Calculation of fission product yields for uranium isotopes by using a semi-empirical model

Abstract

A semi-empirical model for calculating the fission product yields (FPY) of neutron induced fissions of uranium isotopes is developed, where the FPY are assumed to be proportional to the level density of a microcanonical ensemble of a compound nucleus at the fission barrier. The fission height that determines the level density is modeled as a sum of two parts; a symmetric part and an asymmetric part. The origin of the symmetric part can be attributed to the liquid drop model, and that of the asymmetric part to the shell effect in the fission products. Our model has essentially just seven adjustable parameters. They are fitted to the ENDF/B-VII.1 fission yield data of various uranium isotopes for the mass number ranging from 232 to 238 induced by thermal and fast (500 keV) neutrons. Five of the resulting parameters are nearly independent of the mass number of the uranium isotopes. Two parameters which change with the mass number of the uranium isotopes can be expressed as a linear function of the mass number. The FPY calculated from our model are found to be in a good agreement with both the ENDF and experimental data.