Background: The ratio between the rates of the reactions ${}^{17}$O${(\alpha ,n)}^{20}$Ne and ${}^{17}$O${(\alpha ,\gamma )}^{21}$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$(n,\gamma )$ 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 $(\alpha ,\gamma )$ 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.

Method: The ${}^{17}$O$(\alpha ,n_{(0+1)})$ reaction has been measured with 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 ${}^{20}$Ne. 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.

Result: The reaction channels ${}^{17}$O${(\alpha ,n_0)}^{20}$Ne and ${}^{17}$O${(\alpha ,n_1\gamma )}^{20}$Ne were measured in the energy range $E_{\alpha }=800$ keV to 2300 keV. A new ${}^{17}$O$(\alpha ,n)$ reaction rate was deduced for the temperature range 0.1 GK to 10 GK. At typical He burning temperatures, the combination of the new $(\alpha ,n)$ rate with a previously measured $(\alpha ,\gamma )$ 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 $(\alpha ,\gamma )$ 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.

• Received 1 November 2012

DOI:https://doi.org/10.1103/PhysRevC.87.045805