Context: the radioactivity level of waste materials from the decommissioning of nuclear power plants and particle accelerators must be measured to classify the materials by comparing the obtained activity value with the legal exemption levels set by the regulatory authorities. Clearance monitors (CMs), commonly used for this purpose, require a scenario-dependent efficiency calibration (e.g. efficiency as a function of mass of the material to be measured) using sensitivity factors related to a reference nuclide called the key (or leading) nuclide. In commercial CMs, however, these factors are typically fixed for a given nuclide, regardless of the different measurement scenarios.
Objective: the objective of this study was to determine the dependence of the sensitivity factors on the mass of measured material for a specific scenario (copper parts placed inside a stainless steel drum), with the goal of defining a general recommendation on how to select the appropriate sensitivity factor for a given measurement scenario.
Approach: Monte Carlo simulations of the CM were performed for this specific scenario to obtain the sensitivity factor for five nuclides (²²Na, ¹³³Ba, ¹³⁷Cs, ¹⁵²Eu and ²³²Th) and various geometries. Measurements were carried out with certified sources of those nuclides for four specific geometries to validate the Monte Carlo calculations. Additionally, clearance measurements using the manufacturer-provided sensitivity factors and the factors calculated in this work were compared to determine in which situations one single sensitivity factor could be used for various geometries.
Results: the results show that, in case of measurements with the same material but mass varying from 10.0 kg to 40.0 kg, the use of a single sensitivity factor for monoenergetic nuclides (e.g. ¹³⁷Cs), if not selected properly, leads to an over- or underestimation of the activity level of the measured materials. If overestimation is not an issue, the lowest sensitivity factor can be used for all possible measurement configurations. Otherwise, a situation-dependent sensitivity factor should be estimated and implemented. The use of a single fixed sensitivity factor can be easily justified in the case of nuclides emitting gammas in a wide energy range (e.g. ¹⁵²Eu), where the dependence of these factors in the studied mass range is small. Significance: these findings facilitate more accurate decisions regarding the calibration of CMs with the aim of classifying waste from the dismantling of nuclear power plants and particle accelerators, with potential logistic and economic implications.