The effect of dense and dilute collision cascades on helium bubbles in metals

The evolution of small helium bubbles in thin gold and aluminum foils has been followed during irradiation with high energy heavy ions (400 keV Ar+ and 200 keV Xe+) using in-situ transmission electron microscopy. Substrate materials were chosen in order that the heavy ion irradiation would produce dilute (aluminum) and dense (gold) collision cascades. Significant differences in bubble behaviour are observed in the two cases, the major effect of dilute cascades being bubble shrinkage due to helium resolution by direct displacement of the gas out of the bubbles. Effects observed for dense cascades, however, include the disappearance and Brownian motion of bubbles under irradiation. The paper will present recent experimental results as well as simple numerical modelling of the observed behaviour. In particular, our interpretation of the dense cascade effects is consistent with the type of displacement and thermal spike processes that recent molecular dynamics simulations have indicated may be important in radiation damage in metals.