Background: The origin of fluorine is a widely debated issue. Nevertheless, the ${}^{15}\mathrm{N}(\alpha ,\gamma ){}^{19}\mathrm{F}$ reaction is a common feature among the various production channels so far proposed. Its reaction rate at relevant temperatures is determined by a number of narrow resonances together with the direct capture and the tails of the two broad resonances at $E_{\mathrm{c}.\mathrm{m}.}=1323$ and 1487 keV.

Purpose: The broad resonances widths, ${\mathrm{\Gamma }}_{\gamma }$ and ${\mathrm{\Gamma }}_{\alpha }$, have to be measured with adequate precision in order to better determine their contribution to the ${}^{15}\mathrm{N}(\alpha ,\gamma ){}^{19}\mathrm{F}$ stellar reaction rate.

Methods: Measurement through the direct detection of the ${}^{19}\mathrm{F}$ recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a ${}^{15}\mathrm{N}$ beam impinging onto a ${}^4\mathrm{He}$ windowless gas target. The observed yield of the resonances at $E_{\mathrm{c}.\mathrm{m}.}=1323$ and 1487 keV is used to determine their widths in the $\alpha$ and $\gamma$ channels.

Results: We show that a direct measurement of the cross section of the ${}^{15}\mathrm{N}(\alpha ,\gamma ){}^{19}\mathrm{F}$ reaction can be successfully obtained with the recoil separator ERNA, and the widths ${\mathrm{\Gamma }}_{\gamma }$ and ${\mathrm{\Gamma }}_{\alpha }$ of the two broad resonances have been determined. While a fair agreement is found with earlier determination of the widths of the 1487 keV resonance, a significant difference is found for the 1323 keV resonance ${\mathrm{\Gamma }}_{\alpha }$.

Conclusions: The revision of the widths of the two more relevant broad resonances in the ${}^{15}\mathrm{N}(\alpha ,\gamma ){}^{19}\mathrm{F}$ reaction presented in this work is the first step toward a more firm determination of the reaction rate. At present, the residual uncertainty at the temperatures of the ${}^{19}\mathrm{F}$ stellar nucleosynthesis is dominated by the uncertainties affecting the direct capture component and the 364 keV narrow resonance, both so far investigated only through indirect experiments.

• Received 31 October 2016

DOI:https://doi.org/10.1103/PhysRevC.95.045803