On the investigation of resonances above and below the threshold in nuclear reactions of astrophysical interest using the Trojan Horse Method.

Abstract

The occurrence of resonances in reactions of astrophysical interest might significantly enhance the astrophysical factor with respect to the direct reaction contribution, divert nucleosynthesis path and change the energy production, with significant impact on astrophysics. Moreover, the determination of resonance parameters, that is, energy, spin-parity and partial widths, allows one to perform nuclear structure studies leading, for instance, to determine the cluster structure of the state under investigation. However, nuclear reactions in stars take place at energies well below ~ 1 MeV owing to the typical temperatures characterising these environments. Therefore, the Coulomb barrier exponentially suppressing the cross section and the electron screening effect, due to the shielding of nuclear charges by atomic electrons, make it very difficult to provide accurate astrophysical factors. The THM is an indirect method allowing to overcome such difficulties. It makes use of quasi-free reactions with three particles in the exit channel, a + A → c + C + s, to deduce the cross section of the reaction of astrophysical interest, a + x → c + C, under the hypothesis that A shows a strong x + s cluster structure, right at astrophysical energies. By using a generalised R-matrix approach, the resonance parameters can be deduced from THM data allowing one to perform a full spectroscopic study of low-energy and sub-threshold resonances. In this work, we will discuss two examples of reactions of astrophysical interest, whose cross sections show a resonant behaviour: the¹⁹F(p, α)¹⁶O cross section that displays resonances at energies above the particle emission threshold and the¹³C(α, n)¹⁶O reaction, dominated by the −3 keV sub-threshold resonance due to the 6.356 MeV level in¹⁷O.