Determination of radon-222 and radium-226 in water samples by Cerenkov counting

Cerenkov radiation produced by the high-energy β-particles emitted from 214Bi, a daughter of 222Rn (and 226Ra), was utilized for the determination of 222Rn and 226Ra in water samples. For the determination of 222Rn the gas is extracted into an organic solvent and counted 2h after extraction using a liquid scintillation counter. For the determination of 226Ra a sample storage time of 25 d is necessary to allow secular equilibrium to be established. The effects of extraction procedures, storage conditions and the nature of the organic solvent were evaluated using standard 226Ra solutions. Toluene was found to be the most suitable solvent, both as an extractant and as a transparent medium for Cerenkov radiation. Minimizing the volume of the air space above the liquid phase and centrifuging the organic solvent after extraction led to a good recovery of radon and a clean extraction, a detection limit of 35.1 mBq l-1 being achieved. Some results for water samples in the UK are presented. (author)