Study of tunneling effect in Nuclear Physics through the measurement of heavy ion fusion; consequences for Astrophysics

The present thesis reports on the recent measurement of the fusion excitation function for the system 12C+24Mg (Qfus= +16.3 MeV) down to around 10 µb, in order to investigate whether fusion hindrance shows up, phenomenologically marked by the observation of a maximum of the S-factor vs. energy. The aim was to search evidence for fusion hindrance in medium-light systems with positive Q-values, besides the heavier cases where hindrance is recognized to be a general phenomenon. The system 12C+24Mg is very close to the 16O+16O and 12C+12C systems that are important for the late evolution of heavy stars. The experiment has been performed in inverse kinematics using the 24Mg beam from the XTU Tandem accelerator of Labaratori Nazionali di Legnaro (LNL) in the energy range 26-52 MeV with an intensity of 4-8 pnA. The targets were 12C evaporations 50 µg/cm2 thick, isotopically enriched to 99.9%. The fusion-evaporation residues were detected at small angles in a E-∆E-ToF telescope following the electrostatic beam deflector in use at LNL. Previous measurements of fusion cross section for 12C+24Mg were limited to above-barrier energies. The new results show that the S-factor develops a clear maximum vs. energy, indicating the presence of hindrance in this system below the Coulomb barrier energy. The hindrance threshold in this system follows rather closely a systematics obtained for several medium-light systems together with a phenomenological estimate that fits well also the new data point for this case. The calculated fusion cross section at threshold is very large (σs=1.6mb) and it is indeed the largest found for medium-light systems. This has allowed to identify the hindrance phenomenon in a favorable condition. It may even be possible to extend the measurements further down in energy to better establish the position of the S-factor maximum. The experimental data have been compared with the results of coupled channel calculations including the lowest quadrupole vibrations of 24Mg. With this comparison, the hindrance behavior has been better evidenced at deep sub-barrier energies.