Measurement of neutron active interrogation contraband signatures using organic scintillators

Fast neutron active interrogation is a powerful method for detecting shielded contraband, including explosives, narcotics, and special nuclear material. Pulsed neutron sources can produce intense, time-dependent gamma-ray signatures of contraband, but the detection of those gamma rays is frequently challenging due to high detector pileup. Organic scintillators are fast and cost-effective gamma-ray detectors and thus could be well suited for this application if they can provide adequate gamma-ray spectroscopy. We discuss the use of maximum likelihood estimation maximization based spectral reconstruction techniques for organic scintillators to reconstruct the inelastic scattering and capture gamma-ray spectra produced in neutron active interrogation. Several contraband simulants were irradiated with a pulsed deuterium-tritium neutron generator, and organic liquid scintillators were used to record the emitted radiation signatures. The gamma-ray spectra are time-gated based on the measured neutron pulse time profile, which is shown to be able to improve the signal-to-background ratio for contraband gamma-ray signatures. We isolated several major gamma-ray signatures in the reconstructed spectra that correspond to contraband constituent elements, allowing for the identification of potential contraband objects. We additionally show that the reconstructed gamma-ray spectra can be used to measure sample stoichiometry and discriminate benign from contraband simulant objects.