Positron reemission brightness enhancement method

Numerical results are presented for a one-dimensional diffusion model for freely diffusing positrons in thin films with an initial exponential positron profile. The results are for two boundary conditions which are experimentally realizable. Since thermalized positrons generally obey this diffusion model, it should be possible to extract the bulk diffusion constant by fitting experimental measurements to the model's predictions. Recently, a scheme has been proposed for the brightness enhancement of slow positron beams. We discuss experimental methods for implementing this scheme and the various problems that may arise.Summer student at Brookhaven National Laboratory, 1980     

The shape of the allowed positron spectrum of Na22

The shape of the allowed positron spectrum from the decay of Na²² has been studied under various experimental conditions in an ironfree, double-focusing spectrometer in search of instrumental distortions of the spectrum. The possibility of positrons back-scattering from the material directly behind the source on the optical path of the positrons was studied. The beta spectrum shape was measured with end-plates (located ～6.5 cm behind the source) of aluminum, lucite and lead. Different spectrometer baffle settings were also used. No differences were observed in the spectrum shapes taken under the different experimental conditions. The experimental shape factor could be fitted with an empirical shape of the form of (1+b/W) with 0.10?b?0.3 in agreement with earlier work.     

液滴法制备空心玻璃微球的模拟计算

 通过对液滴法制备空心玻璃微球的成球过程进行分析,建立了成球过程数学模型,定量描述了液滴的形成、液滴的封装、凝胶球壳的形成与干燥,发泡剂的分解,球壳的预热与熔炼、冷却与收集等过程中,液滴/球壳的大小、速率、壁厚、气压等随操作条件的变化,计算了抽气速率、发泡剂浓度、玻璃溶液浓度和初始液滴大小等的改变对成球过程的影响,并进行了初步验证实验。     

Rectifying barrier at GaN/SiC hetero-junction studied with positron annihilation spectroscopy

Positron annihilation spectroscopy on GaN films grown on SiC substrate with MBE are presented. It is shown that the GaN/SiC interface is rectifying towards positrons, such that positrons can only travel from SiC to GaN and not vice versa. Potential steps seen by the positron at the GaN/SiC interface are calculated from experimental values of electron and positron work function. This “rectifying” effect has been successfully mimicked by inserting a thin region of very high electric field in the Variable Energy Positron Fit （VEPF） analysis. The built-in electric field is attributed to different positron affinities, dislocation and/or interface defects at the GaN/SiC interface.     

On radiation damping in planar channeling in the presence of multiple scattering

The effect of radiative damping of transverse oscillations of positrons in the planar channel of a crystal is considered under realistic conditions when multiple scattering of the positrons on the atomic electrons is taken into account.     

双势阱产生正负电子对过程中的正电子波干涉与克莱因隧穿现象

通过将狄拉克场量子化并且数值求解狄拉克方程的方法研究在一维情况下双势阱激发正负电子对产生的过程.研究发现双势阱激发的正电子波在双势阱之间会出现干涉现象,过程中伴随着克莱因隧穿效应,并且在双势阱之间的距离取正电子波对应的驻波条件时,在双势阱之间会表现出驻波形式的正电子干涉波.并且驻波的出现对正负电子对的产生过程也存在相应的影响,驻波最终会通过克莱因隧穿效应衰减消失.     

Positron annihilation in solid krypton and xenon

A computation of the life time of positrons as well as the angular distribution of the resulting gamma pair for polycrystalline krypton and xenon has been made. The calculations are based on the technique employed by Salvadori and Carbotte for the case of solid argon. The field seen by the positrons is constructed from the charge densities derived from Herman-Skillman wave functions. The positron wave functions are obtained using the Wigner-Seitz approximation. For electrons Herman-Skillman wave functions have been used. The computed distributions, when the effects of correlation are ignored, are wider than the experimental ones. The calculated values for the mean lives of positrons for krypton and xenon are 2.2×10⁻⁹ sec and 2.7×10⁻⁹ sec, respectively, and that for xenon is, as usual, longer than the measured value of 0.43×10⁻⁹ sec. Effects of electronpositron correlations on the angular distribution and the life time have been calculated for the case of krypton. When these corrections are taken into account, the life time of positrons in krypton is reduced by a factor of about eight and the angular distribution shows a slight narrowing. Paper A24 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

