Positron states between positronium and “free positron” states in condensed matter

Positronium (Ps), the bound state of an electron and a positron, is formed in many insulating solids and liquids. Recently, several cases of a transition from a Ps state to a non-Ps state have been observed. This has been found for liquid CS₂ if the temperature is decreased from about –40 °C to the melting point, –110.8 °C. In some solid aromatic hydrocarbons, such as naphthalene, biphenyl and p-terphenyl, similar transitions occur upon decreasing the temperature. These transitions are unique in physics since Ps is the only bound state of two light particles in normal low-energy physics. Measurements of the positron lifetime, angular correlation of the annihilation radiation, Ps chemical reactions, and Zeeman effect of Ps give information on the transitions.Deceased.     

Positron and positronium diffusion in nanomaterials

Some problems of positron (positronium) diffusion in powder nanoparticles of solid and nanoparticles of nanomaterials (index g) separated by voids in the case of powders and by interfaces, vacancies, and empty volumes at grain junctions of a nanomaterial (index v) have been considered. It has been shown that the validity of the diffusion approach to describe positron and positronium transformation and the conclusions of diffusion theory can be checked by varying the particle sizes of nanopowder and solid nanomaterials, and the diffusion coefficients D can be rather simply determined.     

GaN-a new material for positron moderation

GaN with its wide bandgap might be of interest as a positron moderation material in much the same way as SiC is. To investigate this, positron beam experiments have been performed to establish the diffusion behaviour and surface branching of positrons implanted with energies varying from 0 to 25 keV into an epitaxially grown layer of semi-insulating GaN on a sapphire substrate. The measured diffusion length of the positrons amounted to 19.3 ± 1.4 nm. The surface branching ratios were as follows: 48% positron emission, 12% positronium formation and 40% trapping at the surface. The positron workfunction was shown to be negative with a value of 2.4 ± 0.3 eV. The materials feasibility for positron moderation and its possible use in field assisted moderation is discussed.     

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).     

Surface positron and positronium states in real ionic crystals

Positron annihilation has been used to examine the positron and positronium states at the surfaces of (100) and (110) faces of KCl and KCl-5% AgCl single crystals. The measured and theoretical parameters differ from the analogous bulk values; exposure of KCl to Ps atoms produces mainly A₊, F₊, and D₊ centers. It is shown that this method can be used to examine surface states and surface electron-density distributions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 76–81, January, 1978.     

Isolation of22Na for intense positron sources

A method of removing²²Na from Al via distillation is investigated. Distillation is rapid (≈10 minutes) and nearly quantitative. When the distilled vapor is directed by a flow of He gas the deposit may be localized fairly well and easily recovered with water.     

Field ionization of strongly magnetized rydberg positronium: A new physical mechanism for positron accumulation

Magnetized Rydberg positronium forms when an energetic positron ( e(+)) slows within a tungsten crystal and picks up an electron ( e(-)) as it emerges in a strong magnetic field. The signature is equal numbers of e(+) and e(-) when a weak electric field is applied, either of which can be accumulated and counted. The new e(+) accumulation technique is simple, robust, and much more efficient than any other demonstrated to be compatible with a cryogenic vacuum. Possible applications include the study of cold single component plasmas of e(+) and the formation of cold antihydrogen.     

Compact positron beam for measurement of transmission moderator efficiencies and positron yields of encapsulated sources

We have constructed a simple positron beam for measuring both the efficiencies of positron moderator foils and the positron yields of encapsulated positron beam sources. The slow-positron emission rate from the moderator is determined from the positron annihilation radiation. This technique is also applied to measure source yields. In addition to the coincidence technique, the source yield is determined by measuring electric currents generated by the annihilating positrons. The performance of the system is demonstrated with different tungsten moderator foils before and after heat treatments, and several sources with primary activities in the range 1-50 mCi.     

