Multifragmentation And Nuclear Temperature In Heavy Ion Collisions At Low And Intermediate Energies

In terrestrial laboratories, the heavy-ion collision (HIC) at low and intermediate energies is the only one way to study the properties of the nuclear matter with high tem-perature and high density. The multifragmentation is the one of important mechanisms in low and intermediate energy HICs. The correct description on this phenomenon is an important test for the theoretical model. The nuclear equation of state for asymmetric nuclear matter is currently a hot topic in nuclear physics research. It is not only very important to understand the structure of nuclei and nuclear reactions far away from β-stability line, but also very important for many astrophysical issues, such as the neu-tron star properties, the dynamical evolution mechanism of the supernova explosion, and so on. For HICs at intermediate and high energies, the system would be heated, and the high-temperature environment would also appear during some stars’ evolution. Therefore, the temperature dependence of the symmetry energy itself, and the effect of the density dependence of the nuclear symmetry energy on the nuclear temperature are also important issues. In this thesis, by using the improved quantum molecular dynamics (ImQMD) model plus the statistical model GEMINI, the multifragmentation process and the nuclear temperature in the low and intermediate energies HICs were investigated.One work of this thesis is the study on the multifragmentation in HICs at low and intermediate energies. Before the simulation, we checked the reliability and stability of ImQMD model by verifying the ground state properties of nuclei, such as the binding energy, the root mean square radius and so on. By calculating the excitation energies of compound systems in three fusion reactions, and comparing to the excitation energies which are calculated according to the Q value of corresponding fusion reaction, two methods to calculate the excitation energy are tested. The results show that the exci-tation energy information given by ImQMD model is reasonable. The 197Au+197Au reactions at incident energies of 35 MeV/u are simulated with the ImQMD model. The correlation between the primary fragment size and excitation energy shows that, in the case of peripheral collisions induced by heavy nuclei, even though the dynamic-s simulation lasts quite long time, there are still quite a lot of primary fragments with rather high excitation energies. Therefore, the statistical decay analysis at the end of the ImQMD simulation is necessary. By using the hybrid model of ImQMD+GEMINI, the charge distribution of fragments in Au+Au at 35 MeV/u can be reproduced reasonably well.In the other work of this thesis, we studied the nuclear temperature in HICs at low and intermediate energies. Firstly, the charge, bound charge and isotope distributions of projectile fragments in 107、124Sn+120Sn reactions at incident energy of 600 MeV/u were simulated with the hybrid model of ImQMD+GEMINI. The calculation results can well reproduce the experimental data. By using THeLi thermometer, we extracted the nuclear temperatures in isoSn+120Sn reactions at incident energy of 600 MeV/u. The behavior of temperature decreasing with increasing bound charge is in agreement with the experimental data. We also studied the isospin asymmetry and density dependence of symmetry energy on the nuclear temperature. We found that the nuclear temperature is related to the isospin asymmetry of the projectile nuclei, and the nuclear temperature in the system with larger isospin asymmetry degree is relative higher than that in the system with smaller isospin asymmetry degree. The nuclear temperatures calculated by using different density dependence of symmetry energy show that: The nuclear temper-atures obtained from the stiffer symmetry energy is higher than those obtained from the softer symmetry energy, especially for the neutron-rich system. In addition, by com-parison to experimental data, we found that a softer symmetry can well reproduce the experimental data, it is consistent with the symmetry energy range which is constraint from other observations constraint.