Are ageing and fading really a problem for the quality of the passive radon measurements?

Since ageing and fading of alpha tracks in CR-39 are acknowledged to modify the sensitivity of radon measurement by an alpha track device (ATD), a Radon Service operating under quality assurance has to state how these effects affect the uncertainty of the measurements, especially when long term exposures are met. A test lasting one year has been carried out, by using a total of 624 PADC detectors, to analyse the effects of three different treatments (temperature, light and as used inside the ENEA holder) together with the factor of the duration of the treatment (1, 3, 4, 6, 9 and 12 months) on the detector response. The replications (N=6)(N=6) and the target exposure (4000kBqhm-3) were so set to achieve a minimum of 2% as significant statistical difference between treatment and control exposures. The shift of the track effective minor axis distribution has been investigated and related to the reduction of the sensitivity, which resulted in correlation with the surface removed layer of the bulk material. The use of a sensitivity value dependent on how fading, ageing and other factors modify the track diameter distribution was able to reduce the bias of the measurement.