Investigation to assess the feasibility of using the GRI+ Compton Camera for Radioactive Waste Characterisation

Platt, Jaimie
(2020) Investigation to assess the feasibility of using the GRI+ Compton Camera for Radioactive Waste Characterisation. PhD thesis, University of Liverpool.

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The work presented in this thesis examines key scenarios that a gamma-ray imaging system may face when characterising radioactive waste contamination. The aim of this project is to show that a mobile Compton Camera Gamma-Ray Imaging (GRI+) system developed by the University of Liverpool for medical and nuclear decommissioning applications, can quantitatively assess data collected from industrially relevant measurements. GRI+ consists of Si(Li) and HPGe planar, electrically segmented detectors, and a third coaxial HPGE detector. The three detectors are mounted onto a mobile cart system, along with their mechanical cooling systems, power supply and the digital data acquisition system. The mobility of the GRI+ system allowed for the first set of ‘in situ’ measurements to take place in a room with a significantly high level of background radiation. A replication of a select few standard laboratory measurements showed no outstanding effect from the presence of the background radiation from analysis of the peak-to-total values. This could be significant in waste contamination scenarios where the influence of high-levels of background radiation could potentially affect the quality of the data. The imaging response of the detector system for a range of standoff distances was explored, with distances ranging from a minimum of 45 cm (the closest feasible distance the cart could achieve in measuring a source in a contamination scenario) to a maximum of 250 cm. 137Cs point sources were measured in various positions across the detector’s field of view at a standoff distance of 45 cm to assess its capability for measuring a non-uniform distributed sample. The average angular resolution across the range of 68 cm to 250 cm was determined to be 7.19 ± 0.26 degrees, a leading performance for a gamma-imaging system used today. The detector’s sensitivity to concrete material has been examined to provide a better understanding of the influence of scattering material on coincidence energy spectra, with a view of utilising this in future to increase the number of useable imageable events. The peak-to-total ratio was determined for the 662 keV full-energy peak from a 15.6 MBq 137Cs point source in each coincidence energy spectrum produced for differing thicknesses of concrete. A linear relationship is observed between the natural log of the peak-to-total ratio and the thickness of concrete, with a gradient of −0.066 obtained. These results indicate that GRI+ would be capable of waste characterisation measurements. However, further optimisation of the system is required, with the main focus being to overlay Compton images onto real photographic images. Potential improvements to GRI+ have been suggested and would be recommended before any future offsite measurements take place.

Item Type: Thesis (PhD)
Divisions: Fac of Science & Engineering > School of Physical Sciences
Depositing User: Symplectic Admin
Date Deposited: 18 Jan 2021 15:50
Last Modified: 03 Mar 2021 09:28
DOI: 10.17638/03112641
  • Boston, Andrew
  • Page, Robert