【作者】 贾一剑；

【导师】 刘福虎； 李祝霞；

【作者基本信息】 山西大学 ， 粒子物理与原子核物理， 2009， 硕士

【摘要】 输运理论模型作为模拟重离子反应的一种重要工具,已经被广泛应用,而且取得了很大成功。碰撞项作为输运理论的两部分输入量之一,随着反应能量的增大,其贡献也越来越大,尤其是在中能重离子反应中。长期以来,人们对平均场做了大量的研究,通过密度依赖引入了介质效应;而对碰撞项的研究还很少,大多采用的仍然是自由的核子-核子（NN）散射截面（即实验测量得到的NN散射截面）。介质中的NN散射截面无法从直接实验上测量得到,只能依靠理论计算给出。但是,目前理论计算的结果差别很大,不同的理论或相同理论计算细节不同给出的结果存在比较大的差别。这种差别,不仅存在于定量上,而且存在于定性方面:主要是在大小和同位旋依赖方面。为了消除这种差别,亟需通过寻找灵敏实验观测量,通过实验与理论计算的比较,间接获取介质中的NN散射截面的信息。本工作主要基于改进了的量子分子动力学模型（ImQMD05模型）,对介质中的NN散射截面进行了研究。我们研究了入射能量为400AMeV的⁹⁶Ru+⁹⁶Ru及⁹⁶Zr+⁹⁶Zr两个对称反应系统中出射的自由中子、自由质子和它们的比值n/p,在不同的截面下的动能分布、角分布等,从中寻找了一些敏感的观测量来对介质中的NN散射截面进行约束。关于截面的选取,一方面围绕截面的同位旋依赖的不同,选取R_iso^med=σ_np^med/σ_pp（nn）^med=0\0.10\0.1\0.333\1\2\∞七种不同的同位旋依赖的截面;另一方面围绕截面的大小不同,选取R^med=σ_NN^med/σ_NN^free=1\（1+0.4ρ/ρ₀）\2三种大小不同的截面。首先,通过对⁹⁶Ru+⁹⁶Ru系统出射的自由核子在动量空间的出射数目、经历的最大密度、经历的碰撞次数、达到最大密度的时间等物理量的研究,全面分析了反应系统的出射机制。通过分析发现,受截面直接影响比较大的自由核子,主要从横向出射;由于它们经历的碰撞次数少,所以能量耗散少,出射的动能比较大。其次,结合出射核子的能谱和角分布的分析得到,在同位旋相关的不同截面下,这部分自由中子与自由质子的比值n/p,敏感地依赖于介质中同位旋相关的NN散射截面;随着R_iso^med的增加,n/p单调地减小。同时,通过比较⁹⁶Ru+⁹⁶Ru及⁹⁶Zr+⁹⁶Zr两个系统发现,丰中子系统表现出更为明显的同位旋依赖性。对截面大小的分析方面,同样结合自由核子的出射机制、能谱、角分布进行分析,发现截面的大小变化对自由核子的中质比（n/p）几乎没有影响,对核阻止本领的影响非常明显。通过对两个阻止本领的观测量Q_zz和E_RAT进行分析得到,O_zz/A随R^med的增大单调减小,E_RAT随R^med的增大单调增大。比较两个不同丰中子程度的系统发现,系统的丰中子程度对阻止本领随截面大小的变化没有影响,只是存在数值上的差别。更多还原

【Abstract】 Transport theory is an important method to simulate the process ofheavy-ion reactions, and the collision term is one of the input parts in thetransport models. Free nucleon-nucleon cross section has been always used inthe collision term. However, the contributions from nucleon-nucleoncollisions are as important as that from the mean field contributions atintermediate and high energy heavy ion collisions in the transport modelapproaches. The information on the isospin dependence of the in-mediumnucleon-nucleon cross section is very important for us to constrain thedensity dependence of symmetry energy, especially at super-saturationdensities. Up to now, there is not much study on isospin dependence of thein-medium NN cross section through heavy ion collisions and very largeuncertainty exists concerning the isospin dependence of the in-medium NNcross section. Therefore, to find sensitive observables and then to constrainthe isospin dependence of the in-medium NN cross section is necessary forfinally accurately constrain the density dependence of the symmetry energy.The aim of this work is to search for the sensitive observable for the isospindependence of the in-medium nucleon-nucleon cross sections.The influence of the isospin dependence of in-medium nucleon-nucleoncross sections on the n/p ratios for emitted nucleons in reactions ⁹⁶Zr+⁹⁶Zrand ⁹⁶Ru+⁹⁶Ru at E/A = 400MeV is investigated by mean of the ImproveQuantum Molecular Dynamics model. The reason for select E/A = 400MeVis that the nucleon-nucleon cross sections are important and elastic collisionchannel is still dominant at this incident energy.Firstly, we investigate the nucleon emission mechanism. Our resultsshow that the emitted nucleons with higher kinetic energies carry clearinformation of the isospin dependence of in-medium NN cross sectionsbecause they experience less two-body scatterings. Therefore, the n/p ratiofor emitted high energy nucleons is selected to study the isospin dependence of the in-medium nucleon-nucleon cross sections.Secondly, the influences of isospin dependence of in-mediumnucleon-nucleon cross sections on the n/p ratios of emitted nucleons aresystematically analyzed by varying the ratio R_iso^med=σ_np^med/σ_pp（nn）^med.Thecorrelation between the n/p ratios for emitted nucleons with E_k＞300MeV andR^med=σ_NN^med/σ_NN^free shows that n/p ratio depends on R_iso^med and the isospinasymmetry of the reaction system. Therefore, we propose that the n/p ratio ofemitted high energy nucleons in very neutron-rich reaction system at severalhundreds AMeV can be taken as sensitive observable to constrain the isospindependence of in-medium nucleon-nucleon cross sections.Based on the analysis for the difference of the energy distribution andangular distribution on the NN cross section, we find the very small influenceof the cross section on the free neutron and proton ratios, and the obviousinfluence of the stopping power. The calculation result is that the variableQ_zz/A decrease with increasing R^med and E_RAT increase with increasingR^med. For the different neutron-rich systems, we obtain the small influence ofthe tendency of stopping power on the NN cross section, and the differenceexists in the calculated values.更多还原