Size Distribution of Unattached and Aerosol-Attached Short-Lived Radon Decay Products: Some Results of Intercomparison Measurements

Abstract

Abstract Within the framework of radiation protection programmes supported by the CEC, the US-DOE, and the Australian Government, intercomparison measurements were performed in a house with elevated radon concentrations in Northern Bavaria (Germany) in October 1991. Besides the research aspects of aerosol sciences, the purpose of this joint measurement was to compare dose conversion factors calculated from the results obtained by these three laboratories. In low ventilated rooms with moderate aerosol particle concentrations (Z=4000-8000 cm-3) about 40% of the 218Po activity is associated with clusters, narrow in shape (sg<1.2) and with a median diameter of 0.9 nm. There are strong indications for an additional, fairly broad mode (sg>1.2, fraction=10%) of the 'unattached' part of the 218Po distribution with a median diameter of 3-4 nm. The averaged mode (3 days) derived effective dose conversion factors (HE-DCF) from the 218Po values - measured by the three groups - differ less than 30%. However, the daily averaged values sometimes differ by a factor of 2. In general, it does not appear to make much difference to the derived conversion factors if the ultrafine mode (<10 nm) is unimodal or bimodal. The median diameters of the aerosol-attached fraction of the short-lived radon decay products ranged between 200 and 350 nm, depending on the different methods used by the three laboratories. However, these fairly large differences have only little influence on dose conversion factor calculations. This joint exercise clearly showed that accurate particle size measurements in the diameter range 10-100 nm (nucleus mode), which requires combining impactors and diffusion battery techniques, is a difficult task, not fully solved as yet.