Prompt fission neutron energy spectra induced by fast neutrons

Abstract

Prompt fission neutron energy spectra for 235U and 239Pu have been measured for fission neutron energies greater than the energy of the incident neutrons inducing fission. The measurements were undertaken to investigate the shape dependence of the fission neutron spectra upon both the incident neutron energy and the mass of the nucleus undergoing fission. Measurements were made for both nuclides at incident neutron energies of 0.50, 1.50, 2.50 and 3.50 MeV. The data are presented either as relative yields or as ratios of measured spectra to that of 235U at 0.50 MeV. Incident neutrons were produced by the 7Li(p,n)7Be reaction using a pulsed, bunched proton beam from the 5.5 MV Vand de Graaff accelerator at the University of Massachusetts Lowell Radiation Laboratory. Fission neutrons were detected by a thin liquid scintillator with good time resolution capabilities. Neutron energies were determined by time-of-flight techniques with pulse-shape discrimination to reduce gamma-ray background levels. The measurements are compared to calculations using the Los Alamos model of Madland and Nix to test the predictive capabilities of the model. The calculations were performed prior to the measurements. The data are fit by the Watt equation to determine the mean energy of the spectra and facilitate comparison of the results to previous measurements.