Effect of the preequilibrium process upon fast neutron fission spectra from238U

Abstract

A preequilibrium process for the prefission neutron which is emitted before scission is calculated with the model of Feshbach, Kerman, and Koonin. A forward-peaked angular distribution of the neutron emission from ${}^{238}\mathrm{U}$ bombarded by 14 and 18 MeV neutrons is expressed with the statistical multistep compound process and the one-step direct process. The fission neutron energy spectra are calculated with the model of Madland and Nix, with some modifications by Ohsawa et al. The calculated total neutron emission spectra and their energy-angle distributions (double-differential cross sections) are compared with the experimental data, and a strength of the residual interaction ${V}_{0}$ is estimated. The comparisons of the calculations with the experimental data show that the 14 MeV data are well reproduced but the 18 MeV data are underestimated. Anisotropy is seen in the angle-differential fission spectra, and this is due to an existence of the prefission neutron.