The Measurement of Total Delayed-Neutron Yields as a Function of the Energy of the Neutron Inducing Fission

Total delayed-neutron yields from ²³³U, ²³⁵U, ²³⁸U, ²³⁹Pu, and ²⁴²Pu have been measured as a function of the energy of the neutron inducing fission. The measurements extend from 0.1 to 6.5 MeV for ²³³U and ²³⁵U, from 1.6 to 6.5 MeV for ²³⁸U, from 0.1 to 1.8 MeV for ²³⁹Pu, and from 0.7 to 1.3 MeV for ²⁴²Pu. No variation in yield with energy for any of these isotopes was found below 5 MeV. Between 5 and 6.5 MeV the yields for ²³³U, ²³⁵U, and ²³⁸U, were found to decrease by approximately 30%. The absolute yield for ²⁴²Pu was measured for the first time, resulting in a value of 0.016 ± 0.005 delayed neutrons per fission. The average energies of the delayed neutrons from ²³³U, ²³⁵U, ²³⁹Pu, and ²⁴²Pu were estimated to be slightly less than 0.5 MeV for fissioning neutron energies below 1.8 MeV. These delayed-neutron results are generally consistent both with earlier measurements and qualitative theoretical predictions.