Experimental Determination of 𝛼 Widths of 21 Ne Levels in the Region of Astrophysical Interest: New 17 O +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 17 O (𝛼,𝑛) 20 Ne and 17 O (𝛼,𝛾) 21 Ne reactions that determine the potency of the 16 O 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 21 Ne . In this Letter, we report on the first experimental determination of the 𝛼-particle spectroscopic factors and partial widths of these states using the 17 O ( 7 Li, 𝑡) 21 Ne 𝛼-transfer reaction. With these the 17 O (𝛼, 𝑛) 20 Ne and 17 O (𝛼, 𝛾) 21 Ne reaction rates were evaluated with uncertainties reduced by a factor more than 3 with respect to previous evaluations and the present 17 O (𝛼, 𝑛) 20 Ne reaction rate is more than 20 times larger. Finally, 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.