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

The activity concentration of an 225 Ac 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 229 Th. The computation of the counting efficiency is rather complex and requires a correction to allow for the short-lived 213 Po. 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 CeBr 3 solid scintillator as a gamma detector. The offline analysis was used to obtain a time-difference spectrum, using signals from the 213 Po 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) 213 Po alpha decays as the stop signal. After fitting an exponential function with a constant background, the half-life of 213 Po was determined to be 3.709(12) μs, which is in good agreement with the evaluated value. The half-life of 225 Ac 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).