Reconstruction of a Radioactive Source Distribution using a Tomographic Spatial-unfolding Method with Compton Gamma Imager Measurements

Advanced detection and reconstruction techniques are utilized to characterize the distribution of radioactive materials in radiological safety and security operations. Most radioactive materials can be detected through their gamma-ray emissions. There may arise circumstances in which the measurement of these gamma rays must be made from behind a pre-established perimeter, however. To address this, Natural Resources Canada (NRCan) and the National Research Council Canada (NRC) have jointly developed novel Compton gamma imaging technology for use in safety and security applications. Compton gamma imaging utilizes the physics of Compton scattering to locate gamma-ray sources, making the technology well-suited to localization at a distance from the suspected source. A fast tomographic reconstruction method has been developed for use with Compton gamma imaging measurements to provide rapid, in situ source localization. For a more precise o ine reconstruction of the data, a spatial unfolding technique has also been developed that  ts the observed experimental tomogram with tomographicresponse templates produced by Monte Carlo simulation of spatially-localized sources of radioactivity. These methods have been applied to two sets of experimental data using measurements acquired with the detector developed by NRCan and the NRC. In one experiment, measurements of a Cs-137 point source were acquired in a laboratory setting. In a second experiment, measurements of a spatially-extended La-140 source were acquired in realistic outdoor operational conditions. The tomographic reconstruction of the sources takes only minutes and demonstrates good localization capability. The spatial-unfolding method shows good localization of experimentally-measured sources but underestimates their activity, while accurately recovering the activity of synthetic sources. The underestimation of experimentally-measured source activities is ascribed to limitations in the simulated representation of the detector response and environmental terrain not included in the simulated world. The simple tomographic reconstruction method will be useful in guiding radiological safety and security operations in near real-time. The post-process unfolding method will further be useful in informing follow-on consequence management operations and remediation activities.