The ratio between the rates of the reactions O-17(alpha,n)Ne-20 and O-17(alpha,gamma)Ne-21 determines whether O-16 is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by O-16(n,gamma) 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. Recent results on the (alpha,gamma) channel have made it necessary to measure the (alpha,n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars. We present a new measurement of the O-17(alpha, n) reaction using a moderating neutron detector. In addition, the (alpha, n_1) channel has been measured independently by observation of the characteristic 1633 keV gamma-transition in Ne-20. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (alpha,n) and (alpha, gamma) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature. A new O-17(alpha,n) reaction rate was deduced for the temperature range 0.1 GK to 10 GK. It was found that in He burning conditions the (alpha,gamma) 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 (alpha,gamma) 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.
Download