Delayed Neutrons from Low Energy Photofission

The time behavior of delayed neutrons from the photofission of ²³⁸U and ²³⁵U was measured for bremsstrahlung endpoint energies of 8- and 10-MeV. The data were analyzed to determine the relative abundances (β_i) of the delayed-neutron groups assuming the observed time distribution can be adequately described by six groups. A comparison of the ²³⁸U results at 8- and 10-MeV, and other results at 15-MeV show no strong dependence of the β_i on endpoint energy. In the case of ²³⁵U, no large differences were observed between the β_i at 8- and 10-MeV, however there are marked variations for several groups at 15 MeV. Possible causes for the observed differences in the β_i with endpoint energy are discussed. No evidence was found for the existence of a delayed-neutron group with a half-life in the 10 to 100 msec region