Bombardment with 15 to 350keV 209Bi ions, and transmission electron microscopy, were used to obtain detailed information on collision cascades in these materials after low-fluence (1011 to 1012/cm2) irradiation at temperatures of between 35 and 40K. The object was to determine the nature of the damage that was produced by individual collision cascades, as well to study the early stages of cascade overlap. It was found that, at high values of average deposited-energy density in the cascade (which corresponded to low-energy heavy-ion implantation), the visible damage that was produced by a cascade consisted mainly of a single damage region. With decreasing deposited-energy density (increasing ion energy), there was an increasing tendency for multiple damage regions (sub-cascades) to form within the main cascade. The visible damage regions appeared to be amorphous. It was also found that, as the Bi ion energy was increased (decreased deposited-energy density), the fraction of the theoretical collision cascade volume that was occupied by visible damage regions within a cascade rapidly decreased. The crystalline-to-amorphous transformations in samples which were bombarded with heavy ions appeared to be the result of direct amorphization within the collision cascades and of a critical defect concentration being reached in the cascade overlap regions; thus producing additional amorphous regions.

L.M.Howe, D.Phillips, H.H.Plattner, J.D.Bonnett: Nuclear Instruments and Methods in Physics Research B, 1995, 102[1-4], 77-80