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 lifetimes in V3Si

From positron annihilation lifetime measurements on V₃Si single crystals we estimated that about 17.2% of the positrons annihilate with core electrons.     

Scientific tasks and present status of the GAMMA-400 project

The GAMMA-400 telescope is designed to investigate discrete high-energy gamma-ray sources in the energy range of 0.1–3000 GeV, to measure the energy spectra of galactic and extragalactic diffuse gammaray emissions, and to study gamma-ray bursts and gamma-ray emissions from an active Sun. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. Its special assignment is to measure fluxes of gamma rays, electrons, and positrons that could be associated with the annihilation or decay of dark matter particles.     

High-momentum analysis in Doppler spectroscopy

Unique information about the chemical vicinity of positron annihilation sites is provided by the contribution of high electron momenta to the Doppler spectrum, since this momentum range is characteristic for the annihilation with core electrons and hence element specific. However, the corresponding energy region in the spectrum is overlaid by a huge background caused by the annihilation radiation itself and the Compton spectrum of other gamma lines having an energy above 511 keV. Usually these backgrounds are reduced by measuring both annihilation quanta in coincidence.By mathematically analyzing the background contributions, we open another possibility to obtain the high-momentum region employing one single germanium detector. A necessary precondition is employing either background-free positron beams or a low-background positron source, e.g. ⁶⁸Ge, instead of the widely used positron emitter ²²Na. The ⁶⁸Ge-source emits positrons with an endpoint energy of about 1.9 MeV, where as the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low.When analyzing spectra from metals and semiconductors according to the described background subtraction, the same information contained in the momentum range up to 35 × 10⁻³m₀c or beyond can be extracted, as if the spectra were measured employing a coincidence setup with two Ge-detectors.     

Investigations of positronium formation and destruction using 3gamma/2gammaannihilation-ratio measurements

Positronium (Ps) produced by 4 to 40 eV positrons colliding with Ne, Ar, Kr, CO2, and O2 is investigated by measuring the ratio of signals of two gamma rays in coincidence resulting from (a) three gamma annihilation of ortho-Ps and (b) two annihilation gamma rays due to para-Ps decay and destruction of ortho-Ps at an aluminum scattering cell surface. These ratios provide evidence that relates to the kinetic energy dependence of ortho-Ps interactions with an aluminum surface, the Ps formation potential at this surface, and the fact that Ps is being formed with inner orbital electrons for CO2 and O2.     

正负电子三光子湮灭中光子能谱的Monte Carlo计算

用Monte Carlo方法计算正负电子三光子湮灭的光子能谱,并与量子电动力学(QED)理论谱及实验结果进行了比较,三种结果符合甚好。     

Positron annihilation in gamma irradiated crystals

The angular correlation between the two gamma-quanta arising from positron annihilation in untreated and gamma-irradiated alkali halides is reported. For low level gamma irradiation the percentage of positrons annihilating with F-centre electrons is not very significant. When the F-centre concentration is increased to as much as 1.5 × 10¹⁷ per cm³, about 15 per cent of positrons annihilate with F-centre electrons. The angular correlation curve becomes progressively narrower with increase in F-centre concentration. Positrons annihilating in gamma-irradiated crystals reduce the F-centre considerably. The results of a systematic investigation on F-centre bleaching by positron annihilation are reported.     

Defects in Si-Rich SiO2 Films Prepared by Radio-Frequency Magnetron Co-sputtering Using Variable Energy Positron Annihilation Spectroscopy

Si-rich SiO2 films prepared by rf magnetron co-sputtering method are studied by slow positron beams. The nega- tively charge point defects （probably Pb centres or peroxy radicals） at the silicon nanocluster （nc-Si）/SiO2 interface are observed by Doppler broadening spectra. Coincidence Doppler-broadening spectra show that positrons have a higher annihilation probability with core electrons nearby oxygen atoms than silicon atoms. The formation of N-related bonds may be the reason for the prevention of the migration reaction of Si and 0 atoms, hence nc-Si formation is inhibited by annealing in nitrogen compared to in vacuum.     

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.     

511 keV photons from color superconducting dark matter

We discuss the possibility that the recent detection of 511 keV gamma rays from the galactic bulge, as observed by the International Gamma-Ray Astrophysics Laboratory, can be naturally explained by the supermassive very dense droplets (strangelets) of dark matter. These droplets are assumed to be made of ordinary light quarks (or antiquarks) condensed in a nonhadronic color superconducting phase. The droplets can carry electrons (or positrons) in the bulk or/and on the surface. The e(+)e(-) annihilation events take place due to the collisions of electrons from the visible matter with positrons from dark matter droplets which may result in the bright 511 keV gamma-ray line from the bulge of the Galaxy.     

