MEASUREMENT OF UNATTACHED FRACTION OF AIRBORNE RAA

The significance of the unattached fraction, f of airborne RaA in assessing the hazard from the inhalation of radon and its daughter products is discussed, and reference made to some of the measurements of f which various workers have carried out in mine atmospheres. A method developed by the authors for measuring f is described. The method entails sampling the atmosphere through a pair of filters positioned side by side. One of the filters is preceded by a diffusion battery designed to remove most of the unattached daughters but hardly any of the attached ones, and the difference in activities collected on the filters is therefore a measure of the unattached daughter product concentration. Since f is concerned only with RaA atoms it is necessary to distinguish the alpha-activity on the filters due to RaA from that due to RaC'; this distinction was made by alpha-spectrometry. Furthermore, because of the short half life of RaA on the one hand, and the desire for high sensitivity on the other, it was found necessary to measure the activity while sampling was in progress. The method has been used to measure f in the open air and it has been found that only when both f and the RaA concentration are unusually low does the sensitivity become inadequate for a meaningful determination of f. © 1969 Health Physics Society.