Positron annihilation in metals

Abstract

The Doppler broadened annihilation gamma ray line Shape (511-kev) resulting from the interaction of thermalized positrons with a metallic lattice has been measured for a number of metals---Cadmium, Indium, Tin, Aluminium and Lead; either as single crystals, or in polycrystalline form between 4.2° K and their melting temperature by means of a high resolution germanium detector.

Application of the trapping model to parameters derived from changes in the annihilation line profiles provides monovacancy formation energies for thermally generated vacancies.

Other phenomena and their possible effects---e.g. thermal expansion, meta-stable self trapping of positrons, positron zero point motion, di-vacancies-are explored in order to achieve a better understanding of the overall temperature-or defect concentration-dependance of the line profile and thus a better estimate for the vacancy formation energies of the metals studied.

A positron can be demonstrated to be an extremely sensitive probe of the environment in which it annihilates. By using the Convolution technique the probability of positron annihilation with Core and Conduction electrons can be calculated throughout the temperature range-or defect concentration in question.

Inclusion of zero point motion of trapped positrons in the analysis of these line shape profiles lends a possibility to estimating the trapping probability in a more direct fashion for some metals, and therefore an alternative method of testing some of the assumptions and conclusions of the conventional way in which the trapping model is currently, and widely, applied.