Investigating the Stability in Optical Properties of Portable Radiation Detector Components Through Accelerated Ageing
Creators: name: Loughney, Kerri
Publisher: name: University of Surrey
Year: 2023

description: The use of scintillation detectors in different environmental conditions (e.g. vehicle/cargo radiation portal monitors) and radiation environments (non-destructive imaging, high energy physics, environmental contamination monitoring, etc.) requires the components to have a highly stable performance under such conditions for extended durations. Ageing of the components as a result of environmental or radiation damage has the potential to cause a degradation in the overall performance of a detector. It is therefore important to understand both the mechanisms and degree of degradation possible under such conditions. The work reported in this thesis outlines the optical characterisation of scintillators and coupling materials following different accelerated ageing methods in order to determine the degree of damage possible over a detector’s lifetime. The optical properties of select coupling materials and crystal scintillators were characterised pre and post accelerated ageing through thermal cycling, displaying the degree of stability in the optical properties after ageing to the equivalent of a single year in real time. Eljen EJ-560 optical coupling pads and an unconventional 3M VHB double-sided adhesive foam tape displayed the highest level of stability in the transmission properties following this thermal ageing. Extensive optical characterisation of CsI:Tl and GAGG:Ce crystals before and after thermal ageing showed the properties of both scintillators remained stable. In particular, the energy resolution of the ¹³⁷Cs 662 keV peak was shown to remain within an average value of 0.2 % for both scintillators when coupled to a photomultiplier tube. <br>
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description: Radiation damage investigations through gamma and neutron irradiation were used to determine the radiation hardness and stability in optical properties of CsI:Tl and GAGG:Ce scintillators. ⁶⁰Co gamma irradiation of scintillators up to 613 kGy showed GAGG:Ce to remain stable in all optical characterisations. Radiation damage of the CsI:Tl scintillators resulted in a significant degradation across all optical characterisation measurements for higher level gamma irradiation of 10 kGy or greater. A shift in the emission spectra with the increase in gamma irradiation level was also observed for the CsI:Tl samples, correlating to a greater degradation in the energy resolution when coupled to a photomultiplier tube compared to a silicon photomultiplier. For neutron irradiation of CsI:Tl samples up to 10¹³ n · cm⁻², significant degradation in the optical properties was only observed for the energy resolution of the ¹³⁷Cs 662 keV peak. The ¹³⁴Cs activation of the CsI:Tl samples was observed to significantly increase with the increase in neutron irradiation, heavily influencing the measured energy resolution as a result. However, a reduction in the gain for the 662 keV peak with increasing irradiation level suggested a reduction in the light yield or transmission of the CsI:Tl samples following neutron irradiation. Irradiation of GAGG:Ce samples up to 10¹³ n · cm⁻² displayed a high level of radiation hardness across all optical characterisation measurements conducted, showing it to be a suitable scintillator for use in high dose environments.<br>