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

Calculations of the decay widths of three-body resonances are considered using both R-matrix and dynamical three-body theoretical models. The R-matrix approach, which treats the three-body decay as two, ordered two-body decays, has both simultaneous and sequential particle emission pathways, each with an associated decay width. The question of how these two widths should be combined to determine the total resonance width is considered using comparisons with the width deduced from fully dynamical three-body model calculations. We use the decay of the well-understood 6 He(2 + , 1.8 MeV) resonance (into 4 He + n + n) as a benchmark case.

Abstract. The r-process is supposed to be a primary process which assembles heavy nuclei from a photo-dissociated nucleon gas. Hence, the reaction flow through light elements can be important as a constraint on the conditions for the r-process. We have studied the impact of di-neutron capture and the neutron-capture of light (Z 10) elements on r-process nucleosynthesis in three different environments: neutrino-driven winds in Type II supernovae; the prompt explosion of low mass supernovae; and neutron star mergers. Although the effect of di-neutron capture is not significant for the neutrino-driven wind model or low-mass supernovae, it becomes significant in the neutronstar merger model. The neutron-capture of light elements, which has been studied extensively for neutrino-driven wind models, also impacts the other two models. We show that it may be possible to identify the astrophysical site for the main r-process if the nuclear physics uncertainties in current r-process calculations could be reduced.