On-site activity determination of ²¹¹At using absolute 4pi-alpha liquid scintillation counting and HPGe

For accurate activity measurements and to comply with EU regulations traceability is required. For alpha emitters of interest for emerging targeted alpha therapies (TAT), like ²¹¹At, standardised samples are not readily available. The use of gamma spectrometry can come with high uncertainties due to its reliance on decay data for gamma emissions. A new method for determining the activity of ²¹¹At in a clinical setting is proposed, i.e. on-site measurement, using absolute 4pi-alpha liquid scintillation counting (LSC) with thresholding on a portable TDCR counter. The method focuses on measuring only the alpha particles and not the electrons, which emission intensities come with a higher uncertainty. Using the absolute LSC method the relative expanded measurement uncertainty was 2.2 % (k = 2). A comparison using gamma spectrometry and the more readily available high purity germanium (HPGe) detector, the relative expanded uncertainty was 12 % (k = 2). The largest contribution to the much higher uncertainty of the gamma spectrometry comes from the uncertainties in the decay data. Using a portable TDCR-detector is a viable approach to establishing traceability for ²¹¹At, where transportation to a national metrology lab is not feasible. With better decay data the uncertainty of the gamma spectrometry can be reduced by at least one third, which would make it a better alternative when absolute measurements are not available.