Role of the Surface Energy in Heavy-Ion Collisions

The surface energy is one of the fundamental properties of nuclei, appearing in the simplest form of the semi-empirical mass formula. The surface energy has an influence on e.g. the shape of a nucleus and its ability to deform. This in turn could be expected to have an effect in fusion reactions around the Coulomb barrier where dynamical effects such as the formation of a neck is part of the fusion process. Frozen Hartree-Fock and Time-Dependent Hartree-Fock calculations are performed for a series of effective interactions in which the surface energy is systematically varied, using 40Ca + ⁴⁸Ca as a test case. The dynamical lowering of the barrier is greatest for the largest surface energy, contrary to naive expectations, and we speculate that this may be due to the variation in other nuclear matter properties for these effective interactions.