CALCULATION OF MASS YIELD DISTRIBUTIONS IN FISSION INDUCED BY HIGH ENERGY PROTONS

An analytical expression is obtained for the mass-yield distribution in fission induced by high-energy protons, using a two-step model. In the first step, the incident proton penetrates the target nucleus, knocking out some of its constituting nucleons and leaving a residual nucleus in an excited state. In the second step, the residual nucleus fissions. The mass-yield distribution in the fission of this residual nucleus is described within the hypothesis of two independent fission modes, with parameters extracted from the analysis of low-energy fission. Comparison with experimental data of the mass yield in fission of 238 U induced by 190 MeV protons shows that the agreement with experiment can be achieved only if one assumes that a considerable amount of excitation energy is removed from the residual nucleus by nucleon evaporation prior to fission.