Measurements of thorium-B (Pb-212) in the outdoor environment and evaluation of equivalent dose

The activity size distribution of unattached as well as attached Pb-212 to aerosol particles was measured in the open air of El-Minia City, Egypt. The samples were collected using a wire screen diffusion battery technique and a low pressure cascade impactor. The mean activity median thermodynamic di ameter (AMTD) of unattached Pb-212 was determined to be 1.4 nm with a relat ive mean geometric standard deviation (a,) of 1.55. A mean unattached fract ion Cf, of 0.03 +/- 0.007 was obtained at a mean aerosol particle concentra tion of 32 x 10(3) cm(-3) Sometimes the f(p) values were less than the dete ction limit of 0.008. The mean concentration of activity of Pb-212 was foun d to be 9.6 +/- 1.1 mBq m(-3). The mean activity median aerodynamic diamete r (AMAD) of the accumulation mode of attached Pb-212 was determined to be 3 60 nm with a mean (sigma(g)) of 2.7. The mean value of specific air activit y concentration of Pb-212 associated with that mode was determined to be 30 3 +/- 12 mBq m(-3). With a dosimetric model calculation (International Comm ission on Radiological Protection. Human respiratory tract model for radiol ogical protection. Oxford: Pergamon Press, ICRP Publication 66, 1994) the t otal deposition fractions as well as total equivalent and effective dose ha ve been evaluated considering the obtained parameters of the activity size distributions. At a total deposition fraction of about 97% for unattached a ctivities the total equivalent and effective doses to the lung were determi ned to be about 0.18 and 0.02 mu Sv, respectively, while total equivalent a nd effective doses of about 0.45 and 0.05 mu Sv, respectively, were determi ned at a total deposition fraction of about 23% for the attached activities . (C) 2000 Elsevier Science Ltd. All rights reserved.