Standardization of ²²⁹Th (SRM 4328d)

A high-purity ²²⁹Th standard is needed as a tracer for alpha- and mass-spectrometry methods, such as ²³⁴U/²³⁰Th radiochronometry (Inn, et al., 2008), as well as for nuclear forensic analyses (Essex, et al., 2017). As a result, ²²⁹Th Standard Reference Materials (SRM) have been developed and disseminated by the National Institute of Standards and Technology (NIST) since the 1980s. The current standard (SRM 4328d) was preceded by four earlier issues, viz.: SRM 4328 in 1984; SRM 4328a in 1994; 4328b in 1996; and SRM 4328c in 2007.The certified massic activity for ²²⁹Th in SRM 4328d was obtained by 4παβ liquid scintillation (LS) counting by comparative measurements in 2022 against SRM 4328c. Both SRMs were prepared from the identical 2007 ²²⁹Th stock solution. SRM 4328c was standardized in 2007 by live-timed anticoincidence counting (a primary standardization method) with confirmatory measurements by four other methods (Fitzgerald, et al, 2010). The comparative measurements were performed using two different commercial LS counters and with LS counting sources prepared with two different cocktail compositions. The certified massic activity of ²²⁹Th in SRM 4328d, based on the decay-corrected certified value for SRM 4328c, is (41.19 ± 0.35) Bq·g^-1 at k = 2 as of 14 July 2022.

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Fitzgerald, R., Collé, R., Laureano-Pérez, L., Pibida, L., Hammond, M. M., Nour, S., & Zimmerman, B. E. (2010). A new primary standardization of 229Th. Applied Radiation and Isotopes, 7–8, 1303–1308. https://doi.org/10.1016/j.apradiso.2010.01.007

Inn, K. G. W., Kurosaki, H., Frechou, C., Gilligan, C., Jones, R., LaMont, S., Leggitt, J., Li, C., McCroan, K., & Swatski, R. (2008). A blueprint for radioanalytical metrology CRMs, intercomparisons, and PE. Applied Radiation and Isotopes, 6–7, 835–840. https://doi.org/10.1016/j.apradiso.2008.02.022