Characterizing low-activity waste containers: A case study for Compton Suppression Systems under challenging signal-to-noise ratio

This work investigates a compact Compton Suppression System (CSS) based on a High-Purity Germanium (HPGe) diode as a primary spectrometer. From a wide range of potential materials for the guard detector, including plastic scintillator, Thallium doped Sodium Iodide (NaI(Tl)), and Lanthanum III bromide (LaBr3), a bismuth germanate crystal (BGO) is selected on account of a high photon mass-attenuation coefficient, thus allowing more compactness. The authors then describe the complete design study of an HPGe/BGO CSS based on the post-processing of Monte Carlo particle track history files (MCNP6.1/PTRAC). The calculation program, simulating the filtering of coincident events on both primary and secondary detectors, is validated through a series of experimental measurements using Am-241, Cs-137, and Co-60 sources in various configurations. On the basis of this empirically validated calculation, an optimization study of the BGO-active shield is performed over two factors of merit: Compton Suppression Factor (CSF) and Full-Energy Peak Efficiency. Optimization steps subsequently involve the scaling of BGO internal and external radii, the thickness of the entrance window, and the diameter of a collimation hole. Eventually, performance figures of the CSS are illustrated in a case study involving the characterization of large waste containers, carrying low levels of uranium activity to seek among natural radiological background. Calculation results show a lowering of minimal detectable activities (MDA) by a factor of 2.5 based on the signature of 1001-keV gamma rays from U-238.