Temperature dependence of 4He/3He Exchange in Ni

The release of 4He implanted to saturation with an energy of 15 keV in polycrystalline Ni by subsequent 3He implantation with equal energy has been measured between 295 and 1053 K. The fraction of 4He that can be exchanged is shown to decrease with increasing temperature up to T≲800 K. At T>800 K, however, it increases drastically. The results can be interpreted by assuming He trapping in two subsystems: gas bubbles and He-vacancy complexes. At sufficiently high density of the complexes He atoms can move along interlinked chains of complexes in "relay-race" type of motion in which each individual He-atom moves only one step and an atom from the complex nearest to the surface is released into the vacuum. At T > 900 K all complexes should be thermally dissociated. In this case bubble migration may be the mechanism responsible for He re-emission.