Optimising the Neutron Environment of Radiation Portal Monitors: a Computational Optimisation Study

• Mark R. Gilbert
• Zamir Ghani
• Lee W. Packer

The CLASP scheme proposal ‘Optimising the neutron environment of Radiation Portal Monitors (RPM)’ [1], which is a collaboration between Glasgow University (Val O’Shea), Sheffield University (John McMillan) and the UK Atomic Energy Authority was awarded funding through STFC in 2012. The objective of the work reported here is to provide optimised information regarding the design of RPMs with a focus on computational simulations using radiation transport modelling with modern nuclear data libraries. The aim was to demonstrate that enhancements to a generic RPM model design and surrounding environment can enable a significant reduction in the measured neutron background, and hence lower their detection limit to a ‘threat’ neutron source. In preliminary work, two environmental neutron-field models were developed to evaluate the detection response for a range of RPM modifications and ground or ‘roadway’ materials. The basic modifications implemented in the models and subsequent optimisation studies demonstrated that, for the modelling assumptions, materials and geometries implemented, the environmental neutron background can be reduced and, with some shielding/collimation modifications to the RPM, neutron alarm levels can be reduced significantly. This report describes briefly the preliminary study, to provide background, and then explains, in detail, the modelling and findings from a sequence of parameter optimisation studies that more fully explore the possible RPM modifications in the context of reducing the detection limits to a threat neutron source in an environmental neutron field.