An evaluation of unattached radon (and thoron) daughter measurement techniques

The collection efficiencies of parallel plate, inertial impactor and wire screen devices reported in the literature for measurement of the unattached fraction of radon (or thoron) daughters are calculated for collection of the attached fraction of the aerosol, using real uranium mine aerosol activity-size distributions. The attached activity collection efficiencies may be as much as 4.3%, which, given the typically low values of the unattached fraction (a few per cent), results in a very substantial error in the measurement of the latter. Published unattached fraction measurements in the environment may therefore be high and should be interpreted with care.