Positron annihilation in the alkali halides

Abstract

The lifetime spectra of positrons annihilating in NaBr, LiBr, RbCl, KCl, Rbl, LiF, KBr, KI and NaCl as a function of pressures ranging from 1 to 55 kilobars are reported. Measurements of the annihilation time and energy spectra are reported in KCl as a function of temperatures ranging from 3 0°C to beyond the melting point. Similar measurements were also taken in KCl colored by exposure to X-rays. The equipment used included a Bridgeman anvil assembly for the generation of the high pressures, a fastslow coincidence system for the lifetime spectra and a Ge(Li) detector to measure the doppler broadening of the.511 Mev. annihilation line. An attempt is made to correlate the results of the high pressure work on NaCl with the behavior of vacancies within the context of the A-center model. From this work an escape rate, -. 2 nsec.-1 for positrons from cation vacancies is postulated, leading to an A-center binding energy \J~. 4 2 eV. The results of the high temperature work on KCl indicate anomalous changes in and near 675°c whereas ^ remains essentially constant up to melting. Changes observed with the solid state detector were insignificant, except at the melting point. Finally, the results of the work on colored KC1 show that r2 diminishes rapidly with exposure to X-rays, an effect which eventually saturates.