低SIS 能区重离子碰撞过程的系统性研究
Systematic Studies of Heavy Ion Collisions in theLow SIS Energy Region
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摘要: 在极端相对论量子分子动力学(UrQMD) 模型中加入势能密度泛函形式的势修正、核子-核子弹性散射截面的更细致的介质修正以及碎片重构模式中的同位旋效应后，重点研究低SIS 能区(约40~400 MeV/u) 重离子碰撞的动力学过程。在较系统地研究此能区重离子碰撞后的轻碎片产生及集体流后，重点研究对高密区对称能密度依赖敏感的观测量。发现：(1) 质量对称的Sn 系列同位素反应系统，其中子平衡能的系统N=Z 分布能敏感探测对称能密度依赖；(2) 横速度/动量分布的中子和质子及氢同位素的椭圆流比vn2 =vp;H2 敏感依赖于对称能。经采用多组核物质标量不可压缩系数K0 值差别很小、对称能斜率参数L 值相差较大的Skyrme 势参数计算并对FOPI/LAND 相应数据做χ2 分析后抽取的L值为(89±45)MeV。After inserting the Skyrme potential energy density functional for potential update, more detailed medium modifications for nucleon-nucleon elastic cross sections, and the isospin effect for cluster recognition into the Ultra-relativistic Quantum Molecular Dynamics (UrQMD), the dynamic process of heavy ion collisions (HICs) at low SIS energies (about 40s400 MeV/u) is primarily studied. And, after systematically studying the emission and collective flows of light clusters from HICs in such beam energy region, the sensitive observables especially to the density dependent symmetry energy at supra-normal densities are focused. It is found that: (1)the initial neutron/proton ratio dependence of the balance energy of neutrons from mass-symmetric Sn isotopes can be taken as a useful probe to constrain the stiffness of the nuclear symmetry energy; (2) the transverse velocity/momentum dependence of the elliptic flow ratio of neutrons and protons or hydrogen isotopes (vn 2 =vp;H2 )is also sensitive to symmetry energy. The 2 analysis from the difference bet e the theoretical (taking Skyrme potential parametrizations with incompressibility K0 being almost same but the slope parameter L of symmetry energy being largely different) and experimental (taking FOPI/LAND data) vn 2 =vH2 values determines the value of L to be (89±45) MeV within in a χ2 uncertainty.Abstract: After inserting the Skyrme potential energy density functional for potential update, more detailed medium modifications for nucleon-nucleon elastic cross sections, and the isospin effect for cluster recognition into the Ultra-relativistic Quantum Molecular Dynamics (UrQMD), the dynamic process of heavy ion collisions (HICs) at low SIS energies (about 40s400 MeV/u) is primarily studied. And, after systematically studying the emission and collective flows of light clusters from HICs in such beam energy region, the sensitive observables especially to the density dependent symmetry energy at supra-normal densities are focused. It is found that: (1)the initial neutron/proton ratio dependence of the balance energy of neutrons from mass-symmetric Sn isotopes can be taken as a useful probe to constrain the stiffness of the nuclear symmetry energy; (2) the transverse velocity/momentum dependence of the elliptic flow ratio of neutrons and protons or hydrogen isotopes (vn 2 =vp;H2 )is also sensitive to symmetry energy. The 2 analysis from the difference bet e the theoretical (taking Skyrme potential parametrizations with incompressibility K0 being almost same but the slope parameter L of symmetry energy being largely different) and experimental (taking FOPI/LAND data) vn 2 =vH2 values determines the value of L to be (89±45) MeV within in a χ2 uncertainty.
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