Delayed neutron group parameters from photofission

The emission of delayed neutrons resulting from fission reactions has been rigorously examined for applications ranging from reactor research to national security and nonproliferation applications [16, 17, 40]. Previous work has shown that the time dependence of delayed neutron emission, which occurs following beta-decay of fission fragments, can be accurately characterized by six delayed neutron groups, each with a fractional group yield, a, and decay constant, lambda[15, 21, 40]. Due to the motivating factors of previous research, there exists substantial data, for delayed neutrons resulting from neutron induced fission. In contrast, there is little data available for delayed neutrons from photofission reactions and that data is often contradictory. The relative lack of sufficient photofission data has led researchers to approximate models based on neutron induced fission data [10, 30]. There are, however, significant differences in the delayed neutron groups from neutron induced fission compared to photon induced fission.; This dissertation presents a theoretical basis and the experimental procedure for the determination of delayed neutron group parameters resulting from photofission for three fissionable materials (238U, 232Th, and 239Pu). The fission reactions were induced by a pulsed bremsstrahlung beam produced by a 25 MeV electron LINAC. The resulting delayed neutron emission was recorded and characterized based on a six group scheme. Group parameters were determined for each of the fissionable targets with a 15 MeV endpoint energy bremsstrahlung beam and demonstrates the significant discrepancies in individual isotopes' group parameters. Furthermore, the energy dependence of group parameters was examined for 238U, which was irradiated with bremsstrahlung endpoint energies of 8, 12, 15, and 18 MeV. Significant variations in group parameters was observed, however, the overall decay rates appear consistent.