PREPARATION AND CERTIFICATION OF NEW THORIUM ISOTOPIC REFERENCE MATERIALS

Preparation of two Isotopic Reference Materials of thorium has been pe rformed, starting from highly enriched Th-232 (99.99%) and Th-230 (99. 8%). After full characterization (chemical and isotopic) of these puri fied base materials the thorium nitrate was transformed to thorium dio xide. Accurately weighed amounts of the two isotopes in the dioxide fo rm were subsequently dissolved in nitric acid and resulted in solution s with amount ratios n(Th-230)/n(Th-232) close to 10(-5) (IRMM-035) an d 3 . 10(-6) (IRMM-036). These gravimetrically prepared ratios were fi nally verified by means of Thermal Ionization Mass Spectrometry (TIMS) . The purpose of the Reference Materials is to calibrate thorium isoto pe amount ratio measurements.