U systems have been measured, at Louvain-la-Neuve, for energies around and below the Coulomb barrier, using an array of Si detectors surrounding a UF4 target. The data taken with 4 He are in good agreement with previous data and with the coupled channel fusion calculation performed with ECIS. The 6 He data show a regular trend with a large enhancement below the barrier which is attributed to the halo structure of the 6 He nucleus. PACS number :The sub-barrier fusion of two n uclei is classically forbidden and can be achieved only by quantum tunnelling. The in uence of the nuclear deformation on the fusion probability has been studied for stable nuclei and a high sensitivity to the nuclear structure has been observed 1 . The major remaining questions concern the in uence of other processes such as transfer or break-up reactions, as well as the e ect of unusual structures of nuclei, such a s neutron skins or halos 2 . Theoretical calculations agree that the large spatial extension of halo nuclei and the coupling with eventual low lying resonant states would increase the fusion cross section 3 6 . However, they strongly disagree on the role of the break-up e ect on the fusion probability. This reaction could be seen as another doorway state to fusion 5 that would lead to an extra-enhancement of the cross section or, on the other hand, as a loss of ux for fusion 4,6 that would decrease the cross-section.These calculations have been performed for the 11 Li+ 208 Pb and 11 Li+ 238 U systems. An experimental study of these systems is currently di cult due to the weak intensities that can be obtained for 11 Li radioactive beams. Measurements of sub-barrier fusion for other halo nuclei have started, in 1991, at GANIL France with the fusion-ssion cross section study of 11 Be + 238 U. However, these results did not lead to rm conclusions due to the lack of statistics 7 . At NSCL MSU in the USA, the fusion-ssion of the stable 32 S and the unstable 38 S nuclei on a 181 Ta target have been measured 8 . An increase of the cross section for 38 S has been observed that could be explained by a simple shift in the barrier height. Similar results have been obtained on the protonrich side for the fusion-ssion of 17 F + Pb 9 . Other groups used the identi cation of the residual nuclei via their alpha emission to reconstruct the fusion probability. A t RIKEN Japan, the 11 Be + 209 Bi system has been studied and the authors conclude that the contribution of the break-up process to the fusion is modest" 10 and that the two cross-sections for 9 Be+ 209 Bi and 11 Be+ 209 Bi are r ather similar." 11 . Studies have also been done with the 6 He Borromean neutron-rich n ucleus that presents a large spatial neutron distribution 12,13 with an inner core of 4 He. It will be called a halo nucleus throughout this paper even if the question is still open. The 6 He + 209 Bi system 14,15 has been measured at Notre Dame USA with a similar technique and a large enhancement of sub-barrier fusion is observed" 15 . Therefore both theoretical and experiment...

Fusion-evaporation cross sections were measured in the two systems 48 Ca + 90,96 Zr in an energy range from well below to well above the Coulomb barrier. The sub-barrier fusion of 48 Ca + 90 Zr is reproduced by coupled-channels calculations including the lowest quadrupole and octupole vibrations of 90 Zr, and using a Woods-Saxon potential with a standard diffuseness parameter a = 0.68 fm. However, the fusion cross sections are overestimated above the barrier. The low-energy slope of the excitation function for 48 Ca + 96 Zr is steeper. This implies a larger diffuseness parameter a = 0.85 fm. Fusion cross sections are well fit in the whole energy range, and the effect of the strong octupole vibration in 96 Zr is predominant. The extracted fusion barrier distributions are reasonably well reproduced by calculations for both systems. A comparison with previous data for 40 Ca + 90,96 Zr is made in an attempt to clarify the role of transfer couplings in sub-barrier fusion.

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The fusion excitation functions of 40,48Ca+48Ca have been measured near and below the Coulomb barrier and the fusion barrier distributions have been extracted from the data. The cross sections for 48Ca+48Ca are well reproduced by coupled-channels (CC) calculations, including inelastic excitations to the 2+ and 3- states of both nuclei and using a standard potential. The cross sections of 40Ca+48Ca are significantly larger than previous data for the same system, especially near and above the barrier. While a much better agreement between present data and CC calculations is found in that energy range, the sub-barrier cross sections are still underpredicted