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

The activity concentration of an Ac-225 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 Th-229. The computation of the counting efficiency is rather complex and requires a correction to allow for the short-lived Po-213. 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 Po-213 gamma-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) Po-213 alpha decays as the stop signal. After fitting an exponential function with a constant background, the half-life of Po-213 was determined to be 3.709(12) mu s, 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 Pomme et al. (2012).