Revisiting the activity standardisation of ¹⁷⁷Lu

Two years ago, we published a paper on our laboratory's first activity standardisation of ¹⁷⁷Lu using the β-γ coincidence technique as well as the TDCR method. The coincidence measurements we carried out with plastic scintillators for beta detection predicted activities 2 % lower and discrepant with those determined with the TDCR method. Similar discrepancies between coincidence counting with solid sources and liquid scintillation measurements had also been reported earlier elsewhere. As part of an international comparison for the measurement of the activity of this radionuclide, we revisit this problem here. A¹⁷⁷Lu solution is standardised using the β-γ coincidence technique, with both plastic and liquid scintillation beta detection, together with the TDCR and CNET methods. Coincidence counting measurements with plastic sources, in this standardisation, explored a higher efficiency range and showed that the efficiency functions behave differently at high and low efficiencies. Extrapolating the efficiency with either a linear function in the high efficiency range, or with a non-linear curve throughout the efficiency span, yield activities that agree with liquid scintillation counting determinations, thus resolving the discrepancy observed in our first standardisation of this nuclide.