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 neutronrich conditions, leading to (n,γ)-(γ,n) equilibrium along isotopic chains which peaks around neutron separation energies of a few MeV. Nuclei with larger Z are usually produced by β −-decay, but under certain conditions also α-induced reactions may become relevant for the production of nuclei with Z + 2. Purpose: The uncertainties of the reaction rates of these α-induced reactions are discussed within the statistical model. As an example, α-induced (α,n) and (α,xn) reaction cross sections for the neutron-rich 86Se nucleus are studied in detail. Method: In a first step, the relevance of (α,n) and (α,xn) reactions is analyzed. Next the uncertainties are determined from a variation of the α-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 α-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 (α,xn) cross sections which is practically identical to the total α-induced reaction cross section. Conclusions: α-nucleus potentials play an important role under certain r-process conditions because the flow towards larger Z depends sensitively on the total α-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