Helium in palladium studied by positron annihilation

Positron lifetime and Doppler broadened lineshape measurements in Pd, helium implanted to concentrations of 100, 150 and 250 appm, are reported. From the post-implantation annealing measurements, the onset of bubble growth is identified to be at 770K. From the variation of bubble radius and bubble density, as deduced from the analysis of lifetime parameters, two growth regimes with different activation energies for the three Pd samples with different He concentrations are identified and discussed