Decay properties of 22Ne + α resonances and their impact on s-process nucleosynthesis

The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the ²²Ne(α,n)²⁵Mg reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing ²²Ne(α,γ)²⁶Mg reaction, is not well constrained in the important temperature regime from ∼0.2–0.4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the reactions. We have established a new n/γ decay branching ratio of 1.14 for the key 11.32 MeV resonance in ²⁶Mg, which results in a new (α,n) strength for this resonance of 42 (11) μeV when combined with the well-established (a,γ) strength of this resonance. Monte-Carlo calculations of the stellar and rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ∼0.2–0.4 GK. Using our new ²²Ne(α,n)²⁵Mg and ²²Ne(α,γ)²⁶Mg reaction rates, we performed new s-process calculations for massive stars and asymptotic giant branch stars and compared the resulting abundances with the abundances obtained using other ²²Ne+α rates from the literature. We observe an impact on the s-process abundances up to a factor of three for intermediate-mass AGB stars and up to a factor of ten for massive stars.

The work at Brookhaven National Laboratory was sponsored by the Office of Nuclear Physics, Office of Science of the U.S. Department of Energy under Contract No.DE-AC02- 98CH10886 with Brookhaven Science Associates, LLC.