Rates for the ${}^4\mathrm{He}$$(2n,\gamma )$${}^6\mathrm{He}$ and ${}^6\mathrm{He}$$(\alpha ,n)$${}^9\mathrm{Be}$ reactions have been calculated, including both resonant and nonresonant contributions. The sequential two-neutron capture process on ${}^4\mathrm{He}$ has also been reevaluated on the basis of new experimental results. It is shown that a one-step dineutron capture reaction may enhance the sequential two-neutron reaction rate by several orders of magnitude. This opens the possibility that reaction flow through ${}^4\mathrm{He}$$({}^{2}n,\gamma )$${}^6\mathrm{He}$$(\alpha ,n)$${}^9\mathrm{Be}$ may occur in competition with the bottle-neck three-body reactions ${}^4\mathrm{He}$(2$\alpha ,\gamma )$${}^{12}\mathrm{C}$ and the ${}^4\mathrm{He}$$(\alpha n,\gamma )$${}^9\mathrm{Be}$ that initiate the α process and provide seed nuclei for the $r$ process. Here we explore the effect of such dineutron capture on $r$-process nucleosynthesis. We show that such reactions have little effect on the final abundance and would change only $r$-process abundances in an extremely neutron-rich low-temperature $r$ process.

• Received 22 February 2005

DOI:https://doi.org/10.1103/PhysRevC.74.015802