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${(\ensuremath{\alpha},n)}^{20}$Ne and ${}^{17}$O${(\ensuremath{\alpha},\ensuremath{\gamma})}^{21}$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$(n,\ensuremath{\gamma})$ 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.Purpose: Recent results on the $(\ensuremath{\alpha},\ensuremath{\gamma})$ channel have made it necessary to measure the $(\ensuremath{\alpha},n)$ reaction more precisely and investigate the effect of the new data on $s$ process nucleosynthesis in massive stars.Method: The ${}^{17}$O$(\ensuremath{\alpha},{n}_{(0+1)})$ reaction has been measured with a moderating neutron detector. In addition, the $(\ensuremath{\alpha},{n}_{1})$ channel has been measured independently by observation of the characteristic 1633 keV $\ensuremath{\gamma}$ transition in ${}^{20}$Ne. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. $(\ensuremath{\alpha},n)$ and $(\ensuremath{\alpha},\ensuremath{\gamma})$ 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${(\ensuremath{\alpha},{n}_{0})}^{20}$Ne and ${}^{17}$O${(\ensuremath{\alpha},{n}_{1}\ensuremath{\gamma})}^{20}$Ne were measured in the energy range ${E}_{\ensuremath{\alpha}}=800$ keV to 2300 keV. A new ${}^{17}$O$(\ensuremath{\alpha},n)$ reaction rate was deduced for the temperature range 0.1 GK to 10 GK. At typical He burning temperatures, the combination of the new $(\ensuremath{\alpha},n)$ rate with a previously measured $(\ensuremath{\alpha},\ensuremath{\gamma})$ 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 $(\ensuremath{\alpha},\ensuremath{\gamma})$ 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 $(\ensuremath{\alpha},\ensuremath{\gamma})$ 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.