Effect of internal pressure on helium bubble growth in palladium

A study of helium bubble growth by positron lifetime measurements made on helium-implanted palladium (100 at. p.p.m. He) is reported. The temperature of formation of stable bubbles which retain helium in post-implantation annealing is identified to be 750±20 K). The average bubble radius, bubble concentration and helium pressure in the bubble are obtained from an analysis of experimental data. The variation in the deduced bubble parameters in the annealing temperature range 973-1473 K is compared with that calculated on the basis of a surface diffusion controlled migration and coalescence model, allowing for pressure effects as suggested by Mikhlin in 1979. The comparison is found to be satisfactory. The observed results are explained as due to suppression of bubble mobility at high internal pressures and subsequent enhancement of the mobility on pressure relaxation. (Author)