SEPARATION OF RA AND TH FROM ROCK MATRICES FOR ALPHA-SPECTROMETRY

Dating rocks using Ra-226/Th-230 isochrons demands radiochemical purif ication of radium and thorium. This paper presents an improved method. Rocks are solubilised by nitric/hydrofluoric digestion followed by di ssolution of insoluble fluorides by boric/nitric acids, and Th is extr acted by passing the 8M nitric solution through an anion column in nit rate form. The eluant contains Ra and Ba which are precipitated as sul phate and redissolved in alkaline EDTA. Complete separation of Ba and Ra from sulphate is on an AG 1 x 8 anion column in EDTA form, which wa s found better than the chloride form. The Ba+Ra is separated on an AG 50W x 8 cation column, and uses CDTA as an eluant for Be. Careful pH control is essential. Ra elutes later with EDTA or 4M HCl and is preci pitated with 125 mu g barium as sulphate to yield a source suitable fo r alpha-spectrometry, or further treated to electroplate the Ra Tire y ield tracers used are Th-228 and Ra-224. Because the sample contains n atural Ra-224 a correction must be applied, calculated from the amount of natural Th-232, in the Th spectrum. Th may be precipitated with 10 0 mu g of ferric iron and gives a spectrometry-quality source, but fur ther purification and electrodeposition was found to be preferable. Va riations on the method for the case of analysis of calcium-rich fish o toliths are described.