Study Of The Transport Theory In Heavy Ions Fusion Reactions And Strongly Damped Reactions

This dissertation includes two aspects of work: the first is about the potential barrier and the dynamical mechanism of heavy-ion fusion reactions, and the second is about the strongly damped reactions of very heavy nuclei.Firstly, static potential barriers at the entrance channel in the synthesis of super-heavy nuclei are studied systematically within the ImQMD model. The study is mainly concentrated on the Coulomb barrier and capture potential well. From the analysis of static entrance-channel potentials as a function of mass asymmetry for 18 reactions leading to the same compound nucleus ²⁶⁵Sg, we have shown that the symmetric system is not suitable for the synthesis of superheavy elements (SHEs). Further, the dynamic barrier in the entrance channel for synthesis of heavier nuclei is investigated. It is found that the dynamic barrier approaches to the adiabatic static barrier with the incident energy decreasing and goes up to the diabatic static barrier with increase of the incident energy. In order to understand the energy dependence of the dynamical barrier, we pay a great attention to study the neck formation and shape deformation during the dynamic lowering of the barrier. Furthermore, a modified Woods-Saxon (MWS) potential is proposed for describing nucleus-nucleus interaction based on the Skyrme energy-density functional approach. Fusion barriers for a large number of fusion reactions from light to heavy systems can be described well with this potential. The suitable incident energies which is between the mean barrier height B_m and the fusion barrier height B_ws for fusion reactions leading to superheavy nuclei are also explored.Secondly, low energy collisions of very heavy nuclei ^238U+²³⁸U and ²³²Th+²⁵⁰250Cf have been studied within the ImQMD model. We first study the probability for producing superheavy fragments with Z≥114 (SHFs) in these reactions and find that it is much higher in asymmetric reaction ²³²Th + ²⁵⁰Cf compared with that in the symmetric reaction ²⁴⁴Pu +²⁴⁴Pu. The charge (mass) distributions of primary fragments and the excitation energy distribution of superheavy fragments are found to be strongly incident energy dependent. We then study the average lifetime of the giant composite system transiently formed in these reactions. It is found that the average lifetime of the giant composite systems can be more than 1100 fm/c when E_cm is about 1180 MeV. The calculation results for single potentials and energies for neutrons and protons in the giant composite system show that the Coulomb barrier of single particle potential well for proton is about 20 MeV, which makes the unbound protons in the composite system to be embedded in the potential well. The shapes of giant composite systems are found to be largely deformed. The elongation orientation of the deformed composite system changes from 0°(180°) to 90°(270°), when the center-of-mass energy increases from 680 to 1880 MeV. The anisotropy of the momentum distribution of the giant composite systems as function of incident energies is investigated and it is found that the giant composite system with roughly isotropic momentum distribution can be formed at suitably chosen energy. The decay process of the giant composite system is studied. We find there is two different decay processes with different decay rate. One is the fast decay process corresponding to the fission of giant composite system, the other is the slower decay process corresponding to the decay of the fission products of giant composite system. Finally, the ImQMD model plus the statistical evaporation model (HIVAP) is applied to calculate the mass distribution of survival fragments for reaction ²³⁸U+²³⁸U at incident energy 7.0 A MeV, and the calculated results are in agreement with the experiment ones.