Measurement of the reaction18O(α,n)21Ne

Background: The reaction ${}^{18}$O${(\ensuremath{\alpha},n)}^{21}$Ne is a part of the reaction chains leading to the production of ${}^{19}$F and ${}^{22}$Ne during He burning in low-mass and massive AGB stars, respectively. Additionally, it has been observed as a strong background source in the measurement of other $(\ensuremath{\alpha},n)$ reactions.Purpose: Previously low-energy ${}^{18}$O${(\ensuremath{\alpha},n)}^{21}$Ne cross section data have only been available in a non-peer-reviewed form. An improved measurement of this reaction has been done to both clarify its astrophysical influence as well as to provide background yield data for future $(\ensuremath{\alpha},n)$ experiments.Method: The ${}^{18}$O$(\ensuremath{\alpha},{n}_{(0+1)})$ reaction has been measured with a moderating neutron detector. In addition the $(\ensuremath{\alpha},{n}_{1}\ensuremath{\gamma})$ channel has been measured independently by observation of the characteristic 350.7 keV $\ensuremath{\gamma}$ transition in ${}^{21}$Ne. The reaction cross section at energies above ${E}_{\ensuremath{\alpha}}=1100$ keV was determined by a simultaneous $R$-matrix fit to both channels. The strengths of the two lowest-energy resonances at ${E}_{\ensuremath{\alpha}}=959$ keV and ${E}_{\ensuremath{\alpha}}=1066$ keV were analyzed separately using individual Breit-Wigner fits.Results: The cross section of both reaction channels, ${}^{18}$O${(\ensuremath{\alpha},{n}_{0})}^{21}$Ne and ${}^{18}$O${(\ensuremath{\alpha},{n}_{1}\ensuremath{\gamma})}^{21}$Ne, was determined from the threshold energies at 851 keV and 1280 keV, respectively, to 2300 keV. A new reaction rate has been deduced for the temperature range of 0.1 GK to 10 GK. A previously reported resonance at ${E}_{\ensuremath{\alpha}}=888$ keV is explained as background from the contaminant reaction ${}^{17}$O${(\ensuremath{\alpha},n)}^{20}$Ne.Conclusions: In general, our reaction rate is slightly lower than the reaction rates in recent compilations. At temperatures below 0.2 GK the present rate is significantly lower because it could be shown that the lowest reported resonance is background from the reaction ${}^{17}$O${(\ensuremath{\alpha},n)}^{20}$Ne that has been wrongly assigned to ${}^{18}$O${(\ensuremath{\alpha},n)}^{21}$Ne.