Direct and semi-direct capture in low-energy (,) reactions of neutron-rich tin isotopes and its implications for -process nucleosynthesis

中性子過剰スズ同位体の中性子捕獲反応における直接及び半直接過程と-過程元素合成における意義

千葉 敏; 小浦 寛之   ; 早川 岳人; 丸山 敏毅  ; 河野 俊彦*; 梶野 敏貴*

Chiba, Satoshi; Koura, Hiroyuki; Hayakawa, Takehito; Maruyama, Toshiki; Kawano, Toshihiko*; Kajino, Toshitaka*

天体物理で重要なエネルギー領域において、スズ同位体の直接及び半直接中性子捕獲断面積を計算して統計模型計算結果と比較した。統計模型は安定核近傍では主要な寄与を与えるが、Sn近傍のアイソトープにおいては急激に減少し、直接過程と逆転することがわかった。したがって直接過程は中性子捕獲天体核反応率に大きな影響を与える。逆転の起こる理由と、それが-過程元素合成に与える意義について議論する。

We calculate the direct and semi-direct (DSD) components of the neutron capture cross sections for a series of tin isotopes and compare them with the Hauser-Feshbach (HF) contribution in the energy region of astrophysical interests. It was found that the HF component, which is known to be dominant for isotopes in the vicinity of the stability line, drops off rapidly for isotope Sn and toward more neutron-rich ones, while the DSD component decreases only smoothly. As a consequence, the direct portion of the DSD component comes to give the largest contribution to the total (,) cross section below a few MeV. Therefore, the direct capture process modifies the astrophysical (,) reaction rates for these neutron-rich isotopes to a great extent. The reason of the difference in the isotopic dependence between the HF and DSD components is discussed, and its implication for the -process nucleosynthesis is given.