ACCUMULATION AND ANNIHILATION PROCESSES OF CASCADES IN METALLIC AND NONMETALLIC CRYSTALS UNDER IRRADIATION WITH IONS AND OR ELECTRONS

In situ observation has been performed under dual-beam irradiation with heavy ions and fast electrons in the HVEM-accelerator facility at Kyushu University. The objectives of the present study are to understand the structure and the accumulation process of cascade damage and the synergistic effect of electronic excitation and/or free point defects on the behavior of cascade damage in metallic and nonmetallic crystals, such as Cu, graphite, SiC, Si, Ge, Ge-20at%Si, MgO, alpha-Al2O3, MgAl2O4 and others. The HVEM-accelerator facility has been confirmed to be extremely useful for observing the behavior of cascade damage: (1) Cascade damage is directly produced without the help from other collision cascades in Cu. (2) Most collisions themselves in Si, Ge and Ge-20at%Si induce invisible structural change through electron microscopy, but they are converted into visible ones by the impact from other collision cascades. (3) No visible cascade damage is formed in covalent crystals with light masses such as graphite and SiC and in ionic crystals such as MgO, alpha-Al2O3 and MgAl2O4. (4) Free interstitials suppress the evolution of visible cascade damage and assist the nucleation and the growth of interstitial loops around collision cascades in metallic and nonmetallic crystals.