Feasibility of prompt correlated counting from photon interrogation of concealed nuclear materials

We describe new Monte Carlo codes for the simulation of coincident counts, both real and accidental, originating from photonuclear and photoatomic events in nuclear, and non-nuclear, materials. The codes perform fully analog simulation of all secondary neutrons, and photons, emitted by photonuclear, and photoatomic, events. Separate algorithms are then used to simulate the accidental coincidences. These new codes have been applied to a realistic measurement scenario. Two possible techniques for minimizing the detection of accidental coincidences are then analyzed: pulse-shape discrimination and detector lead shielding. The results showed that the use of lead-shielded liquid scintillation detectors with pulse-shape discrimination is a promising technique for the detection of actinides in shielded configurations by minimizing accidental counts. The data acquisition time for this approach would be on the order of 1 min or less for the measurement scenario considered here.