Determination of the activity and half-life of 227Th.

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 εT227h++ 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 εT227h++ 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 T1/2 = 18.681(9) d.