Determination of the activity and half-life of 227Th

Highlights: • The activity of ²²⁷Th was measured by means of liquid scintillation counting (LSC). • The solution was not in equilibrium with progeny. • The CIEMAT/NIST efficiency tracing method was applied. • A new method to correct for decay during the measurements is presented. • The half-life of ²²⁷Th was determined using LSC and an ionization chamber. - Abstract: Liquid scintillation samples with ²²⁷Th were prepared a few hours after the separation of the progeny. During the measurements, ²²⁷Th and its daughters are not in radioactive equilibrium. The counting efficiencies of the individual radionuclides of the decay chain differ from each other and the activity of an individual progeny relative to the activity of ²²⁷Th varies with time. Hence, the overall counting efficiency varies with time as well. The counting efficiency ${\epsilon }_{{}_{227}Th++}$ of ²²⁷Th and its progeny was determined by means of the CIEMAT/NIST efficiency tracing method. The free parameter is derived from the quench-indicating parameter, SQP(E), and from ³H tracer measurements. This makes it possible to compute the efficiency ${\epsilon }_{{}_{227}Th++}$ as a function of time. The individual efficiencies of all progeny are to be combined, taking correction factors and activity ratios into account. Thereby, a new, time-dependent correction, namely for the decay during the measurements, is applied. With this method, activity results are obtained that are stable over a long period of time. A least-squares method yields the time of the chemical separation as well as the ²²⁷Th half-life, which was also obtained by means of measurements in an ionization chamber. The weighted mean of the two methods (CIEMAT/NIST efficiency tracing and measurements in ionization chambers) was found to be T_1/2 = 18.681(9) d.