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.