Fusion and particle transfer around the Coulomb-Barrier in intermediate systems

The most important characteristics of fusion reactions below and around the Coulomb-barrier are summarized. Experimental fusion cross sections for typical systems are discussed and compared with current formulae obtained from semi-classical and quantum tunneling approaches. The influence of nucleons transfer in the enhancement of the fusion cross section below the Coulomb-barrier is also shown. Sub-barrier fusion cross sections for the systems 35,37Cl + 58,64Ni and 33S + 90,91,92Zr, and near-barrier cross sections of all important transfer channels have been measured using the XTU-TANDEM at Legnaro, Italy. In 35,37Cl + 58,64Ni systems, the motivation further investigated was the influence of the valence proton in the enhancement of the sub-barrier fusion cross section. The data are discussed in comparison with the similar data of 34,36S + 58,64Ni with the aim of revealing the influence of coupled proton transfer channels. Calculations were performed using the simplified coupled channel code CCFUS including ''pick-up'' of one and two neutrons and ''stripping'' of two neutrons channels. Signatures of positive Q-values transfer channels coupled to fusion were clearly identified. For the 33S + 90,91,92Zr systems taking into account the coupling effects between transfer and fusion and using the semi-classical approach, transfer form-factors were extracted and succesfully employed to described the isotopic effects in fusion enhancement. (Author)