Procedure for simultaneous determination of Ra-223,Ra-224,Ra-226,Ra-228 byalpha and gamma spectrometry

The activities of the different radium isotopes in waters and hence their r adiotoxicities strongly depend on to which natural radioactive series they belong and on the chemical behaviour of certain of their radioactive predec essors. We have therefore developed a method to simultaneously determine Ra -223,Ra-224,Ra-226,Ra-228 activities in freshwater. The radium content is s electively extracted from two aliquots into which we divide each water samp le for analysis, with an interval of at least 3 days between the two separa tions. The Ra-228 activity and chemical yields are determined by gamma-spec trometric analysis of the precipitate. Alpha-spectrometric analysis of eith er of the two precipitates allows us to determine the activity of Ra-226 di rectly. The Ra-223,Ra-224 activities are indirectly obtained from two reaso nably acceptable hypotheses: first, that the integrals of Rn-222, Rn-220 an d Rn-219 alpha-emissions are the same as their respective daughters; second , that Ra-223 is in secular equilibrium with all its daughters. The propose d procedure gives detection limits around 0.1 mBq/l for the alpha-emitting isotopes of Ra, and therefore permits the detection of net activity levels in a large fraction of the freshwater samples analyzed.