Determination of the activity of ²²⁵Ac and of the half-lives of ²¹³Po and ²²⁵Ac

The activity concentration of an ²²⁵Ac solution was determined by means of liquid scintillation counting using three custom-built TDCR counters. The efficiency calculation was carried out in the same way as it had been done in an earlier article on ²²⁹Th. The computation of the counting efficiency is rather complex and requires a correction to allow for the short-lived ²¹³Po. The experimental deadtime was varied to validate the correction. One of the TDCR counters is equipped with a CAEN N6751C digitizer for data acquisition. In addition, the system comprises a CeBr₃ solid scintillator as a gamma detector. The offline analysis was used to obtain a time-difference spectrum, using signals from the ²¹³Po γ-rays at about 440 keV in the gamma channel in coincidence with the preceding beta decay as the start signal, and signals from the subsequent (delayed) ²¹³Po alpha decays as the stop signal. After fitting an exponential function with a constant background, the half-life of ²¹³Po was determined to be 3.709(12) μs, which is in good agreement with the evaluated value. The half-life of ²²⁵Ac was determined from long-term measurements using an ionization chamber (IC) and a TDCR system. The combined result was found to be 9.9179(30) d, which is in agreement with the outcome from Pommé et al. (2012).