A CONTINUOUS MONITOR FOR RADON PROGENY AND ITS UNATTACHED FRACTION

A continuous monitor for measuring environmental radon progeny has been developed. Concentrations of Po-218, 214Pb, 214Bi and their unattached fractions were determined by analysing alpha energy spectra. The concentration of Pb-212 was also estimated by assuming that the concentrations of Pb-212, Bi-212 and Po-212 were in equilibrium with each other in air. The total and unattached Rn-222 progeny concentration below 1 Bq.m-3 can be measured with a relative standard deviation of 50%. From the results of simultaneous measurements with the other methods, the reliability of the monitor was verified for both high and low concentrations of Rn-222 progeny. Continuous measurements of total and unattached Rn-222 progeny concentrations were carried out for several days. A method was proposed to estimate deposition velocities for both attached and unattached Rn-222 progeny by using a mass balance model. Applying the method to the results obtained with our monitor, the deposition velocity of unattached Rn-222 progeny was estimated to be 0.71 cm.s-1.