Experimental and modeling studies of thoron decay products in outdoor air near the ground surface
- New Mexico Institute of Mining and Technology, Socorro, NM (United States); and others
To better understand the physical behavior of thoron decay products in outdoor air at ground level, and the resulting exposure-dose coefficient, measurements of the concentrations of thoron ({sup 220}Rn) gas and its short-lived decay products ({sup 216}Po, {sup 212}pb, {sup 212}Bi) in attached-to-aerosol and unattached states were carried out at a semi-arid field site in central New Mexico under varying atmospheric conditions. A high volume air sampler (100 L min{sup -1}) with a single 635 mesh stainless steel screen (50% penetration for 4 nm particles) and a glass fiber filter were employed. Analysis of the 105 outdoor measurements yielded the following averages values: 9 Bq m{sup -3} for the {sup 220}Rn activity concentration of thoron gas and the unattached fraction of potential alpha energy are similar to those of radon and its decay products at this site but the equilibrium factor and dose rate are smaller. The thoron decay products measurements are interpreted using the computer code TPOUT, which applies one-dimensional eddy transport to calculate the vertical concentrations of {sup 220}Rn and its decay products as a function of atmospheric stability, aerosol concentration, terrain roughness, and surface wind speed. This code successfully predicted the observed trends of unattached and attached decay products concentrations with height and other factors. The resulting effective dose rate is found to be relatively invariant with dry deposition factors such as terrain roughness, surface wind speed, and even the ambient concentration of aerosol particles. Projection of the present results to conditions typical of temperate climates suggests an average annual outdoor effective dose from thoron decay products of about 0.025 mSv, which is a significant component of the total outdoor dose.
- DOE Contract Number:
- FG03-94ER61781
- OSTI ID:
- 270410
- Report Number(s):
- CONF-950999--
- Country of Publication:
- United States
- Language:
- English
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