The nucleation of He bubbles during the isochronal annealing of He-implanted material was studied by using positron annihilation methods. It was found that the evolution of implantation-induced radiation damage before the nucleation stage involved processes such as the dissociation of He from dislocation loops, the coarsening of vacancy clusters, He-vacancy interactions, and loop annealing. The He bubble nucleation stage was characterized by a marked decrease in the positron lifetime at temperatures of between 750 and 850C. Stabilization of the He bubbles in the post-nucleation stage was attributed to the effects of impurity segregation at the bubble surfaces. Positron lifetime data in the He bubble regime were analyzed in terms of the positron surface-state model. This revealed the presence of over-pressurized bubbles. The properties of the present He bubbles were compared with those of previously reported athermal bubbles. A lower limit of 4.7eV was deduced for the He-vacancy binding energy in Nb. The He-vacancy binding appeared to be stronger than H-vacancy binding in this metal.

V.S.Subrahmanyam, P.M.G.Nambissan, P.Sen: Radiation Effects and Defects in Solids, 1994, 132[2], 169-78