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

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". U-238 is assigned a half-life of (4.4683 +/- 0.0096) Ga, i.e. a decay constant lambda(238) = (0.155125 +/- 0.000333) Ga-1. The coverage factor is k = 2 for this and all other estimates presented here. The U-238 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 U-235 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 alpha counting experiments. A provisional value for the U-234 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(U-234)/N(U-238) number-ratios of the natural samples used to estimate the U-235 half-life and those of the four samples used to estimate the U-234 half-life should be compounded into the standard measurement uncertainty of the latter. The resulting provisional uncertainty interval (k = 2) for the U-234 half-life is (244.55-247.77) ka, corresponding to lambda(234) = (2.8203-2.8344) Ma(-1). (C) 2015 Elsevier Ltd. All rights reserved.