Continuous measurement of the equilibrium factor F and the unattached fraction f p of radon progeny in the environment

Abstract

The equilibrium factor F and unattached fraction f p value, which are important in estimating lung dose, are usually obtained separately. Since both parameters are subject to change according to environmental conditions, it is preferable to measure them simultaneously and continuously. An apparatus which can measure unattached and attached fractions of radon progeny simultaneously with radon gas was developed. When air is introduced into this apparatus, unattached fractions are collected on a metal wire screen, and their activities are detected by a silicon semiconductor detector. Remaining unattached fractions, and attached fractions, are collected on a glass fiber filter. The 218Po concentration and equilibrium equivalent radon concentration(EEC Rn) of the unattached and attached fractions are determined by the build-up method. Radon gas flows into a cylindrical vessel (decay chamber) through a drying column filled with CaCl 2. Radon progeny formed in the vessel is collected by an electric field on a detectable surface of a silicon semiconductor detector. Radon concentration is estimated from the net count of collected 218Po. The apparatus provides data every 30 min. Minimum detectable concentration (with a standard deviation of 50%) is 3 Bq m −3 for radon concentration, and below 1 Bq m −3 for both 218Po concentration and EEC Rn. The apparatus was tested in the laboratory and used in an actual dwelling. In the laboratory measurement, an air circulating system was operated. An increase in f p and decrease in the equilibrium factor were observed. In the dwelling, the measurement was continuously performed over a 24 h period. Results show that both the equilibrium factor and the unattached fraction obey a lognormal distribution.