DETERMINATION OF URANIUM, THORIUM AND RADIUM FROM GEOLOGICAL-MATERIALS BY MICROPRECIPITATION AND ALPHA-SPECTROSCOPY

For analysing the isotopic composition of U, Th and Ra contents, amounts of 0.2-2 g of the rock materials or sediment samples are leached with half concentrated HNO3 and the insoluble residues are treated with HF and fused with Na2CO3. Chemical separations are performed by anion exchange of U on Dowex 1 X 2 (8) and subsequently by cation exchange of Ra, Ac (+ Ca) and Th on Dowex 50 X 4. Samples for alpha-spectrometry can be prepared by microprecipitation of U and Th with NdF3 and of Ra with BaSO4. Chemical recoveries of the applied procedures have been determined using U-232 and Th-229 as a spike. To obtain the Ra-226 recoveries, Ra-224 has been taken as an internal standard assuming equilibrium between Th-228 and its daughter, or Ra-225 added in equilibrium together with the Th-229 spike has been evaluated via At-217. Investigations on the leachable and the insoluble fractions show that at least the rock materials have to be quantitatively dissolved to get reliable data about the radionuclide content.