Optimising the neutron environment of Radiation Portal Monitors

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.

This work will be of significant benefit to National Security through the implementation of enhanced performance of existing and future Radiation Portal Monitors whose primary purpose is to detect the presence of radioactive material and so prevent the free transportation of this throughout the world.b This has become a more pressing problem over the past decade due to the fall of thee Soviet Union and the increase in Global Terrorism.
Despite vast numbers of neutron sensitive RPMs being designed and installed, there are very few studies in the open literature of their response to the natural neutron environment. Those that have been published do not appear to be informed by any body of previous research and it thought probable that there is little additional classified research. The most detailed study available, by Frank*, is marred by incorrect cosmic ray neutron production mechanisms and spectra, uses completely unrealistic detector models and contains no attempt at experimental verification.
The collaborators in this proposal from the Universities of Glasgow and Sheffield as well as the Culham Centre for Fusion Energy have prior experience in the modelling and build of instrumentation for neutron generation, detection and experimental design that can contribute to all of the aspects of the work contained in this proposal. This project will build on that experience and lead to improved performance and throughput of current and future RPM installations.

* M.I. Frank. Improving neutron-based detection of fissile materials in the presence of cosmogenic neutron background.
PhD thesis, University of California, Berkeley, 2002. UMI Number 3063368