Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2 annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 Å. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.     

Positron lifetimes in quenched Al+0.09 at.−% Mn crystals

Measurements of positron lifetimes in quenched aluminum +0.09 at.?% Mn single crystals reveal the existence of a long-lived component whose lifetime and intensity increases and decreases with increasing temperature, respectively. The component is attributed to the annihilation of positrons at voids in the sample formed during quenching.     

Annihilation of unthermalized positrons in a silicon single crystal at 77°K

A model is considered for the annihilation of nonrelativistic positrons from quantized states in lattice channels. Annihilation gamma rays of energy over 511 keV have been observed when the positrons from an Na²² source strike a silicon single crystal at 77°K. The experimental results agree well with the proposed model.     

CVD金刚石膜的结构分析

利用x射线广角衍射和低角掠入射散射谱、正电子湮没谱、定性分析软件和Positronfit程序，研究了生长在Si（100）基底上的金刚石膜微结构.研究发现，在样品邻近基底区域为纳米 多晶结构，具有弱的［111］织构；在邻近表面区域为微米多晶结构，具有强的［220］织构 .金刚石膜样品有空位、空位团和空洞3种缺陷，其中主要缺陷是大约10个空位形成的空位团 . 关键词： 金刚石膜 化学气相沉积 x射线掠入射 正电子湮没谱     

Wheeler complexes in crystals

The basic properties of Wheeler complexes have been estimated for crystals that include positrons, positronium, electrons, holes, and excitons. Two basic forms of such quasiparticle states in crystals are considered: a) delocalized states, and b) states localized at defects. It is possible to observe Wheeler complexes by examining the annihilation characteristics of positrons in the presence of external fields such as static magnetic fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 10–14, April, 1981.     

Positronium atom in semiconductors

Evaluation of the lifetimes of positronium is given in relation to binding, dissociation, and annihilation within a wide internal of temperatures. At low temperatures, the overwhelming proportion of positrons becomes bound into atoms of positronium, and from such states is annihilated with the electrons of the crystal. The study of the annihilation of positrons in the semiconductors at low temperatures can give information on the parameters of the positronium atom, and, consequently, also on the exciton states.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 38–40, April, 1974.     

Symmetry and Condensed Matter Physics – A Computational Approach,by M. El-Batanouny and F. Wooten

The behaviour of positrons below the inelastic collision threshold in rare gases can usually be completely accounted for in terms of the annihilation and elastic scattering cross sections for collisions between a positron and a single gas atom. Determining these cross sections has been the subject of many theoretical and experimental investigations. However the basic premise that the positron interacts with the gas atoms, one at a time, has been recently questioned by the yet unexplained results of some low-temperature experiments. This article discusses the annihilation and elastic scattering cross sections as they relate to the conventional picture of positrons in He, Ne, Ar, Kr and Xe gas; as well as the possibility of bound states for positrons in the gases at low temperatures.     

Orthopositronium annihilation and emission in mesostructured thin silica and silicalite-1 films

Mesoporous silica films and MFI-type pure silica zeolite films were investigated using slow positrons. Detection of the 3γ annihilation fraction was used as a quick test to estimate the emission of orthopositronium (o-Ps) into vacuum. Positronium time-of-flight (TOF) spectroscopy, combined with Monte-Carlo simulation of the detection system was used to determine the energy of o-Ps emitted from the films. Evidence for an efficient o-Ps emission was found in both the mesoporous and silicalite-1. A 3γ fraction in the range of 31-36 % was found in the films with the highest o-Ps yield in each type of porous material, indicating that 40-50 % of the implanted positrons form positronium in the pore systems with very different pore sizes. Time-of-flight measurements showed that the energy of the orthopositronium emitted into vacuum is below 100 meV in the film with 2-3 nm pores at 3 keV positron energy, indicating an efficient slowing down but no complete thermalization in the porous films of 300-400 nm thickness.     

Self-trapping of positively charged particles in metals

It is shown that the existence of a metastable state in which positrons in metals are “self-trapped” by strong interaction with the lattice gives rise to an anomalous temperature dependence in positron annihilation properties. The “intermediate” temperature variation of the shape of the annihilation photon line discovered by MacKenzieet al. is well accounted for by this mechanism; alternative interpretations in terms of thermal expansion effects may be refuted. This result calls for considerable revision of some of the published monovacancy formation energies obtained from positron annihilation measurements. Approximate criteria for the existence and the metastability of a selftrapped state of positively charged particles in metals are given. It is found that metastable self-trapping may occur for positrons; hydrogen isotopes and positive muons should be self-trapped in configurations that are always stable relative to the Bloch-wave states of these particles.     

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ₂ = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ₁, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ₁ and the intensity I₂, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.     

Role of binding energy in Feshbach-resonant positron-molecule annihilation

Measurements of positron-on-molecule annihilation have established that positrons bind to a variety of molecules via vibrational Feshbach resonances. Data for deeply bound states in benzene and 1-chlorohexane and for positronically excited (i.e., second) bound states in alkanes are used to establish the dependence of annihilation rates on the binding energy and incident positron energy. With this dependence removed, annihilation rates for a broad class of molecules lie on a universal curve as a function of the number of molecular vibrational degrees of freedom. The implications of these results for theoretical models are discussed.     

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.     

