Measurement of the 134Cs half-life in support of the exponential-decay law

The half-life of ¹³⁴Cs has been determined through 882 and 471 activity measurements of two sources over 12 years with a re-entrant ionisation chamber in a temperature-stabilised room. The data were aggregated from in total 204 565 and 99 486 ionisation current measurements with 0.057 % standard deviation. The decay curves adhere to the exponential-decay law, devoid of any cyclic perturbations at daily, monthly, and multi-annual scales, maintaining an exceptionally low standard deviation of 0.0003 % on the amplitude. A discernible annual cycle with an amplitude of 0.0051 (3) % was effectively mitigated to an insignificant level of 0.0009 (3) % by compensating the residuals proportionally to daily variations in ambient humidity. The study yielded a half-life value for ¹³⁴Cs of 754.200 (53) d or 2.06493 (14) a, with a relative standard uncertainty of 0.0070 %. The result obtained in this work can be recommended as a reference value, owing to its high statistical accuracy and conservative estimate of systematic errors.