A method for determining soil gas 220Rn (thoron) concentrations

A technique has been developed to accurately, precisely, and quickly measure soil gas 220Rn using commercially available grab-sample scintillation detectors. The method requires two counting periods for each sample: a 1-min count as soon as the sample has been drawn into a scintillation cell, followed by a 5- or 10-min count at least 5 min after the soil gas sample has been obtained. The 222Rn concentration is determined from the second count. The counts in the first counting period (1-min) due to 222Rn and its progeny are calculated from the known 222Rn concentration and then subtracted from the total counts obtained in the first counting period. The remaining counts are due to 220Rn and its progeny and are used to calculate the 220Rn concentration. The overall uncertainty when using this method to measure typical soil gas 220Rn and 222Rn concentrations [> approximately 5 kBq m-3 (130 pCi L-1)] was determined to be 19.8% and 10.8% (90% confidence levels), respectively, from analyses of duplicate field measurements. The lowest 220Rn concentrations that can be measured using this technique while maintaining an overall error no greater than about 30% is approximately 500 Bq m-3 (13 pCi L-1), assuming an approximately equal 222Rn concentration. The 220Rn measurement uncertainty at this concentration level decreases to approximately +/- 20% if at least three measurements sampled in series are arithmetically averaged. The uncertainty in the 220Rn measurement at low levels is proportionately affected by the relative concentration of 222Rn present.