By Séan Collins — One of the missions of PRISMAP is to provide accurate and precise standards and decay data that will underpin the development of the new generation of radionuclides that the consortium is making available for research and development. For any activity determination of a radionuclide, a critical component is the ability to define the activity at a point in time without which we cannot know the activity accurately. This is an important consideration throughout the lifecycle, whether during the production and purification of the radionuclide to when determining the activity of a radiopharmaceutical being administered to a patient. A starting point for providing an accurate standard for a radionuclide is to ensure that the radioactive half-life being used is accurate and precise, without which how can we have confidence in the standard or if any deviations are due to radioactive impurities?
In the ongoing efforts at NPL and CHUV’s Institut de radiophysique (IRA) to provide traceability to the terbium theranostic quartet of Tb-149, Tb-152, Tb-155, Tb-161, and other radionuclides provided by PRISMAP, we have investigated these half-lives. Many of the half-lives for these radionuclides have not been measured since the 1960s or 1970s and their relative uncertainties are not conducive to providing suitably precise traceable standards. Using multiple independent techniques, including ionisation chamber measurements, gamma-ray spectrometry and liquid scintillation counting, the radioactive decay rate has been followed over many half-lives. As radionuclide contaminants can be present in these samples, careful consideration of their influence must be made, along with many other factors that can impose biases in the determined half-life.
In these investigations, we have found large differences between those published in previous studies. For example, in the case of Tb-161 we determined a half-life that was significantly longer than the currently evaluated value whilst for Tb-155 a half-life was determined that was 1.6 % shorter than previous studies. Making use of the high-purity samples being provided PRISMAP we have been able to determine these half-lives with high precision, with improvements of almost an order of magnitude over those found in nuclear data evaluation databases.
These half-lives being determined in PRISMAP provide new accurate and precise data that will underpin standards being produced by NPL and IRA, and provide confidence in the activity measurements made for these radionuclides by all.
Comparison of the uncertainty propagation of the standard uncertainty of the half-life for 155Tb, and 161Tb to the uncertainty of the activity, over approximately five half-lives, from the evaluated half-lives (black line) and those reported in this work (red line).