The performance of continuous radon monitors (CRMs) is usually evaluated under controlled conditions in a radon chamber during calibrations or intercomparison exercises. The impact of thoron on CRMs response is rarely evaluated; in case the evaluation is performed, it is carried out in a controlled atmosphere with relatively constant, homogeneous, and generally high thoron concentrations and very low radon levels. In a real indoor environment, both radon and thoron concentrations are extremely variable, so the thoron interference evaluations reported in the literature are generally not applicable to CRMs used to measure radon concentration indoors. For this reason, an experimental study was carried out with four different CRMs in an indoor environment (an office room) where medium-to-high concentrations of both radon and thoron were expected. Thoron concentration has been separately evaluated throughout two different active monitors. Three CRMs resulted in overestimations of radon concentration by about 10% due to thoron interference, whereas such interference results were negligible for the fourth CRM. However, the thoron interference can also be used to assess thoron concentration by using CRM not specifically designed to do so. Based on the results of this study, an indirect method to assess thoron concentration is indeed proposed, relying on the combination of two identical monitors (one placed right close to the wall and the other one far enough from there).
Keywords: indoor radon concentration; radon monitor; thoron interference; thoron sensitivity.