Helium gas-bubble superlattice in copper and nickel

Abstract

The microstructural changes occurring in metals during ion bombardment with helium or hydrogen isotopes have important implications in several areas of technology¹ such as in the deterioration of the accelerator targets suggested for use as intense neutron sources in cancer therapy²; and in the plasmawall interactions which could strongly influence the operation of future thermonuclear fusion reactors (see ref. 3). Research has shown that helium ion irradiation of Mo^4,5, Cu, Ni and stainless steel¹ to high doses can result in a superlattice of small (∼2 nm diameter) helium bubbles in very high concentration (∼10²⁵ m⁻³) even at temperatures up to 0.35 T_m (refs 1,4). The mechanisms involved in the bubble growth⁶ and in the superlattice formation have been discussed but the role of displacement damage collisions on bubble growth has received little study. In the present work, specimens containing a superlattice of helium bubbles formed during prior helium ion irradiation have been irradiated with 1 MeV electrons to create a specific collision regime. This was found to induce bubble growth and changes in superlattice parameters, in both copper and nickel.