Annealing behaviour of α-induced defects in tantalum studied by positron annihilation

Abstract

Positron lifetime and Doppler broadening measurements have been carried out to study the isochronal annealing behaviour of defects induced in tantalum irradiated by 32 MeV α-particles. The vacancy migration stage is shifted towards higher temperatures (100°C) and the presence of hydrogen as a common contaminant in this metal is inferred to be the cause behind this observation. Below 300°C, processes such as dissociation of hydrogen-vacancy complexes, vacancy migration leading to the formation of three-dimensional vacancy clusters, and dislocation relaxation forming dislocation or vacancy loops take place. Above this temperature, multiple vacancy-helium complexes are formed. This is soon followed by the release of vacancies from the dissociating vacancy-impurity complexes. Annealing of the defects takes place above 650°C. The effects of isochronal annealing on the positron annihilation parameters demonstrate that helium, trapped inside the metal during the irradiation, plays a significant role in controlling the defect recovery process. The positron lifetime data are further analysed using the three-state trapping model and the results are discussed.