The detection of delayed γ-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 U238, Pb, Fe and Be targets using a pulsed bremsstrahlung beam for several endpoint energies between 8 and 20MeV. In the 33ms 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 ∼12ms 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.