Examining the enhancement of sub-barrier fusion cross sections by neutron transfer with positiveQ values

Abstract

Background: Significant enhancement of sub-barrier fusion cross sections owing to neutron rearrangement with positive $Q$ values were found for many combinations of colliding nuclei. However, several experimental results on fusion reactions were reported recently in which such enhancement has not been observed in spite of a possibility for neutron rearrangement with positive $Q$ values.Purpose: We aim to clarify much better the mechanism of neutron rearrangement in sub-barrier fusion reactions to find the other requirements (beside positive $Q$ values) which favor (or prevent) sub-barrier fusion enhancement.Methods: A channel coupling approach along with the semiclassical model for neutron transfer has been used for analysis of available experimental data on sub-barrier fusion of heavy ions.Results: The role and interplay of different factors determining the enhancement of sub-barrier fusion (such as $Q$ values for neutron rearrangement, properties of collective excitations, and neutron binding energies) have been studied and clarified.Conclusions: (1) Only $1n$ and $2n$ transfers with positive $Q$ values have a noticeable impact on sub-barrier fusion. A positive $Q$ value for neutron rearrangement is a necessary but not sufficient requirement for additional sub-barrier fusion enhancement to take place. (2) The ``rigidity'' of colliding nuclei with respect to collective excitations is important for sub-barrier fusion enhancement due to neutron rearrangement with positive $Q$ values to be clearly visible. (3) The neutron binding energy of the ``donor'' nucleus has a strong impact only in the case of fusion of light weakly bound nuclei.