Decay of Rhenium-187

The end-point energy for ${}_{}{}^{187}{}_{}{}^{}\mathrm{Re}$ has been determined to be 2.62±0.09 keV by use of ${\left({\mathrm{C}}_5{\mathrm{H}}_5\right)}_2$ReH vapor in a proportional-counter spectrometer. The ${}_{}{}^{187}{}_{}{}^{}\mathrm{Re}$ half-life determined from the proportional-counting experiments is (6.6±1.3)×${10}^{10}$ year. This is identified as the ${\beta }^{-}$ half-life for decay to continuum states only. When this is combined with the total half-life of (4.3±0.5)×${10}^{10}$ year obtained from ${}_{}{}^{187}{}_{}{}^{}\mathrm{Re}$/${}_{}{}^{187}{}_{}{}^{}\mathrm{Os}$ ratios in geologically dated minerals, the ratio of bound-state to continuum-state decays is found to be 0.5±0.3.