New s-process path and its implication to 187Re-187Os nucleo-cosmochronometer
Abstract
We study a new s-process path through an isomer of 186Re to improve a 187Re-187Os nucleo-cosmochronometer. The nucleus 187Re is produced by this new path of 185Re(n,γ)186Rem(n,γ)187Re. We measure a ratio of neutron capture cross-sections for the 185Re(n,γ)186Rem and 185Re(n,γ)186Regs reactions at thermal neutron energy because the ratio with the experimental uncertainty has not been reported. Using an activation method with reactor neutrons, we obtain the ratio of Rth = 0.54 0.11%. From this ratio we estimate the ratio of Maxwellian averaged cross sections in a typical s-process environment at kT = 30 keV with a help of the temperature dependence given in a statistical-model calculation because the energy dependence of the isomer/ground ratio is smaller than the absolute neutron capture cross-section. The ratio at kT=30 keV is estimated to be Rst = 1.3 0.8%. We calculate the s-process contribution from the new path in a steady-flow model. The additional abundance of 187Re through this path is estimated to be Ns = 0.56 0.35% relative to the abundance of 186Os. This additional increase of 187Re does not make any remarkable change in the 187Re-187Os chronometer for an age estimate of a primitive meteorite, which has recently been found to be affected strongly by a single supernova r-process episode.
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