Measurements of positronium formation cross sections for positron-ar and-K scattering

We have made preliminary measurements of positronium (Ps) formation cross sections for 9 to 452 eV positrons scattered from Ar atoms and for 1 to 20 eV positrons scattered from K atoms. Our experimental approach involves setting lower and upper limits on Ps formation cross sections using a combination of (1) the detection of the coincidences of 511 keV annihilation gamma rays produced by the decay of para-Ps and by the interaction of ortho-Ps with the wall of the scattering cell in which the Ps is formed, and (2) the determination of scattering cross sections associated with the measured transmission of the positron beam through the gas in our scattering cell with the angular discrimination of our apparatus deliberately made as poor as possible. The constraints placed by these lower and upper limits are used to check for consistency with prior experimental and theoretical results for Ar and to provide the first measurements of Ps formation cross sections for K, which are compared with available theoretical results.     

Spin-orbit quenching of positronium during atomic collisions

When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is ortho-Ps-->para-Ps conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.     

Experimental evidence for spin-orbit interactions in positronium-Xe collisions

Spin-orbit interaction of positronium (Ps) with the surrounding atoms, predicted by Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)], has been detected experimentally. We have found that the lifetime of the magnetically unperturbed ortho-Ps in Xe gas of 1 atm decreases significantly when a magnetic field of 1.0 T is applied. This decrease is attributed to the Ps spin conversion caused by spin-orbit interaction during Ps-Xe collision. The annihilation cross section of ortho-Ps due to this interaction has been determined to be 3 times as large as that expected by Mitroy and Novikov.     

Second bound state of PsH

The existence of a second bound state of PsH that is electronically stable and also stable against positron annihilation by the normal 2gamma and 3gamma processes is demonstrated by explicit calculation. The state can be found in the ;{2,4}S;{o} symmetries with the two electrons in a spin-triplet state. The binding energy against dissociation into the H(2p)+Ps(2p) channel was 7.03 x 10;(-4) hartree. The dominant decay mode of the states will be radiative decay into a configuration that autoionizes or undergoes positron annihilation. The NaPs system of the same symmetry is also electronically stable with a binding energy of 1.514 x 10;(-3) hartree with respect to the Na(3p)+Ps(2p) channel.     

Enhancement of positron-atom annihilation near the positronium formation threshold

The behavior of the positron- 2 gamma annihilation rate on an atomic target near the positronium (Ps) formation threshold is determined. When the positron energy epsilon approaches the threshold epsilon(thr) from below, the effective number of electrons contributing to the annihilation, Z(eff), grows as Z(eff) approximately A/square root of [epsilon(thr)-epsilon], where A is related to the size of the Ps formation cross section, sigma(Ps) approximately B square root of [epsilon-epsilon(thr)], by A = B square root of [2 epsilon(thr)]/32 pi (in atomic units). Taking account of the finite Ps lifetime eliminates the singularity in Z(eff) and shows that close to threshold the positron annihilation cross section is identical to the para-Ps formation cross section.     

Mössbauer investigations on intramolecularly phosphoryl coordinated organotin compounds

Recently, high-quality positron annihilation lifetime measurements showed that four components of lifetime spectra for many liquids could be resolved. The well-known three components (para-Ps (positronium), free positron, and ortho-Ps) and a short-lived ortho-Ps component were obtained in many liquids. This short-lived ortho-Ps is probably caused by secondary reactions of ortho-Ps and other reactants, such as a positive ion, in the positron spur. We report the first results of measurements of the yield of this short-lived ortho-Ps component in the system of hexane and the electron scavengers CCl₄, CS₂, C₆F₆, and SF₆.Deceased.     

Decay of the positronium molecule into a positronium atom and photons

Decays of the positronium molecule Ps₂ into para- or orthopositronium Ps in the ground state and photons are investigated. The differential probabilities of the decays are determined. The total probabilitiesw _2γ ^(Ps2) andw _3γ ^(Ps2) of Ps₂ annihilation with the production of two and three photons and positronium are calculated to be $$w_{2\gamma }^{(P_{S_2 } )} = 1.6 \cdot 10^{10} \sec ^{ - 1} ,w_{3\gamma }^{(P_{S_2 } )} = 0.43 \cdot 10^8 \sec ^{ - 1} $$ . The curve of the angular correlation of the γ rays on the decay of Ps₂ into two photons and parapositronium is studied. The width of this curve is Δ¦P¦=0.128 a.u. (¦P¦ is the total photon momentum), which corresponds to a deviation of the emission angle of the γ rays from π: θ ? 0.934 mrad. The maximum in the distribution of the photons with respect to the momenta ¦P¦ in the center of mass of the annihilating pair is attained at ¦P¦=0.175 a.u. The calculations were made on an M-222 computer, and their accuracy is determined by the choice of the wave function of the positronium molecule and the accuracy in the computer calculation of the integrals.     

Explosive dark matter annihilation

In this Letter we study pair annihilation processes of dark matter (DM) in the Universe, in the case that the DM is an electroweak gauge nonsinglet. In the current Universe, in which the DM is highly nonrelativistic, the nonperturbative effect may enhance the DM annihilation cross sections, especially for that to two photons, by several orders of magnitude. We also discuss sensitivities in future searches for anomalous gamma rays from the galactic center, which originate from DM annihilation.     

On previous and present investigations of resonance gamma-ray interaction with nuclei at the Institute of Theoretical and Experimental Physics (ITEP, Moscow)

A brief survey of theoretical and experimental work that is devoted to studying the resonance absorption and scattering of gamma rays by nuclei and which was initiated at the Institute of Theoretical and Experimental Physics (ITEP, Moscow) in the 1950s and has been continued to date is given. Investigations of various versions of interaction in beta decay, magnetic-field-perturbed angular distributions of resonantly scattered gamma rays, the problem of the Mössbauer gamma resonance of long-lived isomeric states of nuclei, and the resonance scattering of annihilation photons by nuclei are described.     

