Measurements of deposition velocity of radon decay products for examination of the correlation between air activity concentration of radon and the accumulated Po-210 surface activity

The retrospective determination of radon exposure levels in dwellings by means of the measurement of the Po-210 surface activity is subject to various uncertainties. These result partly from the values assumed for the equilibrium factor F and for the unattached fraction fp, and, more importantly, from differences in the deposition velocities of short-lived decay products of Rn-222, caused by varying conditions of turbulence. In order to evaluate the actual range of the variation which occurs under German living conditions, measurements for the deposition velocity parameter were carried out in several dwellings in which increased levels of radon were present. The statistical evaluation of the measurements produced a mean deposition velocity of 1.7 m h for Po-218 and 0.4 m h for Pb-214, and a relative standard deviation in the measured values of as low as approximately 50%. This lay significantly below the uncertainty value expected from the literature and would seem to justify the retrospective determination of the radon exposure from Po-210 surface activity measurement for use in, for example, epidemiological studies.