Activity determination of 229Th by means of liquid scintillation counting

• The activity of a 229Th solution was determined by means of TDCR measurements.
• The relative standard uncertainty was found to be 0.23%.
• A new portable mini-TDCR system was successfully tested.
• Results are in good agreement with the outcome from alpha spectrometry.

Liquid scintillation measurements of 229Th 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 213Po requires great care, since it often occurs during the dead time of the counter systems. Also the rather short-lived 217At may decay during the dead time caused by 221Fr 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 225Ra or 225Ac in equilibrium with their progenies.