Measurement of the 22Na half-life and evidence supporting the exponential-decay law

The half-life of ²²Na has been determined through 523 activity measurements over 14.5 years with a re-entrant ionisation chamber in a temperature-stabilised room. The data were selected and aggregated from in total 127 830 ionisation current measurements with a reproducibility of the order of 0.065 % standard deviation. The ionisation current was collected over an air capacitor and measured as a change of voltage over time, which warrants excellent linearity with activity throughout the experiment. The residuals to an exponential decay curve show a distinct annual cycle of 0.0055 (5) % amplitude, which reduces to 0.0012 (5) % after compensation for a correlation with ambient humidity. The data confirm the validity of the exponential-decay law, in absence of cyclic perturbations at daily, monthly, and multi-annual scale at the level of 0.0005 % standard deviation. The ²²Na half-life value obtained in this study is 950.68 (12) d or 2.60290 (34) a, with a relative standard uncertainty of 0.013 %. This value is consistent with the mean value of other measurement results in the literature, although the latter have an issue with incomplete uncertainty budgets. Owing to the high statistical accuracy and conservative uncertainty estimate of systematic errors, the result obtained in this work can be recommended as a reliable reference value.