The decay of the vacuum in the field of superheavy nuclear systems

In the strong Coulomb field of a nucleus or quasimolecule withZ?172 a change of the QED vacuum has been predicted, signalled by the spontaneous emission of positrons if holes in theK-shell are available. The dynamical semiclassical theory of positron excitation in heavy ion scattering is presented and extended to collisions with nuclear contact. Interference patterns in the energy spectrum of the emitted positrons and, for sufficiently long reaction timeT, the emerging of a characteristic line at the position of the 1sσ-resonance is predicted. The position and width of the 1sσ-resonance is calculated in dependence on the nuclear configuration, the effect of electron screening is taken into account. Recent experimental indications [1, 2] for structures in the spectra of positrons emitted in U-U collisions at energies close to the Coulomb barrier are discussed in terms of spontaneous positron creation. The observed spectra could be explained under the assumption of long lasting nuclear reactions (T?4·10^?20 s) with a cross section in the millibarn region. Various consequences of this interpretation are discussed.     

On the theory of generation of a polarized muon-photon shower in crystals under different initial conditions

The process of formation of a polarized muon-photon shower in crystals has been investigated under the following initial conditions: (i) a shower is generated by a high-energy longitudinally polarized initial muon and (ii) a shower is generated by a circulaly polarized high-energy γ photon. Analytical expressions have been obtained for the distribution functions of polarized shower muons and γ photons in the medium and comparative analysis of the number of shower particles under different initial conditions has been performed.     

Fraction of cosmic-ray positrons in the galaxy: Results from using the fractional diffusion approach

The fraction of positrons in the electron-positron component of cosmic rays is calculated in a scenario where electrons and positrons are generated by Galactic sources with the same spectral index. It is shown that the proposed scenario allows us to reproduce the existing experimental data on the fraction of positrons if the relative yield of positrons in a source is e ⁺/e ^? ≈ 0.3.     

Positron lifetimes and distributions in the infinite-layer compound SrCuO2 and related materials

We have calculated distributions and lifetimes of positrons in the infinite-layer compound SrCuO₂ and those trapped at possible point defects therein. In the delocalized state, positrons show their density maxima at interstitial sites in the Sr planes and have a significant overlap also with Cu and O atoms. The corresponding positron lifetime is 149 ps. It has been revealed that the Sr vacancy strongly localizes positrons with the binding energy of 2.8 eV and the lifetime of 238 ps, while the O vacancy does not trap positrons. Calculations are also performed on related materials Sr₂Cu₄O₆ and Sr₄Cu₆O₁₀, which are characterized by one-dimensional networks of edge-sharing CuO₄ squares. Positrons are predominantly distributed between these networks in these materials and their corresponding lifetimes are 170–171 ps.     

Positron scattering by phonons in metals

Processes involved in positron-matter interaction are studied. The stopping power and mean free path are calculated for positrons with an energy of about 1 eV, which are scattered mostly by phonons. The mean free path and range of positrons in tungsten, as well as the stopping power of tungsten, are calculated for positron energies between 0.025 and 10 eV.     

Trapped positrons for antihydrogen

Positrons from a 12 mCi²²Na source are slowed by a W(110) reflection moderator and then captured in a Penning trap, by damping their motion with a tuned circuit. Because of the stability of the Penning trap and the cryogenic ultra-high vacuum environment, we anticipate that positrons can be accumulated and stored indefinitely. A continuous loading rate of 0.14 e⁺/s is observed for 32 h in this initial demonstration. More than 1.6×10⁴ positrons have thus been trapped and stored at 4 K, with improvements expected. The extremely high vacuum is required for compatibility with an existing antiproton trap, which has already held more than 10⁵ antiprotons at 4 K, for producing antihydrogen at low temperatures. The extremely cold positrons in high vacuum may also prove to be useful for cooling highly stripped ions.     

