The Use of In Situ222Rn Soil Measurements in the Characterisation of Soil Profiles

Abstract

Abstract Recent in situ222Rn/214Bi soil profile measurements on hermetically sealed and weighed samples have been made using a new, experimental field method based on sequential measurements of the prompt decay of the 214Bi counting rate. At least two 214Bi count rate measurements are made in the first 11/2-2 h after sample collection and sealing, To. Two subsequent 214Bi count rate measurements are made: the first within 4-12 h, and the second approximately 900 h after To. These measurements yield the following information for each sample: (1) the extrapolated, To214Bi count rate and, indirectly, the Toin situ222Rn activity; (2) a minimum 214Bi count rate if 222Rn was lost in the sampling process; and (3) a final, constant and usually a maximum 214Bi count rate reflecting the establishment of secular radioactive equilibrium between the 226Ra, 222Rn and 214Bi in the sealed sample. This information provides the basis for: (1) assesing the state of 226Ra/222Rn radioactive equilibrium in the soil sample for the 2-3 h period immediately preceding sample collection; (2) characterising the potential loss of 222Rn from a soil profile; (3) identifying factors contributing to the mobility and transport of 222Rn; and (4) modelling the 222Rn surface flux values that might be expected under different soil moisture and atmospheric conditions.