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We have applied the Monte Carlo radiation transport code COG to assess the utility of a proposed explosives detection scheme based on neutron transmission. In this scheme a pulsed neutron beam is generated by an approximately seven MeV deuteron beam incident on a thick Be target. A scintillation detector operating in the current mode measures the neutrons transmitted through the object as a function of time. The flight time of unscattered neutrons from the source to the detector is simply related to the neutron energy. This information along with neutron cross-section excitation functions is used to infer the densities of H, C, N, and O in the volume sampled. The code we have chosen to use enables us to create very detailed and realistic models of the geometrical configuration of the system, the neutron source, and of the detector response. By calculating the signals that will be observed for several configurations and compositions of interrogated objects we can investigate and begin to understand how a system that could actually be fielded will perform. Using this modeling capability, many aspects of the design of a system can be optimized early on with substantial savings in time and cost and with improvements in performance. We will present our signal predictions for simple single element test cases and for explosive compositions. From these studies it is clear that the interpretation of the signals from such an explosives identification system will pose a substantial challenge.
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