The resonant cross sections of 10,11B(p,a) reactions at astrophysical energies: indirect measurements via the THM

The study of the nucleosynthesis of the light elements, Lithium, Beryllium and Boron, plays a key role in understanding of several astrophysical problems related to cosmology as well as to stellar structure and evolution. Moreover, the knowledge of their abundances can be a precious tool for the investigation of the inner stellar structure, especially to understand the mixing processes in stellar plasma. These elements, LiBeB, are mainly destroyed in stellar interior through (p,alpha) reactions induced at temperature of the order of 106 K, corresponding to the Gamow energy EG of a few keV's. In this energy region the direct determination of the cross section for charged particle induced reactions is affected by the presence of the Coulomb barrier between the interacting nuclei, and by the electron screening effect. Thus, precise measurements of the cross section for such processes are very complex and sometimes not possible. In the thesis, is presented the study of the 10B(p,alpha)7Be and 11B(p,alpha)8Be reactions which are the main responsible for the Boron destruction in stellar environments. The study of these reactions has been performed by means of the indirect Trojan Horse Method, that allows to overcome the effects due to the presence of the Coulomb barrier and electron screening. The method provides the bare nucleus two-body astrophysical-factor in the energy region of interest for astrophysics, from the quasi-free (QF) contribution of a suitable three-body reaction. The approach allows to avoid the extrapolation, usually made on the direct measurements. The experimental procedure and the results regarding the extracted Sb(E)-factor are reported in the thesis, as well as the electron screening potential Ue determined by means of the comparison with direct data.