Studies On Symmetry Energy Effects In Asymmetric Nuclear Reactionsy

Nuclear equation of state(EOS)is the one of the basic issues in the nuclear physic-s research.The isospin scalar term of EOS has already been well determined,so the isospin vector term,i.e.density dependence of symmetry energy,is an urgent problem to be solved.Because the symmetry energy is closely related to many issues such as the properties of exotic nuclei and reactions,nuclear drip-line positions,dense star prop-erties,and stellar evolution and so on.In this thesis,by using an improved quantum molecular dynamics(ImQMD05)model,the asymmetric nuclear reaction 40Ar+197Au with an incident energy of 35 MeV/u was simulated,and the effects of the symme-try energy on some observations such as mass,charge,isotope distribution,angular distribution,energy spectrum,double differential cross section,etc,were investigated.Following are the main works and results obtained:1.The effects of the length of simulation time of ImQMD model on the yields of the reaction products were investigated.It is found that due to the stability of the nucleus sampled by the model,the mass and charge distribution of the reaction prod-ucts are obviously dependent on the length of simulation time.In order to reduce the influences by the model instability,the stopping time is determined according to the average excitation energy of the primary products.2.The charge and mass distributions of products with different impact parameters calculated by various forms of density dependence of symmetry energy are studied.It is found that,in the central and semi-central collisions,the fragmentation is more obvious with the soft symmetry energy adopted than the case with the stiff symmetry energy.Further,the effects of the symmetry energy on the isotopic distributions of the light nuclei(Z=2,3,4),the projectile-like fragments(Z=9,10,11)and the target-like fragments(Z=58,60,62)are investigated.It shows that the isospin asymmetry degrees of the nuclei at the distribution centers increase with the impact parameter in-creasing and the dispersions of distributions are unsensitive to the impact parameter.By comparing the isospin asymmetry degrees between the isotopes locating at the cen-ters of the three nuclides(Z=58,60 and 62)and the corresponding β-stable nuclei,it is found that the results calculated with the soft symmetry energy(γ=0.5)adopted are more close to the isospin asymmetry degrees of the β-stable nuclei.3.The effects of symmetry energy on the ratio of neutron-proton yield were further studied by analysing the angular distribution,energy spectrum and double differential cross section.The results show that nucleons with different emission angles and emis-sion energies have different sensitivities to the symmetry energy.The ratio of neutron-proton yield with the nucleons emitted into the small angle with intermediate energies shows the highest sensitivity.The sensitivity of the ratio of neutron-proton yield to the symmetry energy can be enhanced by the energy and angle filter.For central or semi-central collisions,the sensitivity can be enhanced by about 35%if the nucleons with energies in the range of 30-60 MeV and with emission angle smaller than 30°are selected.