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

Purpose: The broad resonances widths, Γ_γ and Γ_α, have to be measured with adequate precision in order to better determine their contribution to the ¹⁵N(α ,γ )¹⁹F stellar reaction rate.

Methods: Measurement through the direct detection of the ¹⁹F recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a ¹⁵N beam impinging onto a ⁴He windowless gas target. The observed yield of the resonances at E_{c .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 ¹⁵N(α ,γ )¹⁹F reaction can be successfully obtained with the recoil separator ERNA, and the widths Γ_γ and Γ_α 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 Γ_α.

Conclusions: The revision of the widths of the two more relevant broad resonances in the ¹⁵N(α ,γ )¹⁹F 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 ¹⁹F 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.