222Rn Absorption in Plastic Holders for Alpha Track Detectors: A Source of Error

Experiments show that the plastic holder used for the Swedish Radiation Protection Institute's (SSI) alpha track detector absorbs 222 Rn (radon) and subsequently outgasses it slowly when stored in a lower radon concentration environment. This phenomenon is not unique to the plastic holder used at SSI. A discrepancy in the results found at an international intercomparison of radon detectors highlighted the problem. The observation implies that the practice of enclosing the detectors in plastic bags after exposure should be limited as it can lead to further etching of the detector and an overestimation of the intended measurement. This article describes a procedure for estimating the error that exposure caused by outgassing of radon from the detector housing would give under different exposure conditions. The calculated overexposure agrees with the observed one in the intercomparison. One conclusion is that film and plastic holders should be separated, especially after short-term exposures.