Unmixing Of Radionuclide Signatures In The Presence Of Natural Background And Shielded Materials

National security relies on several layers of protection. One of the most important ones is the traffic control at borders and ports that exploits Radiation Portal Monitors (RPMs) to detect and deter potential smuggling attempts. Two relevant issues for the identification of radioactive materials at RPMs are the presence of natural background radiation and shielding materials surrounding the sources. In this paper, we study the robustness of the unmixing algorithm that we have developed for source identification in both cases: (1) gamma-ray bare source unmixing in the presence of natural background, and (2) identification of shielded neutron sources. Neutron sources are more difficult to shield than gamma-ray sources and therefore can be easier to detect during passive inspection. This would be a particularly challenging scenario for the unmixing algorithm because of the shielding material’s ability to act as a spectrum modulator. For the first application, we experimentally studied the robustness of the unmixing algorithm to different radiation background spectra, due to varying atmospheric conditions, in the 16◦C to 28◦C temperature range. The unmixing algorithm can be used to reliably identify the unshielded gamma-ray radionuclides that triggered an alarm, even with fewer than 1,000 detected counts and in the presence of multiple nuclides at the same time. With fewer than 500 counts available, we found larger differences of approximately 35.9% between estimated nuclide fractions and actual ones. For the second application, we simulated a beryllium-reflected plutonium source shielded by various materials and studied the effect of the spectral modifications induced by shielding on the unmixing response. For shielded neutron source configurations, the algorithm requires as few as 5,000 counts in the whole spectrum to remain effective.