Influence of the Z = 50 shell closure on heavy-ion sub-barrier fusion

The systematics of the lowest 2+ energies of even-even nuclei, and the B(E2) rates connecting those levels to the ground states, give essential information about the degree of collectivity in various regions of the nuclide chart. The B(E2) rates provide crucial tests of theoretical predictions, such as the quenching of shell structure in neutron-rich nuclei. Measurements of such rates are in progress using multi-nucleon transfer reactions and radioactive ion beams. We have measured the fusion excitation functions for the three systems 48Ca + 116Cd, 118Sn, 120Te, in the relevant energy range from above to below the Coulomb barrier. The magic projectile 48 Ca has been chosen in view of its spherical and rigid structure, while the three targets have proton numbers Z = 48, 50, 52. We observe that the cross sections are smallest for the target 118Sn, thus indicating the effect on sub-barrier fusion of crossing the major shell closure at Z=50. This suggests the possibility of using near-barrier fusion reactions of radioactive ion beams with suitable stable targets, as a complementary technique to evidence the low-lying nuclear structure of exotic nuclei, besides well-established methods.