Calibration of an ionisation chamber for parent–progeny decay

Abstract

Abstract For a single radionuclide being measured in an ionisation chamber, a calibration factor can be established that relates the ionisation current to the source activity. The same applies to a decay chain in secular equilibrium, for which the calibration factor comprises the ionisation current produced by the parent and progeny nuclei combined. The calibration of an ionisation chamber for non-equilibrated parent–progeny decay poses a problem because the activity ratio of the parent and progeny nuclei varies with time. This study examines cases in which the half-lives of the parent and only one of the progenies in the decay series are significantly long. Thus, two calibration factors are involved, which combine differently as a function of time. By means of nuclear dating of the material, the parent–progeny activity ratio can be determined, and the respective contributions to the ionisation current can be unambiguously distinguished. Once the ionisation chamber is calibrated, two measurements taken at different times are sufficient to determine the activity and age of the parent–progeny mixture. This study presents equations for calculating the calibration factors and propagating uncertainties, illustrated with a case study focusing on the 227Th/223Ra decay chain used in alpha-immunotherapy.