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

Abstract

Background: The origin of fluorine is a widely debated issue. Nevertheless, the <SUP>15</SUP>N(α ,γ )<SUP>19</SUP>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<SUB>c .m .</SUB>=1323 and 1487 keV. <P />Purpose: The broad resonances widths, Γ<SUB>γ</SUB> and Γ<SUB>α</SUB>, have to be measured with adequate precision in order to better determine their contribution to the <SUP>15</SUP>N(α ,γ )<SUP>19</SUP>F stellar reaction rate. <P />Methods: Measurement through the direct detection of the <SUP>19</SUP>F recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a <SUP>15</SUP>N beam impinging onto a <SUP>4</SUP>He windowless gas target. The observed yield of the resonances at E<SUB>c .m .</SUB>=1323 and 1487 keV is used to determine their widths in the α and γ channels. <P />Results: We show that a direct measurement of the cross section of the <SUP>15</SUP>N(α ,γ )<SUP>19</SUP>F reaction can be successfully obtained with the recoil separator ERNA, and the widths Γ<SUB>γ</SUB> and Γ<SUB>α</SUB> 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<SUB>α</SUB>. <P />Conclusions: The revision of the widths of the two more relevant broad resonances in the <SUP>15</SUP>N(α ,γ )<SUP>19</SUP>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 <SUP>19</SUP>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.