The detection of delayed -rays between intense bremsstrahlung pulses for discriminating fissionable from non-fissionable materials

The detection and identification of delayed fission γ -rays emitted between intense bremsstrahlung pulses was investigated as a method to discriminate between fissionable and non-fissionable materials. Photofission and other photonuclear reactions were induced in U 238 , Pb, Fe and Be targets using a pulsed bremsstrahlung beam for several endpoint energies between 8 and 20 MeV . In the 33 ms between irradiating pulses, correlated photon energy and time spectra were collected using a high-purity germanium detector. When non-fissionable materials were irradiated, the high-energy γ -ray yield decreased a couple orders of magnitude within the first ∼ 12 ms after the pulse. In contrast, the high-energy γ -rays emitted from the decay of fission fragments were nearly constant throughout the period between bremsstrahlung pulses. Hence, a basic energy and time discrimination provides a unique high-energy delayed γ -ray signature for fissionable material detection.