Two-Neutron Capture Reactions And The R Process

Rates for the He-4(2n,gamma)He-6 and He-6(alpha,n)Be-9 reactions have been calculated, including both resonant and nonresonant contributions. The sequential two-neutron capture process on He-4 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 He-4((2)n,gamma)He-6(alpha,n)Be-9 may occur in competition with the bottle-neck three-body reactions He-4(2 alpha,gamma)C-12 and the He-4(alpha n,gamma)Be-9 that initiate the alpha 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.