ANOMALOUS HELIUM-BUBBLE GROWTH IN PALLADIUM

Helium-bubble growth in alpha-implanted palladium has been investigate d by positron-lifetime measurements as a function of isothermal anneal ing time, t. The bubble radius shows first an expected t1/6 variation and then an anomalous time dependence faster than t1. It is shown that a model based on surface-diffusion-controlled migration and coalescen ce, taking into account the dependence of bubble diffusivity on the ti me-varying helium density in the bubble and ledge nucleation, explains the observed behavior.