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

T. Hayakawa·T. Shizuma·T. Kajino·S. Chiba·N. Shinohara·T. Nakagawa·T. Arima
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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,$γ$)$^{186}$Re$^m$(n,$γ$)$^{187}$Re. We measure a ratio of neutron capture cross-sections for the $^{185}$Re(n,$γ$)$^{186}$Re$^m$ and $^{185}$Re(n,$γ$)$^{186}$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.

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T. Hayakawa, T. Shizuma, T. Kajino, S. Chiba, N. Shinohara, T. Nakagawa, T. Arima. 2005-03-30. New s-process path and its implication to 187Re-187Os nucleo-cosmochronometer. https://doi.org/10.1086/430198

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