Annealing studies of voids in neutron-irradiated aluminium single crystals by positron annihilation

Abstract

Positron lifetimes and the angular correlation of annihilation radiation in neutron-irradiated high-purity aluminium single crystals have been measured, and companion small angle X-ray scattering experiments made to determine the radius of gyration and volume fraction of radiation-induced voids. The lifetime data are consistent with a model in which positrons are trapped both in voids and in other defects which are tentatively identified as vacancy clusters bound to transmutation-produced silicon. The lifetime in the latter defects increased from 230 to 430 psec as the voids annealed out (around 306°C), suggesting that the clusters grow in size as voids disappear. The lifetime in voids remained constant at (550 ± 30) psec. Angular correlation results are consistent with the lifetime results. A magnetic quenching experiment showed less than 1% positronium formation in the voids. The results support the Hodges and Stott model of a surface state for a positron in a void.