Half-life ofPu239by two independent methods

The ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ half-life was measured both by $\alpha$ counting with an intermediate geometry proportional counter and by mass spectrometric measurements of the rate of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ formation, using isotopic dilution with titrated ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ spikes. The plutonium concentration was measured by oxidation-reduction titration, using NBS-certified plutonium as a reference standard. The results from the two measurements were statistically consistent and their average gives for the ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ half-life the value 24131 ± 16 yr, corresponding to a specific activity of (1.3758 ± 0.0009) × ${10}^{11}$ dis/min per gram ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ (one standard deviation error). This result agrees with some recent measurements, but is in marked disagreement with a number of earlier ones.