Determination of 228Ra, 226Ra and 224Ra in natural water via adsorption on MnO2-coated discs

A fast procedure based on sorption of Ra on MnO2 coated polyamide discs is presented for determination of radium isotopes (i.e. Ra-228, Ra-226, Ra-224) in aqueous samples. The sample discs can be used directly for low-level a-spectrometry without the need for further separation and preparation methods to produce planar sample sources. While the activity of alpha -emitting Ra-224 and Ra-226 can be determined during a first measurement, beta -emitting Ra-228 is obtained via ingrowth of the progeny Th-228 On the same sample disc after a standing time of about six months. Calculations are presented for optimizing the analytical accuracy as well as for predicting the sorption yield or chemical recovery of radium on the sample disc as a function of exposure time because the sorption uptake proceeds with first-order kinetics. The analyses can be carried out on small samples of 0.5-11 and, for long counting times of one week and use of high-purity silicon surface barrier detectors, a detection limit of 0.15 mBq l(-1) is obtained for Ra-226. Since the half-life of Ra-224 is only 3.7 d and since Th-228 (as a measure for Ra-228) is built up only partially on the sample disc, a slightly higher detection limit of 0.24 mBq l(-1) results for the latter isotopes. The procedure is therefore sufficiently sensitive to allow the investigation of Ra isotope relationships in aquifers at typical environmental levels. (C) 2001 Elsevier Science Ltd. All rights reserved.