Detecting artificial airborne radioactivity: on-line monitoring of external dose rate near an aerosol filter

An early warning method based on external dose rate monitoring near an aerosol filter can be used to detect artificial airborne radioactivity. A theory is presented for calculating the external dose rate in the measurement geometry involved. Airborne radionuclides and radionuclides collected by the filter and deposited on the ground are considered. Delay time from the arrival of a radioactive cloud to a tentative warning level of 0.4μGy.h-1 is calculated for two cases: for a constant activity concentration of 137Cs and for a group of fission products. The simulations indicate that, in terms of activity concentrations, a warning level of <10 Bq.m-3 of artificial airborne radionuclides can be attained. This activity concentration corresponds roughly to an external dose rate of 1 nG.h-1, which is about 2 orders of magnitude below a typical, natural, ground-level dose rate. (Author)