Determination of uranium series radionuclides Pa-231 and Ra-226 using liquid scintillation counting (LSC)

Methods were developed for determining naturally occurring alpha-active Pa-231 and Ra-226 isotopes in rock samples. The purpose of the measurements is to aid interpretation of uranium series data obtained in the context of analogue studies on radionuclide migration. Isotopes are separated from uranium and thorium by ion exchange chromatography and measured with a low-level, low-background liquid scintillation counter using pulse-shape analysis (PSA). The activity of Ra-226 is measured via its alpha-active daughter nuclide Po-214. PSA allows measurement of a yield traces of different decay mode at the same time: beta-active Pa- together with alpha-active Pa-231 and gamma-emitting Ba-133 with alpha-active Po-214. Simultaneous measurement ensures the reliability of the yield determination. Determination limits were 5.3 mBq for Pa-231 and 3.7 mBq for Ra-226. The limit for Pa-231 equals 0.12 Bq of U-238 in radioactive equilibrium. The disequilibrium between Pa-231 and U-235 in a sample can be distinguished from equilibrium when the disturbance causing the disequilibrium is not older than 120 000 years. Corresponding time for the Ra-226/U-238 activity ratio is about 6500 years. In migration analogue studies around uranium deposits, where concentrations of these isotopes are well above the determination limits, the methods can be applied to obtain information for interpreting uranium series data. (orig.)