Sub-barrier fusion ofSi28,30withMg24,26

Measurements of the fusion cross section of ${}_{}{}^{28,30}{}_{}{}^{}\mathrm{s}$/rSi${+}^{24,26}$Mg have been made for energies ranging from well below to above the Coulomb barrier. The fusion cross sections increased from about 40 μb at the lowest energy, to 400 mb or more at the highest energy measured. Surprisingly, a crossover in the excitation functions for ${}_{}{}^{28,30}{}_{}{}^{}\mathrm{f}$/rSi${+}^{24}$Mg is observed at the lowest sub-barrier energies. The data could not be reproduced by either a one-dimensional tunneling calculation or an approximate coupled-channels calculation within the constant coupling scheme. However, the use of an exact coupled-channels code, together with incoming-wave boundary conditions, led to good agreement between the calculated and measured fusion cross sections.