Underground measurement of hydrogen-burning reactions on 17;18O at energies of astrophysical interest

The 17;18O(p,α)14;15N nuclear reactions play an important role in several astrophysical scenarios, and in Asymptotic Giant Branch (AGB) stars in particular. These stars are the site of several mixing and recirculating processes that transport matter from their hot cores to their cooler surfaces, and vice versa. Some of these mixing processes are still not well understood. Constraining them would improve our knowledge of stars that are in, or will enter, the AGB phase, including our own Sun. An ideal way to trace these poorly understood mixing processes are provided by the rare, stable 17;18O isotopes. Their abundances are strongly sensitive to the 17;18O(p,α)14;15N reactions. At temperatures of astrophysical interest, the 17O(p,α)14N reaction is dominated by a narrow, isolated resonance at Eproton=70 keV. This resonance has been studied several times in the past, using both direct and indirect methods. However, the picture painted in the literature is still not completely satisfying. The situation is more complex for the 18O(p,α)15N, for which an interference pattern between at least three resonances dominates the reaction rate at the temperatures of interest. This thesis work concerns an experimental campaign aimed at measuring both reactions at energies of astrophysical interest. These challenging measurements were performed by exploiting the low radiation background at the underground LUNA accelerator in Gran Sasso Laboratories, Italy. The two reactions were investigated in direct kinematics. A proton beam was accelerated onto solid Ta2O5 targets and the alpha particles produced were detected at backward angles using an array of silicon detectors mounted in a purpose-built scattering chamber. Our results indicate that the 17O(p,α)14N reaction rate at temperatures of astrophysical interest is approximately a factor of two higher than previously reported, solving a long standing puzzle on the origin of some pre-solar grains. For the 18O(p,α)15N reaction, we find a reaction rate largely in agreement with previous investigations, but with a significantly reduced uncertainty which could help improve the accuracy of stellar models of a number of stellar sites.