Radiation Protection Dosimetry 108:331-343 (2004)
Radiation Protection Dosimetry Vol. 108 No. 4 © Oxford University Press 2004; all rights reserved
Radon progeny size distributions and enhanced deposition effects from high radon concentrations in an enclosed chamber
International Academy, 693 Wellerburn Road, Severna Park, MD 21146, USA
Received June 13 2003 in final revised form February 10 2004, accepted February 24 2004
Prior work studying radon progeny in a small enclosed chamber found that at high 222Rn concentrations an enhanced surface deposition was observed. Subsequent measurements for unfiltered air showed minimal charged particle mobility influence. Progeny particle size measurements reported here, performed at the US Department of Energy Environmental Measurement Laboratory (now with Home Security Department), using the EML graded screen array (GSA) system show in unfiltered air that the high 222Rn levels causes a reduction in the attached 218Po progeny airborne particulates and formation of additional normal sized unattached (
0.80 nm) and also even smaller 218Po below 0.50 nm. At a 222Rn level of 51 kBq m–3, 73% of all 218Po are of a mean particle diameter of about 0.40 ± 0.02 nm. At this 222Rn level, the ratio of 218Po to 222Rn airborne concentrations is reduced significantly from the concentration ratio at low 222Rn levels. Similar reductions and size reformations were observed for the 214Pb and 214Bi/Po progeny. The particle size changes are further confirmed using the plateout rates and corresponding deposition velocities. The Crump and Seinfeld deposition theory provides the corresponding particle diffusion coefficients. With the diffusion coefficient to ultrafine clustered particle diameter correlation of Ramamurthi and Hopke, good agreement is obtained between EML GSA and deposition velocity data down to 0.40 nm. Strong evidence is presented that the progeny size reduction is due to, as a result of air ionization, the increased neutralization rate (primarily from electron scavenging of OH molecules) of the initially charged progeny. This is shown to increase with the 
power of 222Rn concentration and relative humidity as well as increased air change rate in the chamber. These results imply that at 222Rn levels above 50 kBq m–3, at relative humidity of 52%, a considerable reduction in lung dose could occur from preferential deposition of the progeny in the nasal and oral passages.
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