Application of Cerenkov radiation for the assay of Ra-226 in natural water

A Cerenkov based method has been developed for the determination of Ra-226 via its beta-emitting daughters, Bi-214 and Pb-214. Radium isotopes are separated from interfering elements by coprecipitation on barium sulphate. The precipitate is dissolved in an alkaline solution of EDTA and the Cerenkov signal produced by the build-up of daughter beta activities is counted after 25 days using the tritium channel of a liquid scintillation counter. To correct for any contribution from the daughters of the radium isotopes Ra-224 and Ra-228 present in the sample, the barium(radium) sulphate is reprecipitated, dissolved in EDTA, and counted 2 days later. Using this procedure, Ra-228 can be determined in the same sample. Cerenkov counting efficiency was found to be 77.25%. A lower limit of detection of 17.4 mBq l(-1) (based on 3 sigma of the background with 500 min counting time) was achieved. Any liquid scintillation counter can be used. Chemical recoveries in the range 75-95% were determined gravimetrically. Data from both artificial spiked samples and natural samples are presented.