The spatial distributions of atomic displacements at the end of the collisional phase of cascade damage were calculated by using a computer simulation code which was based upon the binary collision approximation. The densities of atomic displacements were evaluated for the highly dense regions of cascades in several pure metals and were compared with measured cascade collapse probabilities that were based upon transmission electron microscopic observations of thin metal foils that had been irradiated with low doses of ions at room temperature. It was found that there existed minimum or critical values, of the atomic displacement density, above which the highly dense regions collapsed into vacancy clusters that were visible during transmission electron microscopy. These critical densities were usually independent of the cascade energy for a given metal. A material dependence of the critical density was attributed to differences in the vacancy mobilities at the melting points of the target materials. These critical densities, as deduced from ion-irradiation experiments and binary collision approximation simulations, were used to estimate cascade collapse probabilities in metals that were irradiated by fusion neutrons.

K.Morishita, H.L.Heinisch, S.Ishino, N.Sekimura: Nuclear Instruments and Methods in Physics Research B, 1995, 102[1-4], 67-71