Positron annihilation study of post-irradiation annealing of defects in α-irradiated molybdenum

Abstract

Measurements of positron lifetime and Doppler broadening have been used to study the nature of radiation damage and defect annealing in α-irradiated molybdenum. Vacancies, dislocation/vacancy loops and microvoids were detected in as-irradiated samples. Positron trapping at these defects was predicted to be near saturation by the model proposed for the trapping process. The behaviour on annealing showed vacancy migration and formation of vacancy clusters in the region 150–500°C. Above 650°C microvoid coarsening to form voids was observed in two stages. Dislocation/vacancy loops started to anneal out at ∼650°C, but significant recovery of them was observed above 950°C. Various aspects regarding the effect of helium on defect annealing, positronium (Ps) formation in metal voids and its decay scheme are discussed along with a brief review of previous investigations. The results indicate that Ps decays primarily from a singlet state in clean voids, whereas a triplet state decay is possible in the presence of gas atoms bound to the surfaces of voids.