Measurement of the reaction 17O(α,n)20Ne and its impact on the s process in massive stars

Background: The ratio between the rates of the reactions ¹⁷O(α,n)²⁰Ne and ¹⁷O(α,γ)²¹Ne determines whether ¹⁶O is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by ¹⁶O(n,γ) are recycled into the stellar environment. This ratio is of particular relevance to constrain the s process yields of fast rotating massive stars at low metallicity.Purpose: Recent results on the (α,γ) channel have made it necessary to measure the (α,n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars.Method: The ¹⁷O(α,n₍₀₊₁₎) reaction has been measured with a moderating neutron detector. In addition, the (α,n₁) channel has been measured independently by observation of the characteristic 1633 keV γ transition in ²⁰Ne. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (α,n) and (α,γ) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature.Result: The reaction channels ¹⁷O(α,n₀)²⁰Ne and ¹⁷O(α,n₁γ)²⁰Ne were measured in the energy range E_α=800 keV to 2300 keV. A new ¹⁷O(α,n) reaction rate was deduced for the temperature range 0.1 GK to 10 GK. At typical He burning temperatures, the combination of the new (α,n) rate with a previously measured (α,γ) rate gives approximately the same ratio as current compilations. The influence on the nucleosynthesis of the s process in massive stars at low metallicity is discussed.Conclusions: It was found that in He burning conditions the (α,γ) channel is strong enough to compete with the neutron channel. This leads to a less efficient neutron recycling compared to a previous suggestion of a very weak (α,γ) channel. S process calculations using our rates confirm that massive rotating stars do play a significant role in the production of elements up to Sr, but they strongly reduce the s process contribution to heavier elements.