用量子分子动力学模型研究中能重离子碰撞中的核物质约化粘滞系数  

Studies on the specific viscosity of nuclear matter formed in intermediate energy heavy ion collisions by using the isospin dependent quantum molecular dynamics model

作　　者：周铖龙 马余刚[1] 方德清[1] 张国强[1] 曹喜光[1] 

机构地区：[1]中国科学院上海应用物理研究所,嘉定园区上海201800

出　　处：《核技术》2014年第10期78-86,共9页Nuclear Techniques

基　　金：国家自然科学基金项目(No.11035009、No.11175231、No.10979074、No.11405248);国家重点基础研究发展计划项目(No.2013CB834405、No.2014CB845401);中国科学院知识创新工程资助

摘　　要：剪切粘滞系数(η)和熵密度(s)的比值,简称约化粘滞系数(η/s),是刻画物质的一个基本输运系数,对研究核物质的液气相变及状态方程具有重要的作用。我们在同位旋依赖的量子分子动力学模型(Isospin dependent quantum molecular dynamics,IQMD)的基础上,用有限温度Thomas-Fermi理论提取中能重离子碰撞过程中的热力学性质。在此基础上分别用Green-Kubo方法及Danielewicz参数化公式来提取粘滞系数,进而得到约化粘滞系数。研究发现,随着温度或碰撞能量的增加约化粘滞系数会出现一个极小值,这与中能重离子碰撞过程中的液气相变相一致。Background： Specific viscosity, defined as the ratio of shear viscosity to entropy density, was found as a probe of phase transition. It was observed that the specific viscosity reaches a local minimum at the critical point for both macroscopic and microscopic systems. In addition, a universal lower boundary was predicted by the AdS/CFT theory. In intermediate energy heavy-ion collisions, the studies on the relationship of specific viscosity and nuclear liquid-gas phase transition are still rare. Purpose： This paper gives a brief review for our calculations of the specific viscosity of nuclear fireball created in head-on Au＋Au collisions in intermediate energy, for the study of liquid-gas phase transition. Besides, the influences of nucleon-nucleon cross section and symmetry energy on the specific viscosity are also addressed. Methods： The hot Thomas-Fermi formalism is employed to extract the thermal properties of the nuclear matter which is located in the central region of the collision. Two different methods, the Green-Kubo and Danielewicz＇s parameterized formula, are used to evaluate the shear viscosity. Finally the correlation between specific viscosity and temperature or collision energy is presented. Results： In both cases, a local minimum of specific viscosity which has the value of around 10 times of the lower bound is found at certain temperature or beam energy. It is clearly shown that the calculated results are not sensitive to the symmetry energy but strongly influenced by the nucleon-nucleon cross section. Conclusion： Our calculations demonstrate that a local minimum of specific viscosity can be taken as a probe of nuclear liquid-gas phase transition which takes place at around 10 MeV. Furthermore, the lower bound of specific viscosity is still valid for nuclear liquid-gas phase transition.

关 键 词：中能重离子碰撞 液气相变 量子分子动力学模型 约化粘滞系数 

分 类 号：O571.423[理学—粒子物理与原子核物理]