Natural Uranium Radioactivity Solution Standard: SRM 4321d

Abstract

brian.zimmerman@nist.gov leticia.pibida@nist.gov denis.bergeron@nist.gov A new natural uranium solution standard has been produced and will be disseminated by the National Institute of Standards and Technology (NIST) as Standard Reference Material 4321d. The standard is certified for the massic activities of 234 U, 235 U, and 238 U in solution, and it is based on isotopic mass data for the metallic Certified Reference Material (CRM) 112-A (originally issued as SRM 960) that was obtained from THE U.S. Department of Energy, New Brunswick Laboratory. The metallic CRM was chemically cleaned, dissolved, and gravimetrically diluted to prepare a master solution, which was quantitatively dispensed into 5 mL aliquots that were contained within flame-sealed glass ampoules for each SRM unit. Homogeneity among SRM units, verifying solution homogeneity, was substantiated by photonic-emission integral counting with a NaI(Tl) well counter. Confirmatory measurements were performed by liquid scintillation counting for the total massic activity, and by isotope dilution α spectrometry for the 234 U and 238 U massic activities. 1 A Standard Reference Material® (SRM), trademarked and disseminated by NIST, is a certified reference material that is issued with a certificate that provides the value of the specified property, its associated uncertainty, and a statement of metrological traceability.The standardization of SRM 4321d for the massic activities of 234 U, 235 U, and 238 U is linked, both in terms of origin and certified values, to an isotopic mass standard, Certified Reference Material (CRM) 112-A [5], which was obtained from the U.S. Department of Energy, New Brunswick Laboratory (NBL). This CRM consists of a natural uranium metal bar having an assay purity of (99.975 ± 0.006) % by mass and a relative atomic mass of 238.028 918 ± 0.000 012, with certified values for atom percentages of 234 U, 235 U, and 238 U isotopes (given in the second column of Table 1), 2 as obtained from mass spectrometry. Each unit consists of a metal bar of nominal mass, ranging from 4 g to over 25 g, as listed on the container. The CRM has a significant amount of surface oxide that requires chemical cleaning before use.