Abstract
During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of 16O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the 17O(α,n)20Ne reaction. The efficiency of this neutron recycling is determined by competition between the 17O(α,n)20Ne and 17O(α,γ)21Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies.The 17O(α,γ)21Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV Ecm, reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV Ecm have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that significant s-process nucleosynthesis occurs in low metallicity massive stars.
Highlights
IntroductionAlmost all the elements in the Universe heavier than iron are produced by neutron-capture reactions, either via the r-process (rapid neutron capture) or the s-process (slow neutron capture)
Almost all the elements in the Universe heavier than iron are produced by neutron-capture reactions, either via the r-process or the s-process.While significant uncertainties remain in r-process nucleosynthesis, the s-process is considered generally well understood
Most s-process elements between iron and strontium are thought to have been produced in massive stars, through the weak s-process, and those between strontium and lead via the main s-process in Asymptotic
Summary
Almost all the elements in the Universe heavier than iron are produced by neutron-capture reactions, either via the r-process (rapid neutron capture) or the s-process (slow neutron capture). This recycling of neutrons is determined by competition between the 17 O(α , n) Ne and 17 O(α , γ ) Ne reactions These reaction rates are highly uncertain at the relevant energies and the status of 16 O as a neutron poison, and the impact on sprocess abundances, is as yet undetermined. The second prediction comes from Descouvemont [8], using the Generator Coordinate Method, and suggests the ratio to be of the order of 10−4 at all energies This huge disagreement at low energies results in significant differences in the predicted sprocess abundances. Many resonances were observed and fitted using an R-matrix framework Using both datasets and estimates for the contribution from lower-lying states, Best et al [14] calculated new reaction rates and concluded that the (α , γ ) channel is strong enough to compete with the (α , n) channel leading to less efficient neutron recycling. Neither measurement had sufficient sensitivity to provide any experimental data in the energy region relevant to the s-process during the core helium burning stage
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