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 microscop y. Substrate materials were chosen in order that the heavy ion irradia tion would produce dilute (aluminum) and dense (gold) collision cascad es. Significant differences in bubble behaviour are observed in the tw o cases, the major effect of dilute cascades being bubble shrinkage du e to helium resolution by direct displacement of the gas out of the bu bbles. Effects observed for dense cascades, however, include the disap pearance and Brownian motion of bubbles under irradiation. The paper w ill present recent experimental results as well as simple numerical mo delling of the observed behaviour. In particular, our interpretation o f the dense cascade effects is consistent with the type of displacemen t and thermal spike processes that recent molecular dynamics simulatio ns have indicated may be important in radiation damage in metals.