Criterion of subcascade formation in metals from atomic collision calculation

Atomic collision calculation of displacement cascades is performed, based on the binary collision approximation with systematically varied materials and energy of incident atom, to understand the break up process of a large cascade into subcascades. The mean distance between collisions to transfer a large kinetic energy to a target atom, and the size of the vacancy rich region produced by the target atom are compared as a function of the transferred energy to a target atom. The relation between these two parameters are proposed as a criterion of subcascade formation. When the criterion is applied to several metals, a tendency for larger subcascade energy for heavier atomic species is obtained. The origin of the formation of subcascade and factors which determine the subcascade parameters, such as subcascade energy and subcascade zone size are discussed.