Characterisation of radiation fields with combined fast-neutron and gamma-ray imaging

• Main Author: Beaumont, Jonathan
• Other Authors: Joyce, Malcolm ; Mellor, Matthew ; Shippen, Alan
• Published: Lancaster University 2017
• Subjects: 539.7
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.718655

This research explores the use of organic liquid scintillation detectors coupled with digital pulse-processing electronics, mechatronics and shielding materials to non-destructively characterise radiation sources and their emitted radiation fields through passive imaging techniques. The research sought to expand upon existing gamma-ray imaging techniques, but with focus on fast-neutron imaging techniques which are few in number. The study involved conceptual design, Monte Carlo optimisation and characterisation of collimator and detector geometry, followed by the subsequent design and procurement, assembly and modification to produce several probe configurations. The full system was then realised through control system design, electronic interfacing between custom and commercial off-the-shelf components, communication interfacing and software engineering to produce the data acquisition systems. Coordination with universities and nuclear facilities, logistics and experimental planning enabled the successful deployment of imagers at the University of Lancaster, University of Manchester, the National Physical Laboratory and the Atominstitut at Vienna University of Technology. Data were then analysed by custom code, interpreted and benchmarked to conclude the accuracy of the output images. Three types of imaging devices were investigated. The first was a slot-modulated imaging approach with a tungsten and polythene collimator. This imager was the backbone of the study and underwent significant developments to allow for deployment in different environments. The principle of operation was a heavily shielded single detector which sequentially interrogated space through a small unshielded and sensitive region over the time-scale of a few hours. The objectives were to create a compact, lightweight and portable system which could be used in high-dose or highly-shielded environments to image radiation fields. The second was a slot-modulated imaging approach with a tungsten anti-collimator, effectively using the first imaging system in geometric inversion. As with the first imager, this required sequential interrogation of space over the order of hours, though here the sensitive region was large. This introduced some drawbacks on the image quality but addressed situations where a more compact and lightweight probe was required or where neutron radiation fields were of very high energy (up to deuterium-tritium fusion at 14.1 MeV). The third system was an uncollimated multi-detector system which used readings from four detectors with a real-time algorithm to determine the position of a single source. This configuration was incapable of imaging complex fields, but was effective at tracking the position of a single source every 2 seconds. The bulk of the research was conducted with the slot-modulated imaging approach which was demonstrated with the following radiation sources: a 252Cf source and 241Am/Be source stored in cans, a 252Cf source stored in a steel-shielded water tank and a TRIGA test reactor core. These sources of neutrons and gamma-rays in combination with variation in shielding provided a range of scenarios which were representative of potential industrial deployments in nuclear medicine, nuclear safeguards, nuclear security and nuclear decommissioning. The anti-collimated imaging technique was demonstrated using a 252Cf source stored in a steel-shielded water tank. The uncollimated real-time approach was demonstrated in tracking a single 137Cs source in 3D space which was representative of nuclear security and nuclear medicine applications. The potential applications were explored in the context of other technologies in previous and active research.