Theoretical study of evaporation-residue cross sections for fusion reactions at energies near the Coulomb barrier

The empirical coupled-channel model and the statistical model are used to analyze the measured evaporation-residue cross sections of 48 fusion reactions. The experimental data of these reactions are systematically well reproduced (within one order of magnitude). It is found that the ratio of a(f) (level density parameter in fission channel) to a(n) (level density parameter in neutron-evaporation channel) plays an important role in reproducing the data appropriately. Furthermore, a good exponential holds between the ratio a(f)/a(n) and the fissility parameter.