Measured near-equilibrium delayed neutron spectra produced by fast-neutron-induced fission of 232Th, 233U, 235U, 238U, and 239Pu

Measurements of the near-equilibrium energy spectra of the delayed neutrons associated with fast-neutron-induced fissions of 232Th, 233U, 235U, 238U, and 239Pu are reported over the energy range from 35 to 1500 keV. The fission source spectrum, produced from the 9Be(p,n)9B reaction using 10-MeV protons, approximated a prompt fission spectrum. Over 20% of the delayed neutrons in the measured spectra for each sample appeared below 100 keV. Two large peaks at 41 and 74 keV were observed in the spectra from each nuclide. The measured spectra compare favorably with the data reported by others at energies above 200 keV. However, at energies below this value, our results show a significant increase in the number of delayed neutrons when compared with previously published results