Investigation of deep sub-barrier fusion in asymmetric systems

Enhancement of fusion cross sections below the interaction barrier has been quite successfully explained by coupled-channels methods. However, extending the measurements to deep sub-barrier energies revealed a steeper descent of the excitation function which could not be explained by standard coupled-channels calculations. Though a large number of heavy-ion and light-ion induced reactions have been investigated to understand the dynamics of fusion deep below the barrier, the phenomenon of fusion hindrance has been studied only for a handful of asymmetric systems. We report new measurements of fusion excitation functions for the systems O-16+Cd-116 and O-16+Ce-142. We also present comparisons of the same with the data for existing symmetric systems having nearly similar values of the zeta parameter, characterizing the size of the colliding system. We extracted the logarithmic derivatives of the energy-weighted cross sections and the astrophysical S-factors. Experimental results were reproduced well by coupled-channels calculations. We extrapolated our results, following the systematics, beyond the threshold energy for fusion hindrance for both the systems. From our investigation, we conclude that the present asymmetric systems, as well as the corresponding symmetric systems, show fusion hindrance and this feature is independent of the entrance channel mass asymmetry.