Role of (α, n) reactions under r-process conditions in neutrino-driven winds reexamined

Background: The astrophysical r-process occurs in an explosive astrophysical event under extremely neutron-rich conditions, leading to (n,gamma)-(gamma,n) equilibrium along isotopic chains which peaks around neutron separation energies of a few MeV. Nuclei with larger Z are usually produced by beta(-) decay, but under certain conditions also alpha-induced reactions may become relevant for the production of nuclei with Z + 2. Purpose: The uncertainties of the reaction rates of these alpha-induced reactions are discussed within the statistical model. As an example, alpha-induced (alpha, n) and (alpha, xn) reaction cross sections for the neutron-rich Se-86 nucleus are studied in detail. Method: In a first step, the relevance of (alpha, n) and (alpha, xn) reactions is analyzed. Next the uncertainties are determined from a variation of the a-nucleus potential which is the all-dominant parameter for the astrophysical Z -> Z + 2 reaction rate. Results: It is found that the r-process flow towards nuclei with larger Z is essentially influenced only by the alpha-nucleus potential whereas the other ingredients of the statistical model play a very minor role. This finding is based on the fact that the flow towards larger Z depends on the sum over all (alpha, xn) cross sections, which is practically identical to the total alpha-induced reaction cross section. Conclusions: alpha-nucleus potentials play an important role under certain r-process conditions because the flow towards larger Z depends sensitively on the total alpha-induced reaction cross section. The uncertainty of the reaction rate is about a factor of two to three at higher temperatures and exceeds one order of magnitude at very low temperatures.