Activity determination of ²²⁹Th by means of liquid scintillation counting.

Liquid scintillation measurements of (229)Th in radioactive equilibrium with its progenies were carried out. The counting efficiency was determined by means of a free parameter model. The measurements were made in a custom-built triple-to-double coincidence ratio (TDCR) system. In addition, a new portable mini-TDCR system with three channel photomultipliers and two commercial counters were tested. The decay of the short-lived (213)Po requires great care, since it often occurs during the dead time of the counter systems. Also the rather short-lived (217)At may decay during the dead time caused by (221)Fr decay events. The overall counting efficiency of the TDCR system of PTB was found to be about 700% (depending on the degree of chemical quenching) and the relative standard uncertainty of the activity concentration was found to be about 0.23%. The determined activity concentration was compared with the outcome of alpha spectrometry under defined solid angle and excellent agreement was found. The TDCR efficiency calculations can be easily adapted to activity determinations of (225)Ra or (225)Ac in equilibrium with their progenies.