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 Mo4,5, 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 Tm (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.