【作者】 张英逊；

【导师】 李祝霞；

【作者基本信息】 中国原子能科学研究院 ， 粒子物理与原子核物理， 2005， 博士

【摘要】 核物质作为由强相互作用的粒子组成的复杂的量子体系,其性质随密度、温度的变化呈现包括相变在内的各种变化形态。特别是对于非对称核物质的核态方程而言,由于不同的微观相互作用理论模型所给的对称能的密度依赖形式差别很大,因此对于通过重离子碰撞来研究对称能的密度依赖形式已经成为目前核物理研究的热点领域之一。 本工作首先对于原有的ImQMD模型做了进一步的发展,其主要改进有以下几个方面:1)、势场中的体对称势部分采用了非线性的密度依赖形式;2)、碰撞部分采用了同位旋相关的核子核子碰撞截面以及同位旋依赖的Pauli阻塞。3)、我们在ImQMD模型中引入了基于Skyrme能量密度泛函的更为自洽的同位旋依赖的势场以及动量相关项。4)、在碰撞部分,采用了密度依赖的介质中核子核子碰撞截面;σ_nn^*=（1-αρ/ρ0）σ_nn^free并对Pauli阻塞进行了进一步的改进。 基于上述发展的ImQMD模型,我们研究了^112,124Sn+⁸⁶Kr在入射能量为Eb=25AMeV的擦边碰撞中,两种不同的对称势密度依赖形式（软的对称势以及硬的对称势）对多种实验观测量的影响,为实验上提取对称势的密度依赖形式提供了一些新的物理观测量。研究表明:该能量下的擦边碰撞中,核子以及轻碎块主要从受到挤压的脖子区发射;而受激发的类弹/类靶（PLF^*/TLF^*）的密度基本上维持在正常密度附近。采用软的对称势时计算给出的出射核子的N/Z之比要比硬的对称势所给的相应的N/Z之比要高;从轻碎块的同位素分布上可以看出,采用软的对称势比采用硬的对称势更容易产生丰中子的轻碎块;而对于类弹、类靶的重碎块,情形则相反,即采用硬的对称势比采用软的对称势更容易产生丰中子的重碎块。我们的研究表明,擦边反应中硬的对称势所给的出射核子的N/Z之比随碰撞参数变化的斜率要大于采用软的对称势所给的相应的斜率;采用软的对称势所给的Y（³H）/Y（³He）要高于采用硬的对称势所给的相应的值。对于这两个反应在擦边碰撞中产生的轻碎块以及类弹的重碎块的同位素产额的分析表明,两个不同反应的同一种同位素产额之比满足Isoscaling规律,即R₂₁=Y₂（N,Z）/Y₁（N,Z）=Cexp（αN+βZ）。我们的研究给出采用软的对称势所给的Isoscaling的α参数的值要高于采用硬的对称势所给的相应的值。通过上述擦边反应的研究,我们给出了对对称势比较灵敏、新的实验观测量,即1)、擦边反应中出射核子的N/Z随碰撞参数变化的斜率;2)、中心碰撞时的更多还原

【Abstract】 The current studies on heavy-ion collisions at intermediate energy are of broad scientific interest. One of the main motivation for these studies is to extract information of the Equation of State （EoS） of nuclear matter, especially the density dependence of the symmetry energy term. The symmetry energy term in the EoS are very important for understanding many intersting astrophysical phenomena. But it is also a subject with large uncertainties, especially its density dependence form of the symmetry energy. Therefor, to acquire more accurate knowledge of the symmetry energy term in the EoS becomes one of the main goals in nuclear physics at present, and has driven a lot of theoretical and experimental efforts.We have developed a new version of Improved Quantum Molecular Dynamical model （ImQMD） for studing these problems. The main improvments are as follows: l).In order to study the dynamical effects of the density dependence of the symmetry potential on heavy-ion collisions by means of transport theory, we verify the symmetry energy with the nolinear form of the density dependence of the bulk symmetry potential energy in the potential energy density functional. 2). In collision term, isospin dependent nucleon-nulceon scattering cross sections and the isospin dependent Pauli blocking effect are used in our model. 3). we introduced a self-consistent isoscalar and isovector potential based on the Skyrme energy density functional in our microscopic transport model, and the momentum dependence interaction is also take into accout. 4). new treament of the Pauli blocking in collision part are adopted, and a phenomenological density dependent in-medium nucleon-nucleon cross sections are used.Based on new version of ImQMD model, the peripherial heavy-ion collisions of ^112,124Sn +⁸⁶ Kr at Eb=25AMeV are studied in order to seek those obsevables that are sensitive to the symmetry potential. For peripheral heavy-ion collisions, that nucleons, light charged particles, and very few intermediate mass fragments are emitted mainly in the compressed neck region. The density of PLF^*/TLF^* is always around the normal density and it means that thereis no compression happening for PLF"/TLF*. Our study shows that the N/Z ratio of emitted nucleons, isotope distributions of light charged particles calculated with soft symmetry potential are more neutron rich than that with stiff symmetry, and the heavy residue isotope distributions show an opposite trends, i.e., the stiff symmetry potential produce more neutron rich isotope than the soft symmetry potential for the conservation of the N/Z ratio of the reaction system. Our results show that the slope of the average N/Z ratio of emitted nucleons vs impact parameters for these reactions is sensitive to the density dependence of the symmetry energy. The N/Z ratio increase with impact parameters and the increase slope with stiff symmetry potential is about twice as large as those with soft symmetry potential. The ratio of Y（3H）/Y（3He） in central collisions are also depend on the symmetry potential stiffness, the ratio of Y（3H）/Y（sHe） calculated with soft symmetry potential are larger than that with stiff symmetry potential. The difference between the calculated results calculated with stiff and soft symmetry potential is enhanced for neutron rich reaction systems. We also analysis the isoscaling law of the light charged particles and heavy residues. Our results show that the Isoscaling parameters a values of the light charged particles and heavy residues depend on the symmetry potential. The a values of light charged particles and heavy residues calculated with soft symmetry potential are larger than that with the stiff symmetry potential.We have further studied the dynamical effect of the different Equation of State on heavy ion collisions with our new version of the ImQMD model. We find that charge distributions of products in heavy ion collisions calculated with Skyrme force -Skp parameters （corresponding the soft EoS） are in good agreement with the experiments at incident energies ranging from the Fermi energy to 400AMeV. We pay main attention on studing the influence of different: EoS on the collective flow in HICs. For the stiffcr equation of state can generate larger pressure than the softer EoS, so. the directed and elliptical flow calculated with the stiff EoS are stronger than that with the soft EoS at higher incident energy above the transition energy EiTans. The excitation functions ofelliptical flow alsoshow an isospin effect that neutron rich reaction systems can generate more negative values of v-i- We systematically studied the elliptical flow excitation function for several reaction systems, and the results show the system size dependence of the directed flow and elliptical flow. The influence of the momentum dependent interaction and the in-medium nucleon-nucleon cross section o*nn are also checked, and we find they are very important for correctly describing the excitation function of elliptical flow. We have also studied the influence of different EoS on the yield of light charge particles, the yield of 3H and 3/7e are also depend on the stiffness of the Equation of State. The yield of 3H and 3He calculated with the stiffer equation of state （SIII） are obviously lower than that with the softer EoS（Skp, Skm*, Sly7）.更多还原