Experimental evaluation of combined ageing and fading effects on annual radon concentration measurement based on nuclear track detectors

The effect of ageing and fading on solid-state nuclear track detectors (SSNTD), used for measuring radon concentrations, leads to a decrease in the number of counted tracks and thus to an underestimation of actual radon exposure. The ISO 11665–4:2020 standards for radon concentration measurements using passive devices make no mention of “Ageing and Fading” effects among the “influence quantities” that can bias measurement results. The present experimental study aimed to investigate the presence of the ageing and fading effects in typical indoor environments for SSNTD, particularly for three different radon measurement techniques (detector and track-readout systems). The first two techniques use Poly(allyl diglycol carbonate) (PADC) detectors, also known commercially as CR-39, manufactured by Intercast Spa and Radosys Ltd respectively, while the third technique uses a cellulose nitrate detector similar to LR-115 (manufactured by Dosirad-Kodak). For simplicity, throughout this paper, these detectors will be referred to as CR-39 and LR-115, respectively. For the two different techniques based on CR-39, track counting is done by a fully automated image analysis system, whereas with the LR-115, the tracks are counted using a spark counter. The present study, following previous work, aimed to evaluate the effects of ageing and fading on the three techniques over exposure periods of 3, 6, and 12 months. The findings suggest that ageing and fading do not significantly impact the response of technique based on LR-115 and CR-39 detectors. However, for the technique based on CR-39 detectors produced by Radosys, a considerable underestimation of the actual radon exposure was observed. Specifically, the results show that the measurement of annual integrated radon exposure, decrease by approximately 10%, when performed using two consecutive 6-month monitoring periods (2 detectors, one per semester) compared to four consecutive 3-month monitoring periods (4 detectors, one per quarter). Similarly, a decrease of approximately 20% was observed in the annual integrated radon exposure when performed using a single 12-month monitoring period (one detector in a year) compared to four consecutive 3-month monitoring periods (4 detectors, one per quarter). These findings highlight the importance of carefully considering the detector material and technique used for radon measurement, particularly for extended measurement periods, and then considering the effects of ageing and fading when developing future standards for radon concentration measurements.