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

Abstract

Background: The astrophysical $r$-process occurs in an explosive astrophysical event under extremely neutron-rich 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 $(α$,$x$n) reaction cross sections for the neutron-rich $^{86}$Se nucleus are studied in detail. Method: In a first step, the relevance of ($α$,n) and $(α$,$x$n) 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 $Z \rightarrow Z+2$ 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 ($α$,$x$n) 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.