CCRI.RI(II)-K2.Pa-231 Key Comparison of activity measurements of the radionuclide 231Pa

Main text The introduction over the last decade of radionuclide therapy based on 223 Ra and 227 Th has reawakened interest in the radionuclides of the 235 U decay series (the 4n+3 decay chain). This has coincided with a requirement for improved accuracy in dating of long-lived radionuclides for nuclear forensic and for geological purposes. Thus, 231 Pa has become the subject of revived interest in recent years. The short-term ingrowth of the decay progeny is of interest to nuclear forensic science since it enables the direct calculation of the separation age of enriched 235 U [1] ; separation times based on the 234 U- 230 Th chain may also be calculated, but are more complex due to the reliance on the 238 U- 234 Th- 234m Pa- 234 U- 230 Th decay family. Furthermore, since protactinium fluorides are non-volatile at ordinary temperature, the build-up of 231 Pa in fuel enrichment facilities may provide information on throughput of separation units as well as the whole plant. In the longer term, the characterisation of sedimentation rates is facilitated by a range of natural nuclear chronometers that include 231 Pa/ 235 U to provide information concerning sediment formation, and the measurement of 231 Pa: 230 Th mass ratios (as well as 231 Pa: 235 U and 230 Th: 234 U mass ratios) may also provide information of global temperature trends over the 100-200 ka range [2] . This report summarises the results of an international comparison of the activity per unit mass of the same 231 Pa solution along with a new half-life determination [3] . To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/ . The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).