Nuclear astrophysics and resonant reactions: Exploring the threshold region with the Trojan Horse Method

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 [Formula: see text] 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, we will focus on the Trojan Horse Method. We will briefly discuss the theory behind the method, to make clear its domain of applicability, the advantages and the drawbacks, and two recent cases will be shortly reviewed: the [Formula: see text] reaction, which is an important fluorine destruction channel in the proton-rich outer layers of asymptotic giant branch (AGB) stars, and the [Formula: see text] reactions, which play a critical role in astrophysics to understand stellar burning scenarios in carbon-rich environments.

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Resonant reactions of astrophysical interest studied by means of the Trojan Horse Method. Two case studies

EPJ Web of Conferences ◽

10.1051/epjconf/202022701011 ◽

2020 ◽

Vol 227 ◽

pp. 01011

Author(s):

Marco La Cognata ◽

Claudio Spitaleri ◽

Silvio Cherubini ◽

Marisa Gulino ◽

Livio Lamia ◽

...

Keyword(s):

Cross Sections ◽

Nuclear Reactions ◽

Cluster Structure ◽

Trojan Horse ◽

Electron Screening ◽

Asymptotic Giant Branch ◽

Direct Reaction ◽

Astrophysical Interest ◽

Trojan Horse Method ◽

The Cross

Resonant reactions in astrophysics play and important role as un- expected resonances may enhance the astrophysical factor with respect to the direct reaction contribution, altering the predicted nucleosynthesis scenarios by changing, for instance, the expected nucleosynthesis path. They also are of great interest in nuclear structure studies, since the determination of energies, spin-parities and partial widths sheds light on the occurrence of cluster structures, for instance. However, nuclear reactions in most astrophysical environments usually take place at energies below about 1 MeV, leading to an exponential de- crease of the cross sections due to the effect of the penetration of the Coulomb barrier. Also, at energies so low to be comparable with those associated to elec- tronic degrees of freedom, the effect of atomic and/or molecular clouds cannot be neglected, resulting in a shielding of nuclear charges and in an enhancement of the cross sections with respect to the case of bare nuclei (the so called elec- tron screening effect). Owing to vanishingly small cross sections and ambigui- ties in the extrapolation due to the electron screening, supplying accurate cross sections for astrophysical modeling is extremely challenging. Indirect methods have been introduced to explore the energy range of astrophysical interest with no need of extrapolation, even guided by theoretical arguments. In particular, the Trojan Horse 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. Even if measurements are carried out above astro- physical energies to be free from Coulomb suppression and electron screening, the range of astrophysical interest can be covered thanks to the x - s intercluster motion and binding energy. In these proceedings we will show the application of the THM, in the case of resonant reactions, using the generalised R-matrix approach introduced by A.M. Mukhamedzhanov. We will discuss the possibil- ity to extract resonance parameters from the Trojan Horse data and perform a full spectroscopic study of low-energy and even sub-threshold resonances. In particular, we will focus on the 19F(p, a)16O and the 13C(a, n)16O reactions, of particular importance in the case of asymptotic giant branch stars and in the synthesis of heavy elements by means of the s-process.

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The Trojan Horse Method in Nuclear Astrophysics

EPJ Web of Conferences ◽

10.1051/epjconf/201818401016 ◽

2018 ◽

Vol 184 ◽

pp. 01016

Author(s):

Aurora Tumino ◽

Claudio Spitaleri ◽

Silvio Cherubini ◽

Giuseppe D’Agata ◽

Guardo Giovanni Luca ◽

...

Keyword(s):

Cross Section ◽

Charged Particles ◽

Nuclear Astrophysics ◽

Trojan Horse ◽

Trojan Horse Method ◽

Basic Features ◽

Three Body

The Trojan Horse Method (THM) represents the indirect way to measure reactions between charged particles at astrophysical energies. This is done by measuring the quasi free cross section of a suitable three body process. The basic features of the THM will be presented together with some applications to demonstrate its practical use.

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