Time-Resolved Digital Data Processing Enhancements in Neutron Active Interrogation

Fast neutron active interrogation is a well-established and powerful technique for the detection of concealed contraband and special nuclear material. However, active interrogation signatures of contraband and special nuclear material are often relatively weak, requiring the use of long measurement times and high radiation doses from the interrogating source. We are exploring new digital pulse processing techniques for active neutron interrogation to enhance the detection of concealed contraband and special nuclear material with a focus on prompt gamma-ray signatures for drug and explosive detection and fast neutron detection for special nuclear material. The new detection systems make use of digital data acquisition from inorganic and organic scintillators, the time structure of deuterium-deuterium and deuterium-tritium fast neutron generator interrogation sources to improve signal-to-background ratios, and a reconfigurable water-based fast neutron collimation system to reduce activation backgrounds. Organic scintillators offer several advantages as gamma-ray detectors in this context, including high pileup resistance and cost-effectiveness in scaling to large volumes. However, their gamma-ray response is dominated by Compton scattering which, combined with their relatively poor energy resolution, requires the use of spectral reconstruction techniques to achieve effective gamma-ray spectroscopy in this context. In this work, we made use of a maximum likelihood estimation maximization (MLEM) based spectral reconstruction algorithm to measure several fast neutron active interrogation contraband signatures. The MLEM algorithm inverts the measured light output spectrum into the true incident gamma-ray energy spectrum according to the detector response matrix. Monte Carlo simulation and spectral fitting via genetic algorithm were used to generate the detector response matrices. Validation measurements were conducted using a range of gamma-ray check sources to demonstrate the ability to identify gamma-ray peaks in the reconstructed spectrum and to verify the accuracy of the relative intensity of distinct peaks within the same spectrum. Fast neutron active interrogation measurements were conducted with several contraband simulants, including graphite, sugar, and melamine to cover the range of major drug and explosives