Unified treatment of positron annihilation and positronium formation

The effects of the positron (e(+)) annihilation in e(+) scattering by the H atom are included directly in the Hamiltonian as an absorption potential, and hence the finite lifetimes Gamma(ns) of positronium (Ps) in states ns are automatically taken account of. The Schr?dinger equation is solved using the hyperspherical close-coupling method. The annihilation and Ps formation are shown to be inseparable near the Ps(1s) threshold E(th); Ps formation constitutes just an indirect pathway to annihilation. The annihilation cross section, which would diverge near E(th) if Gamma(1s) were infinite, connects smoothly across E(th) to the cross section for Ps formation, which is meaningful only at energies well above E(th).     

Depth-dependent positronium formation in γ-irradiated polymers after 30-month aging

Depth-dependent positronium (Ps) formation and annihilation study of Co-60 γ-irradiated polyethylene and polytetrafluorethylene were performed. After 30-month aging, no free radicals effect on Ps formation was detected for the studied polymers. Efficient trapping of back-diffusion positrons was observed in heavily irradiated samples, where Ps formation probability near polymer surface and in the bulk turns to be equivalent to each other. Results shown that positron annihilation Doppler broadening in polytetrafluorethylene is prevailingly influenced by high momentum electrons of fluorine atoms instead of Ps intensity.     

Gamma spectra resulting from the annihilation of positrons with electrons in a single core level

The first gamma spectra associated with the annihilation of positrons with individual core levels (Cu 3p and Ag 4p) are presented. The spectra were obtained by measuring the energy of gamma rays time coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level. Relativistic calculations show good agreement with experiment over a limited range of momenta. However, statistically significant differences indicate that the measurements can provide an impetus to new calculations of many body effects in positron-core electrons annihilation.     

Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique

A high-performance positron age-momentum correlation (AMOC) spectrometer was newly developed.The counting rate is increased up to 200 cps much larger than the value 20 cps reported by other international groups.And at the same time,the time resolution still keeps at the international level of 220 ps.Furthermore,positronium (Ps) annihilation in silica aerogel was investigated by AMOC,which indicates:(1) Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps;(2) Annealing below 400 ℃ changes the grain surface conditions,i.e.the desorption of hydrogen and the decrease of the defect centers concentration.     

Effect of interstitial impurities on positronium formed in voids of vanadium

Angular correlation of two-photon annihilation radiation (ACAR) measurements have been performed to study the effect of interstitial impurities (O, C and D) on positronium (Ps) formation in irradiation-induced voids of vanadium. It has been observed that Ps formation is sensitively affected by doping with the interstitial impurities, irradiation dose, irradiation temperature, and also by post-irradiation annealing. The Ps component intensity is found to be related to segregation of the interstitial impurities and provides a new experimental method to study void surfaces.     

Exchange collisions between positronium and Cu2+ ions

Summary The fate and properties of the ortho-Ps in aqueous solutions of CuSO₄ and CuCl₂ have been investigated through lifetime measurements and through a magnetic-quenching experiment. The obtained results are interpreted in terms of exchange collisions between Ps and unpaired electron. An upper limit is given for the chemical-quenching rate. Our measurements have shown that the presence of paramagnetic centres determines a strong shift of the levels of the Ps hyperfine structure. This work was entirely supported by CISM (Centro Interuniversitario di Struttura della Materia) of the Ministero della Publica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

The ideal resolution of a liquid xenon calorimeter

The energy and angular resolution of a liquid xenon calorimeter for gamma rays in the GeV region is studied by a Monte-Carlo simulation. For a size of (70 cm)³, the energy resolution becomes as good as 0.2 % (r.m.s) at 10 GeV. The angular resolution is order of 1° (median angle) at several GeV without using the pair converter separated from the main body of the calorimeter. An effective SΩ of 1 m²sr is attainable. These factors meet the conditions required for observation of line gamma rays emerging from the annihilation of a sort of hypothetical dark matter.     

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.     

Determination of the electron momentum distribution of matter by the positron-annihilation method

A method is developed for recovering the density function of annihilating electron-positron pairs from the angular correlations of the annihilation gamma rays. On the basis of this positron-annihilation mechanism, the electron density functions of the matter are determined from the time spectra of the positron lifetime and the angular distributions of the annihilation gamma rays.The material of this paper was presented at the Fourth International Conference on the Density of Electrons, Spins, and Moments (Minsk, August, 1973).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 103–109, August, 1974.     

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.     

Constraining dark matter models from a combined analysis of Milky Way satellites with the Fermi Large Area Telescope

Satellite galaxies of the Milky Way are among the most promising targets for dark matter searches in gamma rays. We present a search for dark matter consisting of weakly interacting massive particles, applying a joint likelihood analysis to 10 satellite galaxies with 24 months of data of the Fermi Large Area Telescope. No dark matter signal is detected. Including the uncertainty in the dark matter distribution, robust upper limits are placed on dark matter annihilation cross sections. The 95% confidence level upper limits range from about 10(-26) cm3 s(-1) at 5 GeV to about 5×10(-23) cm3 s(-1) at 1 TeV, depending on the dark matter annihilation final state. For the first time, using gamma rays, we are able to rule out models with the most generic cross section (～3×10(-26) cm3 s(-1) for a purely s-wave cross section), without assuming additional boost factors.