The Impacts of Source Location in Tomographic Gamma Scanning - 17324

Canberra Industries has recently designed and built a Tomographic Gamma Scanning (TGS) system for the deployment in a nuclear facility. The TGS technique combines high-resolution gamma spectroscopy with low spatial resolution 3-dimensional image reconstruction to provide increased accuracy over traditional approaches for the assay of non-uniform source distributions in low-to medium-density containers. The TGS system has been optimized to measure 120 liter drums and has been equipped to use various collimators and attenuators to assay containers under a wide range of operational conditions. In this paper, we report on experimental work using TGS systems aimed at evaluating the effects of source locations at the limits of volume elements, or voxels, defined in the image reconstruction. These include sources placed at voxel boundaries within a single vertical layer and sources placed in the lowest and highest possible vertical positions within the containers. Building upon previous studies, the accuracy of the TGS system was also assessed by varying the radial position of a point source within a vertical segment. The robustness of the TGS image reconstruction algorithm to location of the sources location was also analyzed as a function of matrix density and gamma-ray energy. Methods to reduce the biases from source location at extreme positions within the container were implemented and tested. The results from this work have been used to better understand the total measurement uncertainty (TMU) and analysis methodology used in TGS systems currently deployed and under development. (authors)