DETERMINATION OF RN-222 AND RA-226 IN WATER SAMPLES BY CERENKOV COUNTING

Cerenkov radiation produced by the high-energy beta-particles emitted from Bi-214, a daughter of Rn-222 (and Ra-226), was utilized for the determination of Rn-222 and Ra-226 in water samples. For the determination of Rn-222 the gas is extracted into an organic solvent and counted 2 h after extraction using a liquid scintillation counter. For the determination of Ra-226 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 Ra-226 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.