Direct laboratory determination of the ¹⁸⁷Re half-life

Abstract

The long-lived, naturally occurring radionuclide ¹⁸⁷Re is important in geochemistry and cosmology as a nucleochronometer. Until now there have been no direct laboratory measurements which have avoided the difficulties of both low-energy β-counting and dependence on radiometric ages of rocks and meteorites. We report here a half-life of (4.35 ± 0.13) × 10¹⁰ yr, based on the growth of ¹⁸⁷Os over a 4-yr period into a large source of osmium-free rhenium. Since our result agrees with the best geochemically determined values, no significant revision of the present galactic age limits based on the geochemical values is necessary. The agreement at the 5% level of the decay rates determined on laboratory and meteorite-age timescales places a tight constraint on cosmological models in which fundamental ‘constants’ are allowed to vary. In particular, the fine-structure constant, α, must have changed by less than one part in 10⁵ over the past 4.5 × 10⁹ yr.