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

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

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.