The low energy (Eβmax = 66.945 keV ± 0.004 keV) β-emitter 63Ni has become increasingly important in the field of radionuclidic metrology. In addition to having a low β-endpoint energy, the relatively long half-life (101.1 a ± 1.4 a) makes it an appealing standard for such applications. This paper describes the recent preparation and calibration of a new solution Standard Reference Material of 63Ni, SRM 4226C, released by the National Institute of Standards and Technology. The massic activity CA for these standards was determined using 4πβ liquid scintillation (LS) spectrometry with 3H-standard efficiency tracing using the CIEMAT/NIST method, and is certified as 50.53 kBq ·g−1 ± 0.46 Bq · g−1 at the reference time of 1200 EST August 15, 1995. The uncertainty given is the expanded (coverage factor k = 2 and thus a 2 standard deviation estimate) uncertainty based on the evaluation of 28 different uncertainty components. These components were evaluated on the basis of an exhaustive number (976) of LS counting measurements investigating over 15 variables. Through the study of these variables it was found that LS cocktail water mass fraction and ion concentration play important roles in cocktail stability and consistency of counting results. The results of all of these experiments are discussed.

The National Institute of Standards and Technology recently participated in an international measurement intercomparison for 63Ni and 55Fe, 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 63Ni) and by low-Z (atomic number) electron capture (like 55Fe). The intercomparison findings exhibit a very good agreement for 63Ni 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 55Fe 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.