Experimental Determination of α Widths of Ne21 Levels in the Region of Astrophysical Interest: New O17+α Reaction Rates and Impact on the Weak s Process

The efficiency of the weak s process in low-metallicity rotating massive stars depends strongly on the rates of the competing O17(α,n)Ne20 and O17(α,γ)Ne21 reactions that determine the potency of the O16 neutron poison. Their reaction rates are poorly known in the astrophysical energy range of interest for core helium burning in massive stars because of the lack of spectroscopic information (partial widths, spin parities) for the relevant states in the compound nucleus Ne21. In this Letter, we report on the first experimental determination of the α-particle spectroscopic factors and partial widths of these states using the O17(Li7,t)Ne21 α-transfer reaction. With these the O17(α,n)Ne20 and O17(α,γ)Ne21 reaction rates were evaluated with uncertainties reduced by a factor more than 3 with respect to previous evaluations and the present O17(α,n)Ne20 reaction rate is more than 20 times larger. The present (α,n)/(α,γ) rate ratio favors neutron recycling and suggests an enhancement of the weak s process in the Zr-Nd region by more than 1.5 dex in metal-poor rotating massive stars.