Measurement of 1323 and 1487 keV resonances in N 15 (α,γ) F 19 with the recoil separator ERNA

Background: The origin of fluorine is a widely debated issue. Nevertheless, the N15(α,γ)F19 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 Ec.m.=1323 and 1487 keV. Purpose: The broad resonances widths, Γγ and Γα, have to be measured with adequate precision in order to better determine their contribution to the N15(α,γ)F19 stellar reaction rate. Methods: Measurement through the direct detection of the F19 recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a N15 beam impinging onto a He4 windowless gas target. The observed yield of the resonances at Ec.m.=1323 and 1487 keV is used to determine their widths in the α and γ channels. Results: We show that a direct measurement of the cross section of the N15(α,γ)F19 reaction can be successfully obtained with the recoil separator ERNA, and the widths Γγ and Γα 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 Γα. Conclusions: The revision of the widths of the two more relevant broad resonances in the N15(α,γ)F19 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 F19 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.