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Title: Characterisation of an electrically cooled planar germanium detector for use in the GRI+ Compton camera system
Author: Rintoul, Ellis
ISNI:       0000 0004 9359 2147
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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Nuclear decommissioning presents a challenge in the UK in terms of safe and costeffective clean-up. An aspect of this involves the precise and accurate imaging and quantification of γ-emitting radioisotopes present in a decommissioning environment. In this work, the GRI+ Compton camera system is presented. It is a transportable γ-ray imaging system, comprising of two layers of planar position-sensitive semiconductor detectors; a Lithium-drifted Silicon (Si(Li)) detector and a High Purity Germanium (HPGe) detector, and a tertiary germanium coaxial detector. The work performed in this thesis focuses on improving the image quality and efficiency achievable by the system through study of the system's HPGe planar detector. This was achieved through development and implementation of Pulse Shape Analysis (PSA) methods, used to improve the position resolution of γ-interactions within the detector, and therefore the imaging quality of the system. A characterisation process was performed using collimated γ-ray beams to investigate the signal response across the detector surface and through its depth. This was the first such process performed on an electrically cooled HPGe detector manufactured by Mirion Technologies Ltd, that makes use of boron implanted p⁺ and amorphous germanium based n⁺ strip contacts. The overall performance of the detector was assessed and the charge collection properties investigated. Parametric methods of PSA currently used in the system were optimised using this information. An alternative method of PSA for use in the planar HPGe detector, signal database comparison, was developed. The signal database was produced through simulation and validated using characterisation scan data. An adaptive grid-search algorithm was then developed to process both single-site and double-site interactions within the HPGe detector. The two methods of PSA were compared when applied to imageable events, consisting of single interactions in the Si(Li) detector and either single interactions or double interactions in the HPGe detector. The optimal angular resolution of the system was achieved when signal database comparison PSA was applied to the HPGe detector and parametric PSA to the Si(Li) detector. Double-interactions occurring across two strips in the HPGe comprised around 60% of imageable events at 662 keV and so their inclusion yielded a large increase in imaging efficiency. Imaging a 10 cm stand-off ¹³⁷Cs source produced γ-images with an angular resolution of 8.5° when using single-interactions in the HPGe detector, and of 9.6° when using both single and double-interactions in the HPGe detector. A user-friendly form of the grid-search algorithm was produced and recommendations made for future use and applications.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral