Based on the fractal nature of collisions, the fragmentation of displacement cascades is investigated. Introducing
the caliper diameter to take into account large displacements of atoms in a collision cascade, it is
possible to describe this fragmentation in terms of a threshold energy Ec. From the knowledge of Ec, the
number and the volume fraction of subcascades produced is then derived. This last information allows us to
classify the efficiency of impinging particles to induce structural changes of solids.