Positron interactions in condensed matter

Abstract

Positron annihilation has been studied in a number of solids, and at some condensed monolayer surfaces. The bulk specimens, which included cadmium, tin, selenium, and graphite, yielded information on lattice defects and phase transitions. Two dimensional layers of condensed gases, including argon, nitrogen, oxygen and helium result in positronium formation suggesting the existence of positron surface traps. The Doppler broadening method has been applied to these studies. The 511-Kev Gamma-rays resulting from the annihilation of positrons with electrons was detected by a Germanium detector with high resolution. The cadmium and tin specimens were plastically deformed at 77K and measurements with increasing temperature provided information on phase transitions, annealing processes and on the nature of the defects concerned. The application of the positron trapping model provides values of the vacancy formation energy (enthalpy) and of the concentration of monovacancies and divacancies. The energy spectra were analysed, with Gaussian and Parabolic components convoluted with instrumental resolution function, to indicate the proportion of annihilation of positrons with core and conduction electrons. An important result of the work on tin was the observation of the phase transition of white-tin to gray-tin at 240K. Positron trapping at graphite surfaces, and the formation of positronium has been observed by introducing a parameter R, related to the positronium fraction in this work. The growth of monolayers of gases condensed on graphite has been observed in the changing of the total area of the annihilation line shape spectrum, and also in the changing positronium fraction parameter. Estimated values of adsorption energies are discussed.