Factors controlling the emanation of radon and thoron in soils of the eastern USA

Emanation coefficients (proportion of generated Rn atoms that escape from a solid material) have been measured for Rn-222 (t(1/2) 3.8 days) and Rn-220 (thoron, t(1/2) 55 s) on 68 disaggregated soil samples from 12 soil profiles representative of the eastern U.S.A. Average values for sites are 0.20 for Rn-222 and 0.16 for Rn-220. Based on distribution of Ra-226 among the exchangeable, organic, Fe-oxide, sand, silt and clay fractions of the soils, a multiple regression indicates that the Rn-222 of 0.46, and organic-exchangeable fraction, occurring mainly as coatings on grains, has an emanation coefficient for the silt-clay fraction has an emanation coefficient of 0.22. The latter value is verified by experiments. The organic component makes the largest single contribution to Rn in soil gas. Mineral grains have twice the (222)n emanation as Rn-220, implying that about half the Rn atoms are emanated directly to pore space, and the remainder are freed by track-etching and diffusion over a period of days. Emanation for Rn-220 appears to be dominated by Fe-oxides, the main host for pedogenic (232) Surface A- and Th. E-horizons have low emanation coefficients for Rn-220, probably because of a lack of Fe-oxides in these horizons.