Cosmic gamma-rays associated with annihilation of relativistic e+ −e− pairs

Results of electron-positron annihilation spectrum investigations are reported. The radiation spectra of an e⁺ ?e^? relativistic maxwellian plasma are presented. We consider the particular case of positron annihilation on electrons at rest. It is shown that for positrons with a suprathermal power law distribution the radiation due to annihilation exceeds bremsstrahlung up to at least ω ≈ 100 mc².     

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.     

正负电子三光子湮灭特性的计算机模拟研究

给出了正负电子三光子湮灭模拟实验中描述三个光子发射角的随机变量α₁、α₂、α₃和描述三个光子能量的随机变量w₁、w₂、w₃的抽样方法.根据该抽样方法所产生的子样,得到了关于α₁、α₂、α₃和w₁、w₂、w₃的边缘分布和联合分布。有关结论与已知的量子电动力学(QED)理论预言及实验数据作了比较.     

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.     

Doppler broadening of in-flight positron annihilation radiation due to electron momentum.

We report the first observation of electron momentum contributions to the Doppler broadening of radiation produced by in-flight two-photon annihilation in solids. In these experiments an approximately 2.5 MeV positron beam impinged on thin polyethylene, aluminum, and gold targets. Since energetic positrons easily penetrate the nuclear Coulomb potential and do not cause a strong charge polarization, the experimental annihilation line shapes agree well with calculations based on a simple independent-particle model. Moreover, annihilations with the deepest core electrons are greatly enhanced.     

A MEASUREMENT OF THE LINEAR POLARIZATION OF PHOTONS EMITTED FROM THREE PHOTON ANNIHILATION OF POSITRONS & ELECTRONS

The gamma rays emitted from three photon annihilation of positrons and electrons were selected with a triple coincidence method and the linear polarization of one of the three photons was investigated by using Compton scattering. Under two geometric conditions of three photons annihilation (α₁=α₂=α₃=120° and α₁=90°, α₂=α₃=135°), the ratio of the scattered photons which were parallel with the annihilation plane to those perpendicular with the annihilation plane were 1.82±0.06 and 1.53±0.08, thus the linear polarization of the photons to the annihilation plane were 0.56±0.04 and 0.40±0.09.     

Contribution of valence s+p type electrons to the high momentum components of the electron-positron momentum density in gold

It is generally believed that the high momentum components (HMCs) of the angular correlation of annihilation radiation or Doppler broadening spectra map the positron annihilation with predominately core electrons, containing only a small fraction coming from localised d+f-type valence electrons. In present work, we study how far the contribution of valence electrons to the HMCs of the electron-positron (e-p) momentum density can be neglected. Calculations are performed for gold within both the independent particles model (IPM) and including the e-p correlation effects non-locally, using the weighted density approximation (WDA). In particular, the HMCs due to valence s+p-type electrons are compared with their core and d+f-type valence counterparts.     

Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

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.     

Cosmic Ray Electrons and Protons,and Their Antiparticles

Cosmic rays are a sample of solar, galactic, and extragalactic matter. Their origin, acceleration mechanisms, and subsequent propagation toward Earth have intrigued scientists since their discovery. These issues can be studied via analysis of the energy spectra and composition of cosmic rays. Protons are the most abundant component of the cosmic radiation, and many experiments have been dedicated to the accurate measurement of their spectra. Complementary information is provided by electrons, which comprise about 1 % of the cosmic radiation. Because of their low mass, electrons experience severe energy losses through synchrotron emission in the galactic magnetic field and inverse Compton scattering of radiation fields. Electrons therefore provide information on the local galactic environment that is not accessible from the study of the cosmic ray nuclei. Antiparticles, namely antiprotons and positrons, are produced in the interaction between cosmic ray nuclei and the interstellar matter. They are therefore intimately linked to the propagation mechanisms of the parent nuclei. Novel sources of primary cosmic ray antiparticles of either astrophysical (e.g., positrons from pulsars) or exotic origin (e.g., annihilation of dark matter particles) may exist. The nature of dark matter is one of the most prominent open questions in science today. An observation of positrons from pulsars would open a new observation window on these sources. Several experiments equipped with state-of-the art detector systems have recently presented results on the energy spectra of electrons, protons, and their antiparticles with a significant improvement in statistics and better control of systematics. The status of the field will be reviewed, with a focus on these recent scientific results.