The energy deposition of a heavy ion in a latent track gives rise to a central partially melting core zone surrounded by a larger region where some physico-chemical modifications take place. When these latent tracks start to overlap some interference effect occurs. A previous irradiated zone of one latent track can be eliminated by the core zone of others. In the present, work a Monte Carlo simulation program that permits us to determine the track dimensions is described. The core and chemical radii are obtained from fitting the FTIR absorption as a function of fluence. This new method was applied specifically on ultra-high molecular weight polyethylene (UHMWPE) irradiated with a 72 MeV S beam. The inner core radius value obtained was 5 ± 1 nm.