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 17 O(α,n) 20 Ne and 17 O(α,γ ) 21 Ne determines whether 16 O is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by 16 O(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 17 O(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 20 Ne. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (α,n )a nd (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 17 O(α,n0) 20 Ne and 17 O(α,n1γ ) 20 Ne were measured in the energy range Eα = 800 keV to 2300 keV. A new 17 O(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.