IUPAC-IUGS status report on the half-lives of 238U, 235U and 234U

Abstract

The current state of knowledge on the half-lives of the long-lived U radionuclides has been reviewed by the IUPAC-IUGS joint Task Group “Isotopes in Geosciences”. 238U is assigned a half-life of (4.4683±0.0096)Ga, i.e. a decay constant λ238=(0.155125±0.000333)Ga−1. The coverage factor is k=2 for this and all other estimates presented here. The 238U half-life can be used as a reference for the half-lives/decay constants of all other isotopic geochronometers.A revision of the half-life of 235U based on intercomparison of natural geological samples is premature. The improved repeatability of mass spectrometric measurements has revealed Type B uncertainties that had been dismissed as subordinate in the past. The combined uncertainty of these as yet incompletely charted and quantified sources of Type B uncertainty may be no smaller than the currently accepted uncertainty of the α counting experiments.A provisional value for the 234U half-life can be calculated with the assumption of secular equilibrium in the analyzed natural samples. This assumption has not yet been verified independently and its metrological traceability appears sub-optimum. A Type B evaluation suggests that the ca. 0.17% offset between the N(234U)/N(238U) number-ratios of the natural samples used to estimate the 235U half-life and those of the four samples used to estimate the 234U half-life should be compounded into the standard measurement uncertainty of the latter. The resulting provisional uncertainty interval (k=2) for the 234U half-life is (244.55–247.77)ka, corresponding to λ234=(2.8203–2.8344)Ma−1.