The design and testing of a novel compact real-time hybrid Compton and neutron scattering instrument.

Al Hamrashdi, Hajir and Monk, Stephen and Cheneler, David (2020) The design and testing of a novel compact real-time hybrid Compton and neutron scattering instrument. PhD thesis, Lancaster University.

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Abstract

The requirement for multiple-purpose imaging system occurs regularly within the field of radioactive materials safeguard and security applications. Current instrumentation utilised within the field of dual gamma-ray and neutron imaging systems suffer with limited portability, long scan times, and cover limited energy ranges. Conversely, the imaging system designed, built and tested in this work is not only capable of locating both gamma rays and neutrons, but is also capable of operating in near real time, covers a large energy range and is portable to a desktop degree. The imaging concept applied simultaneously combines Compton and neutron scattering techniques within a threelayer design comprising of a unique combination of scintillators backed with pixelated arrays of photodetectors in the form of 8 x 8 Silicon Photomultipliers (SiPMs). The system features the organic scintillator EJ-204, neutron sensitive lithium glass and thallium doped caesium iodide utilised along with associated SiPMs and front-end electronics, all enclosed within a volume of 120 mm x 120 mm x 200 mm. Further backend electronics is situated within a separate unit where each of the data channels are simultaneously interrogated in order to determine the location of the incident gamma rays and neutrons. The validity of the instrument has been computationally verified using MCNP6 and Geant4 Monte Carlo simulation codes and experimentally tested using Cs-137 gamma sources of ~300 kBq and a Cf-252 neutron source featuring an emission rate of 106 neutrons per second. The developed instrument offers a real-time response with a scan time of 60 seconds and a further data analysis time of 60 seconds. The intrinsic efficiency of the instrument has been experimentally measured to be in the order of 10-4 for both gamma rays at 0.667 MeV and fast neutrons at average energy of 2.1 MeV, and 0.78 for thermal neutrons