Sub-barrier fusion of Si+Si systems: Does the deformation of ²⁸Si play a role?

This contribution reports on the results of measurements of near- and sub-barrier fusion cross sections in the system ³⁰Si+³⁰Si performed at the Laboratori Nazionali di Legnaro of INFN. The ³⁰Si beam of the XTU Tandem accelerator in the energy range of 47-90 MeV, was delivered on a metallic ³⁰Si target (50 μg/cm²) enriched to 99.64 % in mass 30. A beam electrostatic deflector was used for the detection of evaporation residues. The excitation function obtained for ³⁰Si+³⁰Si has been compared with the previous data on ²⁸Si+²⁸Si and Coupled Channels (CC) calculations using the M3Y+repulsion potential, taking into account the low lying 2⁺ 3^- excitations. Reproducing the low-energy ²⁸Si+²⁸Si excitation function was only possible by using a weak imaginary potential, probably simulating the oblate deformation of this nucleus. On the contrary, the data on ³⁰Si+³⁰Si are nicely reproduced by the CC calculations without any imaginary potential (³⁰Si has a spherical shape). The astrophysical S-factor does not show a maximum, so that there is no evidence for hindrance in the measured energy range. The logarithmic derivative of the two excitation functions highlights the difference between the two systems. Even above the barrier the two systems behave differently. This is best seen by comparing the two barrier distributions where the high energy peak observed for ²⁸Si+²⁸Si is not found for ³⁰Si+³⁰Si. This is presumably due to the stronger couplings present in ²⁸Si and further theoretical analyses are in progress.