Positron annihilation in cadmium and zinc

Abstract

A study of Doppler broadened annihilation quanta arising from the interaction of thermalised positrons with the electrons of metallic lattices has been made.The study embraced single crystal specimens of cadmium and zinc and a deformed polycrystalline specimen of cadmium. A high resolution intrinsic germanium gamme-ray detector was employed.

The single crystal studies were carried out for the [0001] and [1010] directions over the temperature range from 4.2K to 580K in the case of cadmium and from 4.2K to 600x in the case of zinc. Both these studies reveal anisotropies in the temperature dependencies of the annihilation characteristics derived from the gamma-ray lineshapes.The origin of these anisotropies is discussed with reference to the coefficients of thermal expansion in the relevant directions.Monovacancy formation parameters are deduced for both orientations of each specimen by means of a two state trapping model and by means of a self-trapping model.In the case of cadmium, a comparison is made of the observed anisotropies arising from two distinct specimens.

The study of the deformed specimen of polycrystalline cadmium here presented embraced the temperature range 4.2K to 580K and included a room temperature deformation followed by annealing and a subsequent deformation at liquid nitrogen temperatures. As the annealing subsequent to this latter deformation progressed, observations were made at low temperatures in order to investigate the possible temperature dependence of the positron trapping rate. A convolution procedure based on the superposition of a Gaussian function and an inverted parabola was employed to describe the annihilation lineshapes arising from this specimen in an effort to investigate, by means of the associated zero point motion,the nature of the deformation-induced traps.The results derived from this analysis are discussed.