₂Se₃ droplets with character nanoscale sizes in partially-crystallized Ga₂Te₂₀As₂₈Se₅₀ alloy is shown to be associated with agglomeration of intrinsic free-volume voids, this process being enhanced over microcrystalline scale in Ga₅Te₂₀As₂₅Se₅₀ alloy. Crystallization changes in the void structure of TAS-235 glass are considered in terms of free-volume evolution under the same principal chemical environment responsible for positron trapping in amorphous and partially crystallized substances.     

Computer calculation of positron diffusion and annihilation in atomic hydrogen

We have performed a computer-aided analysis of position behaviour in atomic hydrogen. Effect of electric, magnetic and temperature fields on the diffusion and annihilation of positrons has been studied. Electric field is varied over a wide range of 0–200 V cm⁻¹ amagat⁻¹, magnetic field over 0–30 kG while the temperature range considered is 300–10,000°K. The position decay rate decreases with electric and temperature fields but increases with magnetic fields. However, the effect of these fields is reversed on the diffusion coefficient.     

Positron annihilation in copper single crystals

A point geometry angular correlation measurement of the annihilation radiation from an annealed single crystal of copper is presented. The results are compared with results for an unannealed sample and theoretical predictions obtained from a simple picture of the electronic wave-functions combined with two models for the positron wavefunction.     

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.     

Positron beam studies of cobalt silicides

Silicide formation in Co/Si thin structures synthesized using thermal evaporation, sputter deposition and ion implantation, has been investigated using depth-resolved positron annihilation spectroscopy (PAS) together with other corroborative experimental techniques. S vs. E_p curves and S-W correlation plots have revealed important processes such as defect annealing, interdiffusion, silicide formation and recrystallization of amorphous Si. These studies have shown that there exist differences in the formation temperature of the silicide phases, the sequence of silicide phase formation and defect generation owing to the nature of the deposition methods employed.     

Defect Characterization of 6H-SiC Studied by Slow Positron Beam

利用单能慢正电子束流,对原生的和经过电子辐照的6H-SiC内的缺陷形成及其退火行为进行研究．发现在n型6H-SiC中,经过退火后缺陷浓度降低．这主要是因为在退火过程中缺陷和间隙子的相互作用所引起．n型6H-Si经过1400 oC、30 min真空退火后,在SiC表面形成一个大约20 nm的Si层,这是在高温退火过程中Si原子向表面逸出的有力证明．在高温退火中,在样品的近表面区域有一个明显的表面效应,既在这些区域的S参数整体较大,这种现象与高温退火中Si不断向表面逸出有关．经过10 MeV的电子辐照,在n型6H-SiC中,正电子有效扩散长度从86.2 nm减少至39.1 nm,说明在样品中由于电子辐照产生大量缺陷．但是对p型6H-SiC,经过10 MeV电子辐照后有效扩散长度变化不大,这与其中缺陷的正电性有关．同时还对n型6H-SiC进行了1.8 MeV电子辐照后的300 oC退火实验,发现退火后缺陷浓度不减反增,这主要是因为在退火过程中,一些双空位缺陷和Si间隙子互相作用从而产生了VC缺陷的缘故．     

Positron annihilation in copper: Ak-space analysis

A new approach to the analysis of positron-annihilation long-slit angular distributions which can provide a more direct reflection of Fermi surface profile has been described in a recent publication. The analysis involves the periodic superposition of angular distributions which has the effect of converting a momentum distribution into a distribution in reduced Bloch wave vectorsk. When applied to data for a Cu crystal, having the resolved momentum component parallel to the [110] direction, the analysis results in excellent agreement with the computed surface of Halse and also provides a guide to the form and intensity of the angular distribution for core electron annihilation. A similar treatment of the results for a [100] orientation, at first sight, appears less encouraging. However, a more careful appraisal supports the general value of the approach, the validity of the analysis for the [110] crystal orientation, and gives further clues to the form and anisotropy of the core distributions for Cu single crystals.