An Extra Push from Entrance-Channel Effects

The fusion probability for heavy symmetric systems is known to show certain very specific features. Apart from the large variance of the fusion barrier distribution, it is found that the energy at which the s‐wave transmission is 0.5 is shifted to an energy significantly higher than the nominal (e.g. Bass) Coulomb barrier. This last feature is referred to in the literature as the ‘extra push’ effect. Many models have been devised to explain the origin of these findings. It is worth noting, however, that despite the extra push, the capture cross section is still greatly enhanced at the very lowest energies. This fact cannot be explained within the framework of macroscopic theories involving conditional saddle points or frictional forces. We have performed full coupled‐channel calculations for heavy, symmetric systems treating correctly the long‐range Coulomb excitations of the collective quadrupole‐ and octupole‐phonon states in the target and projectile. The results obtained show that the extra push and t...