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Os同位体の中性子反応とRe/Os宇宙核時計

Neutron induced reaction and Re/Os chronometer

瀬川 麻里子; 永井 泰樹*; 正木 智洋*; 天満 康之*; 嶋 達志*; 牧井 宏之*; 三島 賢二*; 上田 仁*; 仲吉 彬*; 太田 岳史*; 井頭 政之*; 西山 潤*; Goriely, S.*; Koning, A.*; Hilaire, S.*

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.*

宇宙の年齢を決定する最も誤差の少ない方法の一つであるRe/Os核時計を核時計として用いるにはベータ崩壊でのみ生成された娘核$$^{187}$$Osの存在量を知る必要があるため、$$^{186}$$Os(n,$$gamma$$)及び$$^{187}$$Os(n,$$gamma$$)反応断面積を精度よく求める必要がある。加えて、$$^{187}$$Osは星の中で第一励起状態($$^{187}$$Os')からも中性子捕獲を起こし存在量を減少させるため、この効果を見積もる理論模型の精度向上が急務となっている。そこでわれわれは上記の実験に加え次の二つの実験により得ることで、理論模型に制限を加えることに成功した。(1)$$^{187}$$Os非弾性散乱断面積測定,(2)$$^{187}$$Osと核構造が類似している$$^{189}$$Osの(n,$$gamma$$)反応断面積測定。核の類似性についてはJRR-3中性子源を用いて確認した。本講演では$$^{186,187,189}$$Os(n,$$gamma$$)反応断面積及び$$^{187}$$Os(n,n')反応断面積の実験結果と、これらの実験値と精度よく一致した理論模型による$$^{187}$$Os'(n,$$gamma$$)反応断面積値及び宇宙年齢導出の精度向上に貢献した結果を示す。

$$^{187}$$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 1$$^{187}$$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}$$Os has been measured with use of four 6Li-glass scintillation detectors. Based on these new capture cross sections, we reestimate on the basis of a careful reaction cross section calculation the correction factor F for the neutron capture on the 9.75-keV first excited state in $$^{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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