Differential cross sections for elastic scattering and positronium formation for positron collisions with Ne,Ar, Kr and Xe

Differential cross sections for elastic scattering and Ps(1s) formation are calculated in the truncated coupled-static (TCS) approximation. Comparison is made with the elastic scattering measurements of Dou et al. on Ar and Kr. There is no support from the TCS approximation for the view that structure seen in the experimental results for Ar in the energy range 55–60 eV may be due to a resonance associated with coupling to the positronium formation channel. Rather, we believe that Dou et al. are right in correlating this feature with the maximum in the ionization cross section. In the experiment on Kr structures are observed at 25 and 200 eV. It is tentatively suggested that the structure at 200 eV may be the resonance seen in the TCS calculation at 100 eV, but modified by polarization and absorption effects. It is also suggested that the feature at 25 eV could be associated with coupling to excited states of positronium. The TCS results for the Xe target predict some very pronounced behaviour which would be worth experimental investigation. TCS differential cross sections for Ps(1s) formation by capture of an electron from the outer p-shell of the atom are presented for impact energies up to 75 eV. A noticeable property of these cross sections is that they do not usually peak at the forward direction; this is consistent with an experimental observation by Laricchia et al. on He and Ar targets. The importance of also looking at electron capture from inner shells is emphasized and illustrated by the cross section for electron capture from the 3s shell of Ar.     

Positron annihilation in ionic media and an optical model of the positron

It is proposed that positron motion in quasiatomic positron + anion systems formed in anionic media can be described by a potential of the form V_eff(r) = Z_eff/r²-/r, where Z_eff is the effective charge of the nucleus, and n is the effective charge of the anion. It is shown that the positron wave function of the ground state of the quasiatomic positron + anion system in the field of such a potential is _X(r) = l/4·A_nx·r^X·e^–ar. Thus the validity of selecting a test variation positron wave function (r) = l/4·A·r·e^–ar is demonstrated for the potential V_eff = at r = 0 and V_eff = –/r for r > 0 (Gol'danskii-Prokop'ev optical positron model, Fiz. Tverd. Tela,8, 515 (1966)), belonging to the class of functions _X(r). Having the wave function _X(r) and Slater wave functions _ns,p(r) of the electrons, annihilation photon angular distribution (APAD) curves are calculated, together with halfwidths of the APAD curves and positron lifetimes _ns,p.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 52–56, May, 1990.     

Error sources for the analysis of Doppler broadening in positron annihilation spectra

Correction factors for systematic shifts of the second-order moment ² (linearly related toS ^–1) of the Doppler broadened annihilation gamma ray lineshape are presented. They allow to compare ² values obtained under different experimental conditions. The height of the observed peak, the number of keV/channels used and the number of channels over which the Doppler broadening dsstribution should be considered are bounded by the a priori settled relative uncertainty ofgs ².Fellow of the CIC, PciA Bs. As., Argentina     

Positron annihilation characteristics of positronium oxides,sulfides, etc. [X=e+]: Implications for oxide and sulfide crystals

Theoretical Hartree-Fock calculation are summarized for the [X⁼e⁺] systems with X = O, S, Se, and Te which establish the stability of bound states for the system. The two-photon annihilation lifetime, τ, and angular correlation, N(θ), are calculated and positron annihilation from alkaline earth oxides is discussed.     

Millimetre and FIR sources based on optical techniques

Laser methods of generating millimetre and FIR radiation are considered. Generation by optically pumped lasers and frequency mixing in non-linear crystals are promising and are capable of providing large output power and many other useful features. Design aspects and the results of some experiments performed to date are discussed     

分子XH(X=O,S, Se和 Te)中的正电子能级与正电子原子

基于全对称群的Dirac方程, 研究当存在核场时的正电子能级及其与核场的关系, 即计算在分子OH, SH, SeH和 TeH的核场下的正电子能级. 这时正电子的能量约为 e⁺=-1.022 MeV. 对于低能级, 当核场强增大时, 其能量有所升高, 而对较高能级, 核场强增大时, 其能量无明显变化. 正负电子的湮没过程为三光子湮没过程(the three-photon annihilation). 而当生成e⁺- e^-< 关键词： Dirac方程 三光子湮没过程 正态的正电子原子 电荷宇称守衡     

Defects in H implanted GaAs studied by ion-beam and low-energy positron techniques

Ion-beam and low-energy positron-beam techniques have been used to study damage and implanted ion distributions and their annealing behavior in semi-insulating GaAs after the room temperature implantation of 3 × 10¹⁵−1 × 10¹⁷ 60 keV H⁺ cm^-2. The redistribution of the implanted H during annealing was observed to be connected to the migration of implantation-induced defect-complexes. A huge increase in the displaced atom concentration in the region of the H concentration was observed after annealings. A monovacancy overlayer, dissociation of H-vacancy complexes, and formation of stable vacancy-H agglomerates were observed in the different parts of the slowing-down region of the implanted H.     

A study of cold cathodes for the plasma sources of hydrogen ion beams

Two types of cold cathodes (the hollow and magnetron) of a duoplasmatron used for the production of proton beams are comparatively studied.