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Scintillation materials for the detection of special nuclear materials (SNMs)

22 February 2012

Chiaki Crews

A feasibility study was carried out to assess the use of a commercially available gadolinium-loaded liquid scintillator (EJ-331), combined with a photomultiplier tube (PMT) and data acquisition equipment, for neutron detection. This was of interest due to its potential application (once scaled up) for the detection of special nuclear materials (SNMs). SNMs are a significant neutron source, and this is the key to their detection due to the difficulty in shielding this penetrating radiation.

Natural background neutron radiation levels are low, so higher neutron radiation levels can be strongly indicative of the presence of SNMs compared to gamma rays, which have little diagnostic power where there are high levels of natural background radioactivity.

The necessary materials were procured and assembled, then tested with a californium-252 neutron source and caesium-137 gamma-ray source in a laboratory at the Home Office Centre for Applied Science and Technology. Three types of calculations were carried out on the recorded data (tail-to-total integrals, constant fraction thresholds and rise times), and a pattern emerged (most clearly in the rise time analyses) where the gamma-ray source was used.

The neutron events were not visible – however, it was concluded that this was due to limitations in the equipment, and simple modifications to the PMT and/or speed of data acquisition would allow these to be identified.