Speaker
Description
Resonant reactions play an important role in astrophysics as they might significantly enhance the cross section with respect to the direct reaction contribution and alter the nucleosynthetic flow. Moreover, resonances bear information about states in the intermediate compound nucleus formed in the reaction. However, nuclear reactions in stars take place at energies well below ∼1 MeV and 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 input data for astrophysics.
Therefore, indirect methods have been introduced; in particular, the Trojan Horse Method (THM) (see Ref. [1] for a review of the method) 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. In recent years, a generalized R-matrix approach has been introduced [2], allowing one to deduce the resonance parameters from the THM cross section accounting for half-off-energy-shell effects. In this way, THM can be used to perform a full spectroscopic study of low-energy and sub threshold resonances. For the latter, the ANC can be deduced as well, establishing an alternative high accuracy approach to determine this crucial parameter and leading to an unification of the two mentioned indirect methods.
In this presentation we will briefly discuss the theory behind the method, to make clear its do- main of applicability, the advantages and the drawbacks, and some experimental applications of this extended approach based on the generalised R-matrix [2]. Two recent cases will be shortly reviewed: the 19F(p,α)16O reaction, which is an important fluorine destruction channel in the proton-rich outer layers of asymptotic giant branch (AGB) stars, and the 12C+12C reaction, which plays a critical role in astrophysics to understand stellar burning scenarios in carbon-rich environments.
19F(p,α)16O data stop at about 200 keV, making it necessary to extrapolate its trend at lower energies. The THM was thus used to access this energy region, by extracting the quasi-free contribution to the 2H(19F,α16O)n reaction. The THM measurement shows the presence of resonant structures not observed before, showing up right at astrophysical energies, which cause an increase of the reaction rate at typical stellar temperatures [3]. Recent direct measurements [4] reaching down to about 120 keV confirmed earlier THM predictions, validating such indirect approach.
Finally, the recent investigation of the 12C+12C fusion reactions will be presented [5]. The THM was applied to the 12C(14N,α20Ne)2H and 12C(14N,p23Na)2H three-body processes at 30 MeV of beam energy in the quasi-free kinematical regime, where 2H from the 14N Trojan Horse nucleus is spectator to the 12C+12C two-body processes. The 12C(12C, α)20Ne and 12C(12C,p)23Na cross sections at astrophysical energies were extracted, between 1 and 2 MeV center-of-mass energies, at odds with direct measurements stopping at 2.14 MeV, still at the beginning of the astrophysical region. A strong resonant behavior of the cross section associated to 24Mg levels was observed causing a strong enhancement of the reaction at the relevant temperatures [6].
[1] R.E. Tribble et al., Rep. Prog. Phys. 77 (2014) 106901.
[2] A.M. Mukhamedzhanov, Phys. Rev. C 84 (2011) 044616.
[3] M. La Cognata et al., Astrophys. J. 805 (2015) 128.
[4] M. La Cognata et al., Phys. Rev. Lett. 87 (2012) 232701.
[5] A. Cumming et al., Astrophys. J. 646 (2006) 429.
[6] A. Tumino et al., Nature 557 (2018) 687
