Efficiency of clearance monitors for²³²Th out of secular equilibrium

Clearance monitors (CMs) are used at the Paul Scherrer Institute to characterise radioactive waste, including thoriated tungsten (W-Th) welding electrodes, which contain thorium oxide (ThO2). The decay of²³²Th and its progeny poses challenges to the accurate determination of the CM detection efficiency due to a potential disequilibrium. In this work, a methodology developed for estimating the detection efficiency of CMs for²³²Th not in secular equilibrium with its progeny is presented. A Monte Carlo model of the CM was implemented using FLUKA to determine stand-alone efficiencies for all gamma emitting nuclides in the thorium decay chain. The model accounts for the time evolution of progeny build-up, and a legal figure of merit was used to determine the most conservative efficiency scenario. The methodology successfully estimates the detection efficiency by considering the evolving decay chain of²³²Th with time. Additionally, it enables a conservative assessment of activity in the copper holders of the W-Th electrodes by selecting the nuclide vector relevant at the time of the measurement. The approach improves clearance measurements for materials containing long-lived radionuclides, enhancing regulatory compliance and radiation safety. It can be extended to other radionuclides with complex decay chains and applied to different counter detectors.