Fast neutron induced fission neutron spectra below the incident energy

Fission neutron spectra from neutron induced fission in 235 U and 239 Pu for energies below that of the neutron inducing fission have been measured. The spectra were obtained for 1.5 MeV and 2.5 MeV incident neutrons. Previous accelerator-based fission neutron spectra measurements have been seriously complicated by time-correlated gamma-rays and scattered neutrons from the fission sample. Three barium fluoride detectors were placed near the sample undergoing induced fission and used to identify fission gamma rays. A coincidence of fission gamma-rays was used to gate a liquid scintillator neutron detector to distinguish fission events from other events. The fission neutron spectral shape and average energy measured in this experiment compare well to both previous measurements and prior theory and also suggest a dependence on incident neutron energy and mass of the fissioning nucleus. An overview of the experiment, a discussion of the results, and the importance of this work to homeland security are given.