Helium implanted vanadium studied by the positron annihilation technique

Helium-implanted vanadium, subjected to isochronal annealing, treatment, has been studied using the positron annihilation technique. The helium decorating vacancies and loops become dissociated above 600°C and these defects anneal out upon further annealing up to 950°C. Between 350 and 550°C, a fraction of the voids anneal out and the rest, mainly helium-decorated voids transform into densely filled helium bubbles above 700°C. The properties of these helium bubbles have been studied quantitatively from an analysis based on the positron surface state model, which indicated the presence of overpressurized helium bubbles. In the post-nucleation stage the bubble radius increases and the bubble concentration decreases indicating the bubble growth. Considerable pressure relaxation occurs upon annealing in the post-nucleation stage.