Li 6 , 7 + Al 27 reactions close to and below the Coulomb barrier

The reactions $^{6,7}\mathrm{Li}+^{27}\mathrm{Al}$ were compared with the reactions $^{17,18}\mathrm{O}+^{16}\mathrm{O}$ which form the same compound nuclei $^{33,34}\mathrm{S}$. Cross-section data for the reactions $^{6,7}\mathrm{Li}+^{27}\mathrm{Al}$ were derived from experimentally determined \ensuremath{\gamma}-ray production cross sections for transitions in several residual nuclei. For the reactions $^{17,18}\mathrm{O}+^{16}\mathrm{O}$ experimental results from the literature were used. It could be shown that the weakly bound projectiles $^{6,7}\mathrm{Li}$ undergo not only fusion processes, but also breakup reactions quite in contrast to the tightly bound projectiles $^{18}\mathrm{O}$ and $^{17}\mathrm{O}$, respectively. Especially below the Coulomb barrier such direct reactions play an important role in competition to complete fusion. Calculations based on the statistical model agree well with the available data for $^{17,18}\mathrm{O}+^{16}\mathrm{O}$, but failed to represent the behavior of the experimentally determined production cross sections for the evaporation residues in the reactions $^{6,7}\mathrm{Li}+^{27}\mathrm{Al}$. But coupled-channel codes and calculations based on a nucleus-nucleus proximity potential are able to reproduce the energy dependence of the complete fusion cross sections.