The Development Of Quantum Molecular Dynamics Model And Its Application In Low Energy Heavy Ion Reactions

We have developed a new microscopic dynamical model called improved quantum molecular dynamical model (ImQMD). This model can describe the fusion process at energies near the Coulomb barrier as well as the multifragmentation process at intermediate energies in heavy-ion collision (HIC) uniformly. In fusion reaction, the dynamical effects, isospin effects and mass asymmetry effects etc. can be treated comprehensively and self-consistently in ImQMD model, which offers a new way to study fusion reaction and to explore the mechanism of synthesis of superheavy elements hopefully.Our improvements mainly include: Updating the interaction and introducing the surface term, momentum dependent term and surface-symmetry term of Skyrme effective interaction; introducing system-size-dependent initial wave-packet width; performing the phase space occupation constraint, and proposing a new set of parameters for ImQMD model.The ImQMD model greatly improves the fermion properties of nuclei, the momentum distribution and density distribution of ground state nuclei, and thus increase the stability time of single nucleus by an order. By using this model, the ground state properties of nuclei and the Coulomb barrier can be described very well. We have applied it to fusion reactions at near-barrier energies, and find that in most case the cross sections of fusion reactions (including some of neutron-rich nuclei reactions and that of newly measured 132Sn+64Ni fusion reaction) can be reproduced remarkably well. It is encouraged that the ImQMD model can also reproduce the charge distribution in multifragmentation.By applying this model, the dynamical effects on fusion reactions at energies near the barrier are studied. We find that the dynamical barrier in fusion reaction is closely related to the incident energy, impact parameter, N/Z ratio as well as the structure of system. Generally speaking, the deformation of projectile and target (elongate to prolate ellipsoid) during fusion process can lower the dynamical barrier clearly and enhance the fusion cross section at subbarrier energies. We have further studied the dynamical behavior of the neck in fusion reactions, and find that the neck growth is closely related to the incident energy and mass-asymmetry and isospin of projectile and target. Through studying the neck dynamical behavior for neutron-rich nuclei fusion reactions, we have found that there exist neutron-flow (i.e. large N/Z ratio at neck region) at the early stage of fusion which cause the dynamical barrier lowering and thus enhance the fusion cross sections of most neutron-rich nuclei fusion reactions (48Ca+48Ca is an exception). In addition, the quasi-fission and breakup process in heavy nuclei fusion are studied by using our ImQMD-II model. The time of quasi-fission process and the dependence of lifetime of composite system on the incident energy, impact parameter, system size, shape and N/Z ratio of composite system are studied, and the further analyse is on progress.