| Based on the updated improved Quantum Molecular Dynamics model (ImQMD-II), a lot of dynamical information about heavy ion fusion reactions can be obtained. In the model, Skyrme interaction potential energy is introduced directly. By including the surface term, pr term and adopting phase space occupation constraint method in the QMD model, the density distribution and momentum distribution of nucleon inside nuclei have been greatly improved. Considering the isospin effect of projectile and target, the symmetry potential-energy term is also added into the model. So the density distribution and momentum distribution of proton and neutron are further improved, Through above improvments, the stability of initial projectile and target nuclues has been improved greatly. Therefore, the ImQMD-II model becomes one of better microscopic models for studying heavy ion fusion reactions.Using the Quantum Molecular Dynamics model, the mechanism of fusion reactions, especially the mechanism of neutron-rich fusion reactions, has been studied. As an example, the study for reactions of 40,48Ca +90,96 Zr has been performed. Firstly, we sample the position and momentum of each nucleon in nuclei to prepare initial nuclei. Only those prepared nuclei for which the bulk properties (binding energy and root-mean-square radius) are good enough and their time evolution remains stable and long enough (at least 2000fm/c), are selected as 'good initial nuclei'. Then the four reactions are simulated at different incident energies and impact parameters. Concerning the definition of fusion event, we consider as follows: When the distance between two centers of mass of projectile and target is approached to less than the bigger radius of the projectle and target, the time evolution is still continued to 500fm/c (the period of one rotation or several vibration). Here we consider any event, for which the number of nucleons that escape during the process of forming compound nuclei is equal to or less than 6, as a fusion event. In this way, we calculate the fusion cross section of fusion reaction. We find that the calculted excitation functions for allfour reactions of 40,48Ca +90,96 Zr are in good agreement with experiment data. In order to understand the reason why the cross section of sub-barrier fusion of neutron-rich reaction of 40Ca +96 Zr is enhanced compared with non-neutron-rich reaction 40Ca +90 Zr and the fusion cross section for neutron-rich reactions 48Ca +90,96 Zr are not enhanced, the further investigations are carried out. we will study the influence of neutron-rich effect and shell effect on the fusion process. From the point of view of fusion dynamics, the neutron-rich effect leads to the formation of neck. Thus it results in the lowing of dynamical barrier and enhancing the fusion cross section. However, the difference in neutron transfer orientations (from target to projectile or from projectile to target) can lead the fusion barrier of the system lowing or rising. In the fusion reaction 40Ca +96 Zr, the neutron transfer from 96Zr to 40Ca is preferable and this kind of neutron transfer can lead the decrease of fusion barrier. Therefore, the fusion barrier of 40Ca +96 Zr is lowest in the four reactions 40,48Ca +90,96 Zr. Futhermore, we analyze the Q value for neutron transfer. We find that the Q value of neutron from 96Zr to 40Ca in reaction 40Ca +96 Zr is positive and those for other reactions in 40,48Ca +90,96 Zr are negative. Considering the relation between the Q value and the structure of nucleus, we can deduce that the fusion cross section is not only dependent on the neutron-rich effect but also the structure of projectle and target. |
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