Origin of the annihilation radiation from the galactic center region

Observations show that there are two sources of positron annihilation radiation from the region of the Galactic Center: a variable, compact source at or near the Galactic Center and a steady, diffuse source in the galactic disk. We suggest that the annihilating positrons responsible for the variable, annihilation line radiation, observed from 1977 through 1979, result from photon-photon pair production, most likely around an accreting black hole, and that the annihilating, interstellar positrons, responsible for the steady, diffuse galactic annihilation radiation, result from the decay of radionuclei produced by thermonuclear burning in supernovae.     

Relevance of slow positron beam research to astrophysical studies of positron interactions and annihilation in the interstellar medium

The processes undergone by positrons in the interstellar medium (ISM) from the moments of their birth to their annihilation are examined. Both the physics of the positron interactions with gases and solids (dust grains), and the physical conditions and characteristics of the environments where the processes of energy loss, positronium formation, and annihilation taking place, are reviewed. An explanation is given as to how all the relevant physical information are taken into account in order to calculate annihilation rates and spectra of the 511 keV emission for the various phases of the ISM; special attention is paid to positron interactions with dust and with polycyclic aromatic hydrocarbons. An attempt is made to show to what extent the interactions between positrons and interstellar dust grains are similar to laboratory experiments in which beams of slow positrons impinge upon solids and surfaces. Sample results are shown for the effect of dust grains on positron annihilation spectra in some phases of the ISM which, together with high resolution spectra measured by satellites, can be used to infer useful knowledge about the environment where the annihilation is predominantly taking place and ultimately about the birth place and history of positrons in the Galaxy. The important complementarity between work done by the astrophysical and the solid-state positron communities is strongly emphasized and specific experimental work is suggested which could assist the modeling of the interaction and annihilation of positrons in the ISM.     

Annihilation of positrons in aluminium oxide and phase transformations in this compound

Measurements have been made of the angular correlation N(θ) of positron annihilation quanta, the triquanta annihilation intensity I_3γ by the peak-to-peak method, and the average lifetimes of positrons in A1₂O₃. The measured quantities depend on the temperature at which the Al₂O₃ was obtained. It is assumed that there are two conversion centers of ortho -Ps. One is molecular oxygen adsorbed on the oxide surface and the other is the O^? ion which exists in A1₂O₃ obtained at lower temperatures, both on the surface and in the interior of the grains. The temperature ranges have been determined within which the changes in properties of the oxide with respect to the annihilation of positrons, occur slowly as well as those within which the changes occur rapidly. Among these properties are the vanishing of the postulated O^? ions with rise in temperature a probable diffusion of aluminium ions, and the vanishing of vacancies in the lattice structure (I_Ps). From the measurements of the rate characteristics the complete annihilation intensity I_xxx was calculated within the range from 0.34 to 96ns, and also annihilation intensity from para states of positronium I_p.     

Positron annihilation in ultrafine particles of Fe

Measurements of two-photon annihilation in ultrafine particles of Fe (～ 100 A in diameter) show that a considerable fraction of the positrons annihilate in the state of positronium. The emission of slow positrons from a metal surface can account for these results.     

Stringent constraint on galactic positron production

The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. If these positrons are injected at even mildly relativistic energies, higher-energy gamma rays will also be produced. We calculate the gamma-ray spectrum due to inflight annihilation and compare it with the observed diffuse Galactic gamma-ray data. Even with a simplified but conservative treatment, we find that the positron injection energies must be less than or similar to 3 MeV, which strongly constrains models for Galactic positron production.     

Orientation dependence of relativistic-positron annihilation in single crystals

An effect of the orientation dependence of the cross section for the single-photon annihilation of relativistic positrons with atomic electrons in a crystal is predicted. It is shown that the probability for the single-photon annihilation of a channeled positron in a crystal may be either suppressed in a crystal in relation to a homogeneous medium or, on the contrary, enhanced. The reason is that, depending on their incidence angle, the positrons may be either in the vicinity of ion planes of the crystal, where the electron density is higher, or far away from them, where the electron density is lower.     

Investigation of structural vacancies in titanium monoxide by electron-positron annihilation

Local atomic environment of vacancies in nonstoichiometric titanium monoxide ranging in composition from TiO_0.74 to TiO_1.26 was studied by electron-positron annihilation. Analysis of the Doppler broadening spectra of the annihilation gamma line for titanium and liquid oxygen showed that positrons in titanium monoxide are trapped by titanium vacancies. Experiments revealed that the lifetime of positrons in ordered and disordered titanium monoxide TiO _y increases with increasing oxygen content y and varies from 184 to 210 ps. Data on the valence electron density permitted the prediction that the lifetime of free positrons in stoichiometric titanium monoxide is about 140 ps and the lifetime of positrons localized in an oxygen vacancy is about 170 ps. The method used to analyze the gamma-line Doppler broadening spectra makes it possible to determine the type and number of atoms around a vacancy and to investigate order-disorder phase transformations in nonstoichiometric compounds.     

Die Vernichtung von Positronen im Fluge

The differential cross section at 0° for annihilation in flight of positrons has been measured with foils of lucite, Al, Cu, Ag, Sn, Au, and Pb. Positrons from Cu⁶⁴ and Co⁵⁶ with energies of 400, 600, 800, and 1000 keV were selected by means of a magnetic spectrometer of the Kofoed-Hansen type, and the annihilation was detected in a telescope consisting of three scintillation counters. The experimental results were compared with theoretical values computed from the two-quantum cross section by taking into account the effect of multiple scattering of the positrons. The agreement is good for light elements within a statistical accuracy of 10%. For heavy elements the experimental value was found to be 15% greater than the theoretical value which perhaps could be attributed to the one-quantum annihilation.