Abstract
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 ...
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