Fractal geometry of collision cascades

The fractal nature of self-ion collision cascades is first described using an inverse power potential and then by the more realistic potential of Biersack--Ziegler. Based on the model of Cheng et al. and TRIM Monte Carlo simulations, the average cascade fractal dimension is a function of both atomic mass and initial energy. The instantaneous fractal dimension increases as the cascade evolves. A critical energy E/sub c/ for producing a dense subcascade is derived and it is shown that E/sub c/ agrees well with the onset energy for constant damage efficiency