Positron Facilities in Europe

A complementary study of vacancy defects in Si substrates by using scanning positron microscope (SPM) and electron beam induced current (EBIC) method were demonstrated for the same samples and in the same chamber. Both the S parameter and EBIC contrast were found to be enhanced in the regions containing vacancy defects introduced by ion implantation. That is, the SPM provides a criterion if the spatially resolved carrier recombination centres by the EBIC method are originating from vacancy defects or not.

The study of' the angular correlation of gamma rays from positrons annihilating with electrons in metals can be used as a tool in the investigation of the momentum distribution of conduction electrons in a metal. This method was used in the study of three metals, aluminum, bismuth, and ytterbium, at pressures up to. 100 kilobars.

Positron and positronium annihilation in silica-based thin films studied by a pulsed positron beam

Low-energy nuclear reactions are known to be extremely dependent on the local crystal structure and crystal defects of the deuterated samples. This has a strong influence on both hydrogen diffusion and the effective electron mass. The latter determines the strength of the local electron-screening effect and can change the deuteron–deuteron reaction rates at very low energies by many orders of magnitude. In the present study, zirconium samples were exposed to various conditions and energies of deuteron beams using the unique accelerator system with ultra-high vacuum, installed in the eLBRUS laboratory at the University of Szczecin. Irradiated and virgin samples were investigated by means of the X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS). While the first method delivers information about changes of crystal lattice parameters and possible production of hydrides accompanying the formation of dislocations that are produced during irradiation of the samples, the second one can determine the depth distribution of crystal defects, being especially sensitive to vacancies. The studied Zr samples were also implanted by carbon and oxygen ions in order to simulate the real situation taking place in nuclear reaction experiments and to investigate their influence on the kinetic of produced vacancies. The observed enhancement of the electron-screening effect in the deuteron fusion reaction at very low energies could be explained by formation of a high number of vacancies during the deuteron irradiation of samples. Possible carbon and oxygen impurities can affect this process in various ways by changing the depth distribution of vacancies and their diffusion, but they play only a minor role in the strength of the electron-screening effect.

This article reviews some recent materials analysis results using high‐energy positron beams at Lawrence Livermore National Laboratory. We are combining positron lifetime and orbital electron momentum spectroscopic methods to provide electron number densities and electron momentum distributions around positron annihilation sites. Topics covered include the correlation of positron annihilation characteristics with structural and mechanical properties of bulk metallic glasses and compositional studies of embrittling features in nuclear reactor pressure vessel steel.

Combined effect of MgO sintering additive and stoichiometry deviation on YAG crystal lattice defects

The large critical current in type-II superconductors is due to the pinning of the flux line lattice (FLL) by the crystal lattice defects. The interaction between crystal lattice defects and the FLL has been directly demonstrated by decorating the FLL in a thin-foil superconductor with ferromagnetic particles and then observing the FLL arrangement together with the lattice defects in the foil with transmission electron microscopy [1]. When a transport current is passed through the superconductor, the FLL experiences a body force F = B × J, where B is the externally applied magnetic induction and J is the current density. Under the influence of this force, the FLL would move, and a voltage would be generated if it were not for the pinning effect of the crystal lattice defects. Measurements of the critical current density then provide information on the maximum pinning-force density.

It is shown that a relativistic electron-positron plasma formed by the coalescence of two counterstreaming intense relativistic electron and positron beams can collapse into a very-small-diameter filament. The collapse is accompanied by the emission of intense coherent $\ensuremath{\gamma}$ radiation. The radius of the collapsed state is determined by the quantum-mechanical uncertainty principle. At its maximum contraction the electron-positron plasma can approach nuclear densities. The predicted effect may have many interesting applications, some of which are briefly mentioned. The generation of the intense beams seems to be possible with existing techniques.

The positron annihilation in a metal-oxide semiconductor was studied by using a pulsed monoenergetic positron beam. Lifetime spectra of positrons were measured as a function of incident positron energy for a polycrystalline Si(100 nm)/SiO2(400 nm)/Si specimen. Applying a gate voltage between the polycrystalline Si film and the Si substrate, positrons implanted into the specimen were accumulated at the SiO2/Si interface. From the measurements, it was found that the annihilation probability of ortho-positronium (ortho-Ps) drastically decreased at the SiO2/Si interface. The observed inhibition of the Ps formation was attributed to an interaction between positrons and defects at the SiO2/Si interface.

Surface and bulk investigations at the high intensity positron beam facility NEPOMUC

Construction of the Helsinki University of Technology (HUT) pulsed positron beam

Performance analysis of the intense slow-positron beam at the NC State University PULSTAR reactor

Positron and Positronium Annihilation in Low-Dielectric-Constant Films Studied by a Pulsed Positron Beam

The effect of environment on the production of thermal lattice defects in crystals

The density of crystal lattice defects in Si: Er layers grown through sublimation molecular-beam epitaxy at temperatures ranging from 520 to 580°C is investigated by a metallographic method, and the Hall mobility of electrons in these layers is determined. It is found that the introduction of erbium at a concentration of up to ∼5 × 1018 cm−3 into silicon layers is not accompanied by an increase in the density of crystal lattice defects but leads to a considerable decrease in the electron mobility.