Two-neutron capture reactions and the r process

Abstract

Rates for the He4(2n,γ)He6 and He6(α,n)Be9 reactions have been calculated, including both resonant and nonresonant contributions. The sequential two-neutron capture process on He4 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 He4(2n,γ)He6(α,n)Be9 may occur in competition with the bottle-neck three-body reactions He4(2α,γ)C12 and the He4(αn,γ)Be9 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. © 2006 The American Physical Society.