SIZE DISTRIBUTION OF UNATTACHED AND AEROSOL-ATTACHED SHORT-LIVED RADON DECAY PRODUCTS - SOME RESULTS OF INTERCOMPARISON MEASUREMENTS

Within the framework of radiation protection programmes supported by t he CEC, the US-DOE, and the Australian Government, intercomparison mea surements were performed in a house with elevated radon concentrations in Northern Bavaria (Germany) in October 1991. Besides the research a spects of aerosol sciences, the purpose of this joint measurement was to compare dose conversion factors calculated from the results obtaine d by these three laboratories. In low ventilated rooms with moderate a erosol particle concentrations (Z = 4000-8000 cm(-3)) about 40% of the Po-218 activity is associated with clusters, narrow in shape (sigma(g )<1.2) and with a median diameter of 0.9 nm. There are strong indicati ons for an additional, fairly broad mode (sigma(g)>1.2, fraction = 10% ) of the 'unattached' part of the Po-218 distribution with a median di ameter of 3-4 nm. The averaged (3 days) derived effective dose convers ion factors (HE-DCF) from the Po-218 values-measured by the three grou ps-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 d ifference to the derived conversion factors if the ultrafine mode (<10 nm) is unimodal or bimodal. The median diameters of the aerosol-attac hed fraction of the short-lived radon decay products ranged between 20 0 and 350 nm, depending on the different methods used by the three lab oratories. However, these fairly large differences have only little in fluence on dose conversion factor calculations. This joint exercise cl early 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 a s yet.