利用重离子碰撞确定对称能的密度依赖形式

Extracting the Symmetry Energy Information with Heavy Ion Collisions

对称能的密度依赖形式对于中子星结构及其演化规律、不稳定核结构及其反应等都有非常重要的影响。由于目前理论上预言的对称能密度依赖形式有很大的不确定性,因此确定对称能的密度依赖形式已经成为目前核物理研究的一个重要课题。丰中子核重离子碰撞是目前在实验室条件下确定对称能密度依赖形式的一种重要手段,其主要的理论依据就是微观输运理论。首先回顾了目前输运理论的两个处理方法,即Boltzmann-Uehling-Uhlenbeck(BUU)和量子分子动力学(QMD)方法,并重点介绍了改进的量子分子动力学模型(ImQMD05)。利用ImQMD05对中能重离子碰撞的一些基本观测量,如碎块分布、集体流、核核碰撞的阻止本领等进行了详细研究,通过与实验数据的比较,确定了ImQMD05中同位旋无关部分的平均场、介质中核子-核子散射截面。为了从重离子碰撞的实验数据确定对称能的密度依赖形式,详细研究了输运理论中对称势以及同位旋相关的介质中核子-核子散射截面对于同位旋敏感观测量的影响。研究显示,对于Fermi能附近的重离子碰撞,输运理论预言的同位旋敏感的观测量,如DR(n/p)和Ri,强烈依赖于对称势的软硬,而不敏感于介质中同位旋相关的核子-核子散射截面。多重碎裂作为中能重离子碰撞的一个主要特点,本文论述的研究显示碎块发射机制对同位旋敏感的观测量,出射核子的双中子质子之比DR(n/p)和同位旋转移之比Ri,都有重要影响。以重碎块的同位旋不对称度为探针得到的同位旋转移之比Ri(δZmax>20)要大于Ri(δ),这些测量能够在NSCL/RIKEN的相关实验上进行测量。同时,这一现象的研究对于理解碎块的发射机制及碎块发射的特征时间等问题将有重要的帮助。为此,采用ImQMD05研究了112,124Sn+112,124Sn在入射能量50MeV/u,不同碰撞参数条件下的多个实验观测量,如DR(n/p)、同位旋扩散Ri以及同位旋转移之比的快度分布。研究表明,模型能够同时自洽地解释这几个实验观测量,并通过与这些实验数据的分析比较,自洽地确定出低密区对称能密度依赖形式的范围。

Nuclear symmetry energy plays an important role in the properties of nuclei and neutron stars.Since theoretical predictions of the density dependence of symmetry energy S（ρ） from microscopic nucleon-nucleon interactions show large uncertainties,especially in the region of suprasaturation density,constraining the density dependence of symmetry energy has become one of the main goals in nuclear physics and has stimulated many theoretical and experimental studies.In this paper,we have reviewed the ImQMD05 code and its description on charge distribution,collective flow and nuclear-nuclear stopping.By comparing ImQMD05 prediction with data,the isoscalar part of nucleonic mean field,the energy and density depen-dence of inmedium NN cross sectoin in the ImQMD05 were determined.In order to extracting the symmetry energy information by comparing the ImQMD05 calculations with the data for 112,124Sn＋112,124 Sn at Ebeam=50 MeV/u,the influence of symmetry potential and in-medium NN cross section on the isospin sensitive observables of intermediate-energy heavy-ion collisions was investigated.Focusing on the region above the Fermi energy,our results show that the symmetry potential plays a more important role in the experimental observables,such as double neutron to proton ratio and the isospin transport ratio Ri,than that the in-medium nucleon-nucleon cross section does.Since the copious production of intermediate mass fragments is a distinguishing feature of intermediate-energy heavy-ion collisions,we also examined the influence of cluster emission on the isospin transport ratio using different isospin tracers.The values of the isospin transport ratios with the tracer defined by the isospin asymmetry of the heaviest fragments with Z≥20 in the projectile region is greater than those obtained from projectile residues（emitting source）.This phenomenon can be tested experimentally.By comparing the ImQMD05 predictions with the data for three observables,the self-consistent constraints on the symmetry energy at subsaturation density were obtained.