Defeating IEDs, SNM and contraband secreting via long range gamma-ray imaging of neutron interrogated materials

A single element of a gamma-ray imaging system, based on fast inorganic scintillation materials, is discussed in the context of the ranged sensing of high explosives and other organic materials, as enabled by an interrogating pulsed 14 MeV neutron source. The mechanical design of a single element of the imager, based on BaF₂ bar detectors bounded on two ends by photomultiplier tubes (PMTs), is described. The photonic flux, the scintillation decay, and the PMT response are modeled analytically and numerically, yielding a response which indicates that the boundaries of the empirical bars are governed by 95 % specular and 5 % diffuse reflection. The position of interaction and deposited gamma-ray energy within the bar are discerned from the fast pulse responses via the comparison of the data pulses with calibrated template pulses, yielding user-selectable fine position resolution, with an associated cost in counting efficiency.