Searching the reason for sub-barrier fusion enhancement through multineutron transfer channels

Abstract

Background: The influence of inelastic excitations on sub-barrier fusion enhancement is well established; however, the transfer channel effect can be surprising. Recently, it has been shown that the $2n$ transfer channel is mainly responsible for sub-barrier fusion enhancement despite having a positive-$Q$ value for many-neutron transfer channels. Moreover, some systems do not show enhancement even if they possess positive-$Q$-value transfer channels. It has also been reported that enhancement can be found in those systems for which deformation of nuclei increases after neutron transfer.Purpose: The aim of this work is to examine the role of multineutron transfer channels on sub-barrier fusion enhancement.Method: Fusion cross sections were measured with a recoil mass separator for the $^{40}\mathrm{Ca}+^{70}\mathrm{Zn}$ system and analyzed theoretically within the framework of coupled-channels calculations.Results: Positive $Q$-value neutron transfer channels seem to be essential for the sub-barrier region, since inelastic excitation coupling was unable to reproduce the trend of experimental fusion cross sections. However, up to $2n$ pick-up channel was found to be sufficient for describing the sub-barrier cross sections.Conclusions: Sub-barrier fusion in the $^{40}\mathrm{Ca}+^{70}\mathrm{Zn}$ system is hardly affected by transfer of more than two neutrons. The fusion enhancement due to the transfer channel can be observed in systems for which the deformation of colliding nuclei increases after transfer, though the amount of enhancement does not seem to be proportional to the deformation change.