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Report No.

Neutron induced reaction and Re/Os chronometer

Segawa, Mariko; Nagai, Yasuki*; Masaki, Tomohiro*; Temma, Yasuyuki*; Shima, Tatsushi*; Makii, Hiroyuki*; Mishima, Kenji*; Ueda, Hitoshi*; Nakayoshi, Akira*; Ota, Takeshi*; Igashira, Masayuki*; Nishiyama, Jun*; Goriely, S.*; Koning, A.*; Hilaire, S.*

$$^{187}$$Re-$$^{187}$$Os pair is known as the most promising nuclear cosmochronometer with considerable potential. However, there remains non-trivial problems related to an excited neutron capture reaction of $$^{187}$$Os at a stellar temperature. Firstly, $$^{187}$$Os is produced and depleted by the slow process nucleosynthesis. Hence, it is necessary to obtain both the production and depletion rates of $$^{187}$$Os. Secondly, $$^{187}$$Os is depleted not only by its ground state neutron capture reaction but also by an excited state neutron capture reaction of $$^{187}$$Os in a stellar temperature. In the present study, we have measured the neutron capture reaction cross sections for $$^{186}$$Os, $$^{187}$$Os and $$^{189}$$Os accurately and neutron inelastic scattering reaction cross section for $$^{187}$$Os off the ground state of $$^{187}$$Os to its 9.75-keV first excited statein the neutron energy range from 10 to 100 keV. The measurements of the neutron capture reactions have been carried out by detecting a prompt $$gamma$$-ray by means fan anti-Compton NaI(Tl) spectrometer. The (n, n') reaction cross section for $$^{187}$$Os has been measured with use of four $$^{6}$$Li-glass scintillation detectors. Based on these new capture cross sections, we reestimate on the basis of a careful reaction cross section calculation the correction factor F for the neutron capture on the 9.75-keV first excited state in $$^{187}$$Os as a function of stellar temperature, as required to derive the age of the galaxy within the Re-Os chronology.



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