Medium energy proton induced fission in uranium-238 and bismuth-209

Medium energy proton induced reactions, including fission, in 2380 and 209Bi have been studied. The inelastic interactions of protons with heavy nuclei have been described by a model in which the reaction proceeds in two steps. The first step is the fast cascade where a series of individual particle-particle collision occurs within the nucleus leaving a highly excited cascade residual nucleus assumed to be in thermal equilibrium. In the second step, the excited nucleus disposes of its energy by evaporation of neutrons and light charged particles. The cascade stage is calculated by the Monte Carlo method of computer simulations using the medium energy cascade calculation program MECC-7 originally developed by Bertini at, Oak Ridge National Laboratory. The effects on the cascade stage of the effective mass of the nucleon'in nuclear matter have been included. The evaporation phase of the reaction has been calculated by the statistical theory of evaporation. Only neutrons, protons, deuterons, tritons, helium-3, and alphas are evaporated, and fission is assumed to compete with particle evaporation at all stages. The fission level width is taken from the works of Bohr and Wheeler. The effect of shell structure on the level density parameters has been discussed and a new empirical-formula for the particle emission level density parameter is obtained. Symmetric fission is assumed and the fission probabilities are obtained from the Mlnte Carlo calculations by means of statistical weighting factors. The fission cross sections, the cross sections for the spallation residues, and the number of nucleons lost by the target before and after fission are calculated and compared with experiment for 155 MeV and 1 GeV protons incident on 238U and 209Bi• Some of the model's predictions are in reasonable agreement with the experiment while others are not. However, two possible improvements are suggested. First, the reflection of nucleons at the nuclear surface may be included in the cascade stage and second, the pre-equilibrium emission of nucleons may be incorporated after the cascade stage and before evaporation. We conclude that our two step model provides a reasonable description of the medium energy proton induced fission.U of I OnlyThesi