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

The activity concentrations of solutions containing Ac and Ra 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 Po 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 H as a tracer. For the Ac 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 Ac. We have identified a need for improved Ac decay data to achieve a significant reduction in the overall uncertainty. In the case of Ra, the activity concentrations were determined with relative standard uncertainties below 0.3%. Hence, PTB is prepared to provide calibration services for Ra, which is an isotope of increasing interest in nuclear medicine. The TDCR measurements were also used to determine the half-life of Ra. The decay was followed for about 58 days and a half-life T=11.4362(50)d was obtained.