Authors Referees APS Members |
January 2008, to be published in Physical Review C
Direct and semi-direct capture in low-energy ($n$,$\gamma$) reactions of
neutron-rich tin isotopes and its implications for $r$-process nucleosynthesis
The direct and semi-direct (DSD) components of the neutron capture cross sections are calculated for a series of Tin isotopes by employing a single-particle potential (SPP) that gives a good reproduction of the known single-particle energies (SPE) over a wide mass region. The results are compared with the Hauser-Feshbach (HF) contribution in the energy region of astrophysical interests. The calculated result shows that the HF component drops off rapidly for isotope $^{132}$Sn and toward more neutron-rich ones, while the DSD component decreases only smoothly and eventually become dominant. This result is consistent with those of previous studies, but the dependence of the DSD cross section on the target mass number is a feature of the present SPP that gives a smooth variation of SPE's. As a consequence, the direct portion of the DSD component comes to give the largest contribution to the total (n,$\gamma$) cross section for neutron-rich isotopes below a few MeV. Therefore, the direct capture process modifies the astrophysical (n,$\gamma$) reaction rates to a great extent. The semi-direct component, however, gives a negligible contribution to the astrophysical reaction rates, while its impact is significant above several MeV. The reason of the difference in the isotopic dependence between the HF and DSD components is discussed, and its implication for the $r$-process nucleosynthesis is given. © 2008 The American Physical Society.
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