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

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". 238 U is assigned a half-life of (4.4683±0.0096)Ga, i.e. a decay constant λ 238 =(0.155125±0.000333)Ga -1 . The coverage factor is k=2 for this and all other estimates presented here. The 238 U 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 235 U 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 234 U 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( 234 U)/N( 238 U) number-ratios of the natural samples used to estimate the 235 U half-life and those of the four samples used to estimate the 234 U half-life should be compounded into the standard measurement uncertainty of the latter. The resulting provisional uncertainty interval (k=2) for the 234 U half-life is (244.55-247.77)ka, corresponding to λ 234 =(2.8203-2.8344)Ma -1 .