New s-process path and its implication to 187Re-187Os nucleo-cosmochronometer

Abstract

We study a new s-process path through an isomer of ${}^{186}$Re to improve a ${}^{187}$Re-${}^{187}$Os nucleo-cosmochronometer. The nucleus ${}^{187}$Re is produced by this new path of ${}^{185}$Re(n,$\gamma$)${}^{186}$Re${}^m$(n,$\gamma$)${}^{187}$Re. We measure a ratio of neutron capture cross-sections for the ${}^{185}$Re(n,$\gamma$)${}^{186}$Re${}^m$ and ${}^{185}$Re(n,$\gamma$)${}^{186}$Re${}^{gs}$ 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 $R_{th}$ = 0.54 $\pm$ 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 $R_{st}$ = 1.3 $\pm$ 0.8%. We calculate the s-process contribution from the new path in a steady-flow model. The additional abundance of ${}^{187}$Re through this path is estimated to be $N_s$ = 0.56 $\pm$ 0.35% relative to the abundance of ${}^{186}$Os. This additional increase of ${}^{187}$Re does not make any remarkable change in the ${}^{187}$Re-${}^{187}$Os 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.

Accepted in Astrophysical Journal, 10 pages, 6 figures

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• Frontiers in Nuclear Astrophysics
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• Astromers in the radioactive decay of r-process nuclei
• Tungsten isotopic compositions in stardust SiC grains from the Murchison meteorite: Constraints on the s-process in the Hf-Ta-W-Re-Os region
• Direct determination of the atomic mass difference of Re187 and Os187 for neutrino physics and cosmochronology
• Calculation of The Abundance of ${}^{187}$Re-${}^{187}$Os Nuclear Clock Nuclides in S-process and Sensitivity Analysis of Maxwellian-Averaged Neutron Capture Cross Sections