Neutron-capture cross sections for osmium isotopes and the age of the universe

Abstract

The neutron-capture cross sections for $^{186,187,188,189,190,192}\mathrm{Os}$ have been measured by the neutron-time-of-flight technique from 0.5 eV to 150 keV. Nuclear level spacings were extracted from the low-energy (resonance-region) data below \ensuremath{\sim}1 keV, and average cross sections from the data above 1 keV. The ratio of the Maxwellian-weighted average cross sections for $^{186}\mathrm{Os}$ and $^{187}\mathrm{Os}$ near 30 keV is a vital parameter for the determination of the duration of nucleosynthesis prior to the formation of the solar system, and thus for the determination of the age of the universe by the nuclear-dating technique. The present result of ${10.8}_{\ensuremath{-}2.2}^{+1.4}$\ifmmode\times\else\texttimes\fi{}${10}^{9}$ yr for the duration of nucleosynthesis is in concordance with the value obtained from U-Th dating. The result for the age of the universe (17\ifmmode\pm\else\textpm\fi{}3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ yr) agrees with the value obtained from the globular-cluster method, but clearly exceeds the most recent determination of the Hubble time.NUCLEAR REACTIONS $^{186,187,188,189,190,192}\mathrm{Os}(n,\ensuremath{\gamma})$, $E=2\ensuremath{-}1.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ eV; measured $\ensuremath{\sigma}(n,\ensuremath{\gamma})$, Maxwellian-averaged $\ensuremath{\sigma}(kT=30 \mathrm{KeV})$; extracted average level spacing; deduced age of universe for Re-Os clock.