Activity determination of ²³¹Pa by means of liquid scintillation counting

Protactinium-231 decays through a chain of daughter nuclides to stable ²⁰⁷Pb, resulting in a number of alpha and beta transitions. A method based on liquid scintillation (LS) counting was developed to determine the activity concentration of ²³¹Pa. For α emissions, the corresponding LS counting efficiency was assumed to be unity. A potential loss of the noble gas ²¹⁹Rn was evaluated by an additional experiment. In the case of the short-lived ²¹⁵Po, a dead-time-dependent correction factor was taken into account in the calculation. For beta emitters ²²⁷Ac, ²²³Fr, ²¹¹Pb and ²⁰⁷Tl, the counting efficiencies were determined by the CIEMAT/NIST efficiency tracing (CNET) method with ³H standard as a tracer. The impurities were checked and found that the possible impurity was ²²⁷Ac, with an initial activity ratio A(²²⁷Ac)/A (²³¹Pa) between 0.5% and 0.8%. Finally, the activity concentration of the ²³¹Pa solution was determined with a combined relative standard uncertainty of 0.39%.