Stellar He burning of ¹⁸O: A measurement of low-energy resonances and their astrophysical implications

The ²²Ne(α,n) reaction is the main neutron source for neutron capture nucleosynthesis (s process) in massive stars and plays also a significant role for the s process in thermally pulsing asymptotic giant branch stars. In these scenarios, ²²Ne is produced by the reaction sequence ¹⁴N(α,γ)¹⁸F(β⁺)¹⁸O(α,γ)²²Ne. While the first reaction is well understood, α capture on ¹⁸O was affected by considerable uncertainties. At the temperatures of stellar He burning, the reaction rate is determined by two resonances at α energies of 470 and 566 keV. Since these resonances were not yet successfully measured, the rates had to be based on estimated resonance strengths. In the present work, the first direct measurement of the partial strengths of these extremely weak low-energy resonances is reported. The use of a high-efficiency segmented Ge detector in coincidence with bismuth germanate oxide counters covering a large solid angle led to a significantly improved experimental sensitivity, thus allowing for the clear identification of specific γ transitions. As a result, resonance strengths of 0.71±0.17 μeV and 0.48±0.16 μeV could be obtained for the 566- and 470-keV resonances, respectively. When compared to the previously reported upper limits of ⩽1.7 μeV, these results provide a reliable basis for the determination of the reaction rate during stellar He burning. Accordingly, these data reduce the uncertainties in the s process neutron balance.