Isochronally annealed samples of He-implanted material were studied using positron annihilation techniques. It was found that defect evolution before the bubble nucleation stage involved the coarsening of vacancy clusters to form voids, and the stabilization of He-filled voids to up to about 900C. The He which decorated loops became separated at temperatures above 800C, and the defects annealed out upon further annealing to 1000C. Most of the He-free voids annealed out but the remainder, mainly He-decorated voids, transformed into densely-filled He bubbles at temperatures above 950C.

V.S.Subrahmanyam, P.M.G.Nambissan, P.Sen: Journal of the Physics and Chemistry of Solids, 1996, 57[3], 319-23