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

We study a new s-process path through an isomer of Re to improve a Re-Os nucleo-cosmochronometer. The nucleus Re is produced by this new path of Re(n,γ)Re(n,γ)Re. We measure a ratio of neutron capture cross-sections for the Re(n,γ)Re and Re(n,γ)Re 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 Re through this path is estimated to be Ns = 0.56 ± 0.35% relative to the abundance of Os. This additional increase of Re does not make any remarkable change in the Re-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. Advanced Photon Research Center, Japan Atomic Energy Research Institute, Kizu, Kyoto 619-0215, Japan. National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan. Department of Astronomy, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan. Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan. Tohoku University, Sendai 980-8578, Japan.