Standardization and difficulties of the thoron exhalation rate measurements using an accumulation chamber

Standardization of the thoron exhalation measurements is important for the intercomparison of the results among different laboratories. There are a few important distinctions between thoron (Rn-220) and radon (Rn-222) exhalation measurements using an accumulation chamber, due to a great difference between radon and thoron diffusion lengths in air, namely, short diffusion length of thoron indicates that it is very hard to attain homogenous distribution of the thoron concentration in an accumulation chamber of the usual dimensions, which is often neglected. It will be explained that the same model cannot be used both for radon and for thoron exhalation measurements. The results of the thoron exhalation measurements also depend on the sample thickness which must be considered. A new method for thoron exhalation measurement using an accumulation chamber is proposed. Comparisons between the existing and the proposed methods were performed on five different samples. (C) 2008 Elsevier Ltd. All rights reserved.