A Compendium on the NIST Radionuclidic Assays of the Massic Activity of (63)Ni and (55)Fe Solutions Used for an International Intercomparison of Liquid Scintillation Spectrometry Techniques

The National Institute of Standards and Technology recently participated in an international measurement intercomparison for (63)Ni and (55)Fe, which was conducted amongst principal national radionuclidic metrology laboratories. The intercomparison was sponsored by EUROMET, and was primarily intended to evaluate the capabilities of liquid scintillation (LS) spectrometry techniques for standardizing nuclides that decay by low-energy β-emission (like (63)Ni) and by low-Z (atomic number) electron capture (like (55)Fe). The intercomparison findings exhibit a very good agreement for (63)Ni amongst the various participating laboratories, including that for NIST, which suggests that the presently invoked LS methodologies are very capable of providing internationally-compatible standardizations for low-energy β-emitters. The results for (55)Fe are in considerably poorer agreement, and demonstrated the existence of several unresolved problems. It has thus become apparent that there is a need for the various international laboratories to conduct rigorous, systematic evaluations of their LS capabilities in assaying radionuclides that decay by low-Z electron capture.