Segmented HPGe detectors for post-irradiation examination of nuclear fuel : Design and demonstration in gamma emission tomography

Abstract: Gamma emission tomography (GET) is a proven non-invasive technique for post-irradiation examination of nuclear fuel. In the past, collimated HPGe detectors were used for GET measurements due to their good energy resolution. However, because a large number of projections need to be acquired to achieve high spatial resolution, the use of a single HPGe detector is associated with long measurement times.This thesis investigates the use of an electrically segmented HPGe detector for GET, proposing two conceptual types of segments: 1) scattering segments, each aligned with a collimator slit for localisation, and 2) energy deposition segments for aiding in the full energy deposition. The feasibility of a true coaxial segmented detector for this application was studied using the Monte Carlo particle transport code MCNP. Performance parameters, such as detection efficiency and mislocalisation rate, were obtained using proposed analysis methods. Furthermore, the dimensions of the segmentation pattern of the 18 detection elements were optimised based on the detector's foreseeable use and the performance parameters.For the experimental demonstration, a scaled-down planar prototype detector consisting of 6 scattering segments and 1 energy deposition segment was designed, with a working principle similar to the proposed true coaxial detector. The spatial response of the collimated prototype detector was obtained using MCNP simulations. In the experimental evaluation, energy resolution in different operating modes, count rate capabilities, and mislocalisation rate were obtained. The detector was found 3 times faster than when used in an unsegmented mode upon comparing the simulation and experimental results concerning the relative detection efficiency.An experimental demonstration of the detector in the proposed application was successfully performed at the BETTAN tomography test bench at Uppsala University using three mockup fuel rods (filled with 137Cs source). Images were reconstructed using the filtered back projection algorithm on the projection data. Based on the experimental results, it is recommended to use such detectors for faster data acquisition combined with good energy resolution in GET, which in turn allows for improved spatial resolution in GET examinations of irradiated nuclear fuel.