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Development of Directional Detector System for Localisation in Mixed Field Environments

Randall, GL; (2017) Development of Directional Detector System for Localisation in Mixed Field Environments. Doctoral thesis, UCL (University College London). Green open access

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Abstract

Active interrogation is a method of detecting fissionable materials by directing radiation, normally high energy photons or neutrons, at a volume of interest. The resulting fission events can produce a unique signature in the form of delayed neutrons and gammas that can be used to identify a material. A common application of this technique is in the detection of special nuclear materials (SNMs) such as 235U. A requirement has been identified to provide a directional detector for use in one of these high flux active interrogation environments. Many detectors of ionising radiation exist but few are capable of directionality. Those that are tend to be heavy and inefficient. A novel method for providing directionality for radiation detection, previously developed by the author, is to be used – the RadICAL detector. The idea is based upon the fact that detector response depends upon the radiation pathlength and area presented to a particle flux. Thus a rotating slab of detector gives a characteristic temporal response that can be used to identify the direction of the photon flux. This concept is to be used to locate fast neutrons produced by a material of interest within an active interrogation scenario. The first objective of this study was to model a RadICAL detector. The original model was built using a simple ray tracing method and this was followed by the construction of a more complicated Geant4 Monte-Carlo model. The results of this modelling were used to inform decisions made in the building of a prototype detector. Further modelling was conducted to investigate the optical properties of the detector. The second objective was to design and build a number of these detectors based on this modelling. These detectors were then tested by exposing them to a number of different sources under a variety of different conditions and evaluating the response. The third objective was to develop a detector capable of determining the direction of an active interrogation source. This involved a variation on the RadICAL method, the RadICAL Stack, whereby counts from a stack of stationary detectors were fitted to a standard response curve in order to determine the direction of the source. The detectors were built from Eljen EJ299-33 so that the localisation technique could be combined with pulse shape discrimination techniques to separately determine the location of gamma and neutron sources. Both the RadICAL and the RadICAL Stack detection methods were shown to be effective at determining the direction of a gamma source. A four element detector was built and shown to achieve an angular resolution of approximately 4:4±0:3 when detecting a 3.7 MBq 137Cs source at distances up to 2.1m. The same detector was shown to achieve an angular resolution of approximately 2:95±0:32 when detecting discriminated neutrons from a 150 MBq 252Cf source at distances of up to 4m.

Type: Thesis (Doctoral)
Title: Development of Directional Detector System for Localisation in Mixed Field Environments
Event: University College London
Open access status: An open access version is available from UCL Discovery
Language: English
Keywords: Active interrogation, detector, SNM, Pulse Shape Discrimination, Scintillators
UCL classification: UCL > School of BEAMS
UCL > School of BEAMS > Faculty of Engineering Science
URI: http://discovery.ucl.ac.uk/id/eprint/1553250
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