A positron injector for the LEPTA accumulator

An injector of monochromatic positrons for the low-energy positron accumulator (LEPTA) is being tested at the Joint Institute for Nuclear Research. The source of positrons is the radioactive source ²²Na. At the output of the source, positrons are slowed down in a solid target. Frozen neon is used as a moderator. For this purpose, a system of cryocooling of the source and the neon supply line have been assembled. A method of detection of slow positrons has been developed and tuned. The first experiments with the frozen moderator have been performed. A continuous beam of slow positrons with an average energy of 1.2 eV and spectrum width of 1 eV has been obtained.     

Elastic-scattering cross-section for electrons and positrons from atomic hydrogen

The differential cross-sections of atomic hydrogen for elastic scattering of electrons and positrons have been rederived with the help of a method using a single parameter-dependent unitary shift operator for the calculation of the direct contribution. When the parameter approaches zerc the new method leads to the well-known conventional Glauber results. The numerical calculations include polarization effects and the exchange corrections obtained according to alternative approximation methods. Results calculated with Franco’s exchange show a definite improvement over the earlier results for medium energy electrons at large angles of scattering. Total elastic cross-sections have been calculated for 50 and 100eV electrons and positrons.     

Atomic string holography.

A new diffraction-channeling effect has been discovered, in which Kikuchi or channeling line patterns formed by high energy electrons, neutrons, and positrons are shown to break up into a series of annular disks if the crystal thickness traversed by the beam is small. The disks may be interpreted as Gabor in-line holograms of strings of atoms projected along the beam path. For electrons or positrons the patterns may be detected with little background by detecting characteristic x-ray emission from a thin film as a function of the diffraction conditions of a collimated, ionizing, high energy beam. Uses of the effect for structure determination and atomic-resolution lensless imaging are suggested.     

Formation of the Ordered Positron Flux

A method for forming a continuous flux of monochromatic positrons with a controlled time of arrival at the target, independent of the start time in a limited start time interval. Such an ordered positron (OPF) flux allows one to construct an experimental setup for studies of solid matter structure by positron annihilation spectroscopy (PAS) in its most efficient version—the Positron Annihilation Lifetime Spectroscopy (PALS). Simultaneously, by varying the potential of the target, one can control the energy of the positrons on the target and, accordingly, the depth of their penetration into the sample under study, measuring the distribution of defects in depth with high resolution. The research has been performed at the Dzhelepov Laboratory of Nuclear Problems, JINR and has been presented at the XII International Seminar in memory of V.P. Sarantsev, 2017, Alushta, Crimea, Russia.     

Computer-aided determination of annihilation and momentum transfer rates of slow positrons from argon gas atoms

Annihilation and momentum transfer rates of slow positrons in argon gas have been determined by an iterative computer-based analysis. Agreement of calculated positron annihilation decay constant with experimental measurements at various electric and temperature fields is very good.     

Characterization of fullerenes using positrons

Positron distributions and lifetimes in the K_xC₆₀ crystals for x=0, 3, 4, 6 have been calculated using the superposed-atom model and the numerical relaxation technique. It is revealed that positrons are distributed predominantly at the octahedral interstices in the fcc lattice of pure C₆₀. The distributions and lifetimes of positrons are significantly changed by potassium doping. In the fcc K₃C₆₀, the positron density shows its maxima in the interstitial region between the octahedral and tetrahedral interstices and has some amplitude in the inner space of the C₆₀ molecules. In contrast to this, positrons are distributed one-dimensionally along the [001] direction in the bct K₄C₆₀ while positrons are nearly localized into the C₆₀ molecules in the bcc K₆C₆₀.     

Experimentelle Untersuchung der direkten Elektron-Positron-Paarerzeugung in Kupfer durch Elektronen von 5,8 MeV kinetischer Energie

The direct production of positrons by electrons in copper was investigated using the 5.8 MeV electron beam from the 6 MeV microtron of the Physikalisch-Technische Bundesanstalt. Cross sections were derived from the measurements under the assumption that the positrons produced directly and those produced indirectly by bremsstrahlung photons have the same angular distribution. The values obtained are greater than those calculated numerically by Löns by a factor of 1.3 to 1.45. It is shown that there is an essential contribution of direct positron production to the electron positron conversion factor in thin foils consisting of elements with low atomic number. The experimental set-up is described in detail. Its main feature is the use of a special positron transmission line with bending magnet in order to overcome background difficulties.