Research On The Density Issue About Pions As A Probe Of The Symmetry Energy

The density dependence of nuclear symmetry energy is not only essential for the struc-ture and nuclear reaction of rare isotopes,but also for many interesting issues in astrophysics.To pin down the equation of state(EOS)for asymmetric nuclear matter has always been an important goal in nuclear physics and astrophysics.For a long time,the?~-/?~+ratio is considered as a sensitive probe reflecting the information of the symmetry energy at the high-density.But there are some research works which show that?~-/?~+ratio may reflects the symmetry energy around the saturation density rather than that at the high-density,at least not entirely the information about the symmetry energy of the high-density area.Therefore,which density region of symmetry energy detected by the pion probe is an urgent problem to be clarified.In this thesis,by using the Ur QMD model,the?~-/?~+ratio in the heavy-ion+heavy-ion reactions and the nucleon heavy ion reactions are studied.The effects of the reaction system isospin asymmetry,incident energy,and impact parameter on the?~-/?~+ratio are analyzed.The main works and findings are as follows:1.The Benchmark calculations on Ur QMD model were performed by using the part of spallation reaction nuclear data provided by IAEA.The results show that the double differential cross sections of n,p,?~-,?~+can be reproduced quite well by the Ur QMD model.The Ur QMD model is a very good tool to study the nucleon-heavy ion reactions.2.The?~-/?~+ratio in the heavy-ion+heavy-ion reactions was studied.By the com-parison of the excitation function of the?~-/?~+ratio between the theoretical calculations and the experimental data,it was found that the experimental data in the low incident energy region can be decribed well with the soft symmetry energy adopted,while the ones in the high incident energy region can be described better with the stiff symmetry energy adopted.Further the nucleon densities for the locations where pions produced and freeze-out were studied.The results show that the freeze-out densities is significantly lower than the producing density.The width of freeze-out density distribution increases with the system size increases.As the incident energy increases,the width of distribution increases,but the location of the peak of distribution moves toward the low-density area.Therefore,the?~-/?~+ratio actually reflects the symmetry energy below the saturation density.3.The?~-/?~+ratio in the proton-heavy ion reactions was studied.The producing and freeze-out density distribution of?~-and?~+in the reactions with different incident energies bombarding different targets are analyzed.The results show that the differences between the producing and freeze-out density distributions of pion are not obviouse with different incident energies in the same reaction system.The density distribution basically does not change with incident energy,and the peak of the freeze-out density distribution locates at the sub-saturation density.Further analysis on the distribution of?meson freeze-out density in the reactions of protons bombarding Ca and Sn isotope chains show that,for the same isotope chain,the freeze-out density distributions of?meson are close to each other,or even almost the same,and the peak of the density distribution locates at the sub-saturation density.Therefore,the?~-/?~+ratio actually reflects the symmetry energy below the saturation density.