Activity determination of 227Ac and 223Ra by means of liquid scintillation counting and determination of nuclear decay data

Abstract

The activity concentrations of solutions containing 227Ac and 223Ra in equilibrium with their progenies, respectively, were measured by means of liquid scintillation counting. The counting efficiencies were determined with the aid of a free parameter model. The corresponding calculations comprise the computation of several alpha, beta and beta/gamma branches. For short-lived progenies like 215Po the counting efficiency depends on the counter dead time. Measurements were made in custom-built triple-to-double coincidence ratio (TDCR) systems and various dead-time adjustments were used. In addition, two commercial counters were used to apply the CIEMAT/NIST efficiency tracing technique using 3H as a tracer.For the 227Ac solution, the overall relative standard uncertainty of the activity concentration was found to be 0.93%. The dominant uncertainty components are assigned to the efficiency computation of the low-energy beta transitions of 227Ac. We have identified a need for improved 227Ac decay data to achieve a significant reduction in the overall uncertainty.In the case of 223Ra, the activity concentrations were determined with relative standard uncertainties below 0.3%. Hence, PTB is prepared to provide calibration services for 223Ra, which is an isotope of increasing interest in nuclear medicine.The TDCR measurements were also used to determine the half-life of 223Ra. The decay was followed for about 58 days and a half-life T1/2=11.4362(50)d was obtained.