Ratio of the Prompt Fission Neutron Spectrum of ²³⁹Pu to That of ²³⁵U

The prompt fission neutron spectrum resulting from ²³⁹Pu fission induced by ∼0.55-MeV incident neutrons is measured from 1.0 to 10.0 MeV relative to that of ²³⁵U fission induced by the same incident energy neutrons. The measurements employ the time-of-flight technique. Energy-dependent ratios of the two spectra are deduced from the measured values. The experimentally derived ratio results are compared with those calculated from ENDF/B-V (revision 2) and with results of recent microscopic measurements. Using the ENDF/B-V ²³⁵U Watt parameters for the ²³⁵U spectrum, the experimental measurements imply a ratio of average fission spectrum energies of ²³⁹Pu/²³⁵U = 1.045 ± 0.003, compared to the value 1.046 calculated from ENDF/B-V (revision 2).