A Study of Fission-Neutron Spectra with High-Energy Activation Detectors. Part II: Fission Spectra

The energy spectra of neutrons from the the thermal-neutron-induced fission of ²³⁵U, ²³³U, and ²³⁹Pu have been compared by means of eight activation detectors that cover the energy range 0.8 to 16 MeV. The detectors are exposed to fission neutrons produced at the center of a 10-cm-diam spherical cavity within a heavywater moderator. Comparison of detector responses for the three spectra yield average energy ratios, ²³⁵U: ²³³U: ²³⁹Pu = (1): (1.021 ± 0.005): (1.039 ± 0.002). Differences between the normalized spectra are most pronounced at high energies as exemplified by the relative ²³⁹Pu: ²³⁵U flux ratios 1.17 for 6 < E < 11 MeV and 1.35 for E > 11 MeV. Spectral indexes for the ²³⁵U fission spectrum, based on measurements with monoenergetic neutrons, show progressively fewer neutrons above 6 MeV than given by the usual Maxwellian description of the fission spectrum, χ_235U(E) = (0.770)E^1/2 exp (−0.775E). At lower energies, the observed spectral indexes involving the ²³⁵U, Np, and ²³⁸U fission detectors are significantly discrepant with those predicted.