Standardization of ⁹⁰Sr (SRM 4329a)

A new standardization of ⁹⁰Sr was completed. Strontium-90 is produced by nuclear fission, with a half-life of 28.8 years. It undergoes β⁻ decay into yttrium-90 which in turn undergoes β⁻ decay with a half-life of 2.6684 days. Strontium-90 has applications in medicine; as a radioactive source for superficial radiotherapy of some cancers; and industry as a radioactive source for thickness gauges. It is an isotope of concern in fallout from nuclear weapons, nuclear weapons testing, and nuclear accidents, hence, a good determination is necessary.  The certified massic activity for ⁹⁰Sr in radioactive equilibrium with ⁹⁰Y was obtained by 4pb liquid scintillation (LS) spectrometry with two commercial LS counters. The LS detection efficiency was calculated using the MICELLE2 code for the CIEMAT/NIST method with composition matched LS cocktails of a ³H standard as the efficiency detection monitor and two different LS cocktail compositions. The combined standard uncertainty (k = 2) on the standardization is 0.30 %. This solution is gravimetrically linked to a ⁹⁰Sr stock solution (SRM 4234a) used to prepare the previous NIST ⁹⁰Sr standard SRM 4239. A remeasurement of SRM 4239 at the time of this work in 2020 agreed with the 2008 certified value to within 0.2 %. This determination agrees to the decay corrected SRM 4234a certified value to within 0.002 %.