Metals, the plasma of the poor man?

The electron screening effect in the d(d,p)t reaction has been studied for deuterated metals, insulators, and semiconductors, i.e. 58 samples in total. As compared to measurements performed with a gaseous D2 target, a large effect has been observed in most metals, while a small (gaseous) effect is found e.g. for the insulators, semiconductors, and lanthanides. The periodic table provides the ordering of the observed small and large effects in the samples. An explanation of the large effects in metals is possibly provided by the classical plasma screening of Debye applied to the quasi-free metallic electrons. The data also provide information on the solubility of hydrogen in the samples.

Experimental Nuclear Astrophysics With the Light Elements Li, Be and B: A Review

Light elements offer a unique opportunity for studying several astrophysical scenarios from Big Bang Nucleosynthesis to stellar physics. Understanding the stellar abundances of light elements is key to obtaining information on internal stellar structures and mixing phenomena in different evolutionary phases, such as the pre-main-sequence, main-sequence or red-giant branch. In such a case, light elements, i.e., lithium, beryllium and boron, are usually burnt at temperatures of the order of 2–5 × 106 K. Consequently, the astrophysical S(E)-factor and the reaction rate of the nuclear reactions responsible for the burning of such elements must be measured and evaluated at ultra-low energies (between 0 and 10 keV). The Trojan Horse Method (THM) is an experimental technique that allows us to perform this kind of measurements avoiding uncertainties due to the extrapolation and electron screening effects on direct data. A long Trojan Horse Method research program has been devoted to the measurement of light element burning cross sections at astrophysical energies. In addition, dedicated direct measurements have been performed using both in-beam spectroscopy and the activation technique. In this review we will report the details of these experimental measurements and the results in terms of S(E)-factor, reaction rate and electron screening potential. A comparison between astrophysical reaction rates evaluated here and the literature will also be given.

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

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.

Electron Screening in Nuclear Astrophysics

Electron screening in an important effect that cannot be neglected in nuclear astrophysics, since it influences nuclear reaction cross sections at low energies. We are trying to understand why most measurements in inverse kinematics on solid targets give electron screening potentials more than an order of magnitude above predictions. Below we report our latest results on electron screening in nuclear reactions 1H(14N,γ)15O and 2H(19F,ρ)2°F in both inverse and normal kinematics. The analysis is in progress.