Optimising the neutron environment of Radiation Portal Monitors

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Radiation portal monitors (RPMs) are currently installed at traffic choke points at ports and airports with little regard to their operating environment. Simple analyses suggest that the signal-to-noise ratio of the neutron components of these systems can be improved by judicious shielding, collimation and modifications of the environment, and that these can be done while improving throughput. This proposal will provide a detailed analysis, quantification and costing of these improvements. In addition to the threat neutron flux which the RPMs are intended to detect, there is a background flux of neutrons, predominantly the result of cosmic ray induced reactions in the atmosphere, in the ground below the detectors and also in adjacent infrastructure, in vehicles and their cargos. This background flux has quite a different directional distribution to the expected threat flux, so by shielding and collimation, it is possible to significantly reduce the background counts, improving the overall signal-to-noise ratio. This will result directly in a reduction of the minimum detectable amounts of threat material. Alternatively, the improved statistics can be used to make the same diagnosis in reduced time.