New determination of the 229 Th half-life.

A new determination of the ²²⁹ Th half-life was made based on measurements of the ²²⁹ Th massic activity of a high-purity solution for which the ²²⁹ Th molality had previously been measured. The ²²⁹ Th massic activity was measured by direct comparison with SRM 4328C using 4παβ liquid scintillation counting, NaI counting, and standard addition liquid scintillation counting. The massic activity was confirmed by isotope dilution alpha spectrometry measurements. The calculated ²²⁹ Th half-life is (7825 ± 87) years ( k = 2), which is shorter than the three most recent half-life determinations but is consistent with these values within uncertainties.