A Study of the Position Resolution of the Segmented Inverted-coaxial GerMAnium (SIGMA) Detector for Gamma-Ray Tracking and Imaging

Abstract

The SIGMA detector is a novel p-type segmented germanium detector, manufactured specifically for γ-ray tracking and imaging purposes. The detector combines small point contact technology with segmentation of the outer contacts. The small physical size of the point-like contact provides a low capacitance, resulting in low series noise and superior energy resolution which is key to its application in gamma-ray spectroscopy and tracking. Moreover, the segmentation of the outer contacts, in conjunction with Pulse Shape Analysis (PSA) algorithms, provides precise information about the γ-ray interaction positions within the detector. This enables SIGMA to generate data that can be used to track the scattered γ-ray interactions throughout the crystal. The work detailed in this thesis shows the first experimental investigation of the position resolution of this novel detector. Initially, GEANT4 simulations were conducted to assess the individual contributions of energy and position resolution to the γ-ray tracking and imaging capabilities of SIGMA, aiming to predict its performance and highlight areas of focus for further research and development. The experimental signal response of SIGMA to γ-ray irradiation as a function of γ-ray interaction position was characterised using the University of Liverpool scanning system. A detailed simulated signal database was produced and validated against the experimental signals. This validated database of signals as a function of interaction position can then be used as a search space for identifying where γ-rays interact during experiments . The position resolution of single-site interactions within SIGMA, generated by collimated γ-ray beams at known positions across various detector regions was assessed using pulse shape analysis algorithms with a chi-squared minimisation approach to search the experimentally validated database. It was found that the position resolution for radial positions is between 2.7 mmand4.2 mm, around 7◦ for azimuthal positions, and approximately 2.4 mm for the z-depth positions of the detector. This is the first experimental measurement of position resolution within the detector, which is the performance metric that drives its great potential in γ-ray tracking and imaging. The feasibility of using SIGMA for Compton Imaging was subsequently investigated through implementing PSA algorithms for multi-interaction events. Compton imaging was successfully performed with 22Na, 137Cs, and 60Co point sources, emitting γ-rays with energies of 511 keV, 662 keV, and 1175 keV, respectively. At a stand-off distance of 10 cm, the FWHM of the image resolution was found to be of the order ∼100 mm. Further work is recommended to fully realise the imaging potential of the detector.