Microscopic analysis of fusion hindrance in heavy nuclear systems

Abstract

Background: Heavy-ion fusion reactions involving heavy nuclei at energies around the Coulomb barrier exhibit fusion hindrance, where the probability of compound nucleus formation is strongly hindered compared with that in light- and medium-mass systems. The origin of this fusion hindrance has not been well understood from a microscopic point of view.Purpose: I analyze the fusion dynamics in heavy systems by a microscopic reaction model in order to understand the origin of the fusion hindrance.Method: I employ the time-dependent Hartree-Fock (TDHF) theory as a microscopic reaction model. I extract the nucleus-nucleus potential and energy dissipation by the method combining TDHF dynamics of the entrance channel of fusion reactions with a one-dimensional Newton equation including a dissipation term. Then, I analyze the origin of the fusion hindrance using the properties of the extracted potential and energy dissipation.Results: I obtain finite extra-push energies for heavy systems from TDHF simulations, which agree with experimental observations. Extracted nucleus-nucleus potentials show monotonic increase as the relative distance of two nuclei decreases, which induces the disappearance of an ordinary barrier structure of the nucleus-nucleus potential. This property is different from those in light- and medium-mass systems and from density-constraint TDHF calculations. Extracted friction coefficients show sizable energy dependence and universal value of their magnitude, which are rather similar to those in light- and medium-mass systems. Using these properties, I analyze the origin of the fusion hindrance and find that contribution of the increase in potential to the extra-push energy is larger than that of the accumulated dissipation energy in most systems studied in this article.Conclusions: I find that the nucleus-nucleus potentials extracted in heavy systems show a specific property, which is not observed in light- and medium-mass systems. By the analysis of the origin of the fusion hindrance, I conclude that, as the system becomes heavier, the dynamical increase in nucleus-nucleus potential at small relative distances plays a more important role than the dissipation during the fusion reaction for understanding the origin of the fusion hindrance.