【作者】 李文飞；

【导师】 徐瑚珊； 张丰收； 李君清；

【作者基本信息】 中国科学院研究生院（近代物理研究所） ， 粒子物理与原子核物理， 2004， 博士

【摘要】 正在建造的兰州重离子加速器冷却储存环(HIRFL-CSR)加速器系统可以提供能量为几个MeV/u到1.1GeV/u的重离子束流。其中超重核合成机制研究和非对称核物质状态方程研究是该加速器系统上的两个重要的研究方向。在HIRFL-CSR建造的同时,对相关物理进行系统的有针对性的理论研究是必需的。本论文是为配合在HIRFL-CSR上开展这两方面的物理工作而进行的理论研究。主要内容包括:建立了一个描述超重核合成的理论模型,并基于这一模型研究了实验上合成超重元素的最佳激发能和最佳弹靶组合;基于同位旋相关的量子分子动力学(IQMD)模型,研究了非对称核物质性质,并提供了几种在HIRFL-CSR能区的实验中提取非对称核物质状态方程的可能途径。基于双核系统概念建立了一个描述超重核合成的理论模型。该模型合理地包含了重离子熔合过程中的动力学效应,并且能够充分地体现重离子熔合过程中弹靶的结构效应。并基于该模型重点开展了如下几个方面的工作:1.计算了基于冷熔合反应合成超重核的可能的最佳激发能。由于超重核合成反应的激发函数宽度很窄,因此要求实验能够尽量精确地选择炮弹的入射动能,以给出最高的蒸发剩余核截面。本文最佳激发能的计算可以为超重核合成实验选择束流能量提供参考。2.研究了蒸发剩余核截面的弹靶相关性,并从理论上给出了合成超重核的最佳弹靶组合。如对以Zn同位素为炮弹的冷熔合反应合成112号元素时,选择67Zn为炮弹时蒸发剩余核截面最高。这为实验上挑选弹靶组合提供了依据。3.计算给出了超重核的自旋布居及其对复合核裂变位垒和鞍点态形变的依赖性。发现超重核的自旋布居强烈地依赖于复合核的裂变位垒,高的裂变位垒会给出宽的自旋布居,而超重核的自旋布居对鞍点态形变不是很敏感。因此通过实验测量超重核的自旋布局来提取超重复合核的裂变位垒是可能的。基于该模型还研究了超重复合核的存活几率及其质量、能量及角动量等相关性,发现超重复合核的存活几率存在显著的奇偶效应。同时对多核子转移反应合成超重元素的可能性进行了初步的探讨。目前的结果表明基于多核子转移反应合成108号以上的元素是困难的。非对称核物质状态方程研究对天体物理及理解奇异核结构是十分重要的。本论文基于Skyrme-Hartree-Fock理论以及IQMD模型研究了非对称核物质的性质,并提出几种在HIRFL-CSR能区开展非对称核物质状态方程研究的可能方案。具体工作包括:1.研究了中能重离子碰撞过程中的化学不稳定性,计算表明重离子碰撞中是可以发生化学不稳定性的,且在入射能量较高时化学不稳定性会消失。这对理解实验上观测到的原子核多重碎裂的同位旋效应是很重要的。另外首次研究了高密核物质的化学不稳定性及其发生的条件。2.中子星主要是由致密的极丰中子物质组成,因此提供了研究高密非对称核物质的自然实验室。基于Skyrme-Hartree-Fock理论研究了两种典型的非对称核物质状态方程(软对称势和硬对称势)对中子星中化学组分的影响。结果表明,对具有软对称势的核物质状态方程,中子星内部会出现纯中子物质芯。3.基于IQMD模型研究了同位旋相分化(isospin fractionation)现象并分别从静态和动力学的观点研究了同位旋相分化现象的产生机制。另外计算表明,实验上可以利用同位旋相分化现象来提取非对称核物质状态方程,并且通过与MSU的实验数据比较指出非对称核物质应该有软的对称势。这一结果对非对称核物质状态方程的密度相关形式提供了约束。4.研究了中能重离子碰撞中的径向流现象及其产生阈的同位旋效应。发现软对称势和硬对称势的状态方程给出了截然相反的径向流产生阈的同位旋相关性。这为实验上利用径向流产生阈的同位旋相关性来提取非对称核物质状态方程提供了一条有效途径。本论文工作从理论上为在HIRFL-CSR加速器系统上开展超重核合成及非对称核物质状态方程研究提供了有针对性和有价值的参考。更多还原

【Abstract】 HIRFL-CSR accelerator system can/will provide heavy-ion beams with energies from several MeV/u up to 1.1 GeV/u. The synthesis of the superheavy nuclei (SHN) and the study of the asymmetric nuclear matter equation of state (EOS) are two of the proposed physics programs at it. Before pursuing the experimental research, detailed theoretical study which aimed at the related physics is important. This thesis is devoted to the theoretical study on the two proposed physics programs at HIRFL-CSR. In this thesis, for SHN study, a theoretical model to describe the synthesis of the SHN is founded. Based on this model, the optimal excitation energy and optimal project-target combination to synthesize the SHN are calculated. For exploring the asymmetric nuclear matter EOS, using the isospin dependent quantum molecular dynamics (IQMD) model and Skyrme Hartree-Fock theory, the properties of the asymmetric nuclear matter are studied, and several possible ways to extract the information of the asymmetric nuclear matter EOS at HIRFL-CSR are provided.A model for describing the synthesis of the SHN is developed based on the di-nuclear system (DNS) conception. The advantages of this model are that the dynamical effects are included in the fusion process reasonably and the structure effects of the driving potential can be well maintained. By using this code, the following contents are investigated emphatically based on the requirements from experimental research:1) The optimal excitation energies are predicted for synthesizing the elements of 114,116 and 118 by cold fusion reactions after reproducing the existing experimental data of optimal excitation energies for synthesizing element 104-112. As the excitation function for producing the SHN is very narrow, the result can be treated as a valuable reference for selecting the optimal beam energies for experimental study.2) The projectile or target dependence of the evaporation residue cross-section is studied, and the optimal projectile-target combination for producing a certain SHN can be predicted. The result shows that there exists an optimal projectile-target combination which gives the highest evaporation residue cross-section. This is different from the idea that the cross-section increases with the neutron enrichment of the projectile-target combination in general. For an example, in the case of synthesis of the 112th element through Zn isotope induced cold fusion, the optimal projectile-target combination is 67Zn+208Pb, not the combination of very expensive isotope 70Zn on 208Pb.3) The spin populations of the SHN are studied. The calculations show that the spin population of the SHN depends strongly on the fission barrier. Higher fission barrier gives wider spin population. And the spin population of the SHN is not very sensitive to the deformation of the SHN at saddle point. This indicates that it is possible to extract the fission barrier by measuring the spin populations of SHN in experiment.In addition, the survival probability and its dependences on the mass, excitation energy and angular momentum are investigated, and the pairing effect of the survival probability is studied. Moreover, in order to explore the new mechanism of synthesizing SHN, the possibility for synthesizing SHN by a few nucleons transfer reaction has also been discussed preliminarily. Present results show that it is difficult to synthesize the SHN with charge number Z larger than 108 by a few nucleons transfer reaction.The asymmetric nuclear matter EOS is very important in astrophysics and in understanding the structure of exotic nuclei. However, up to now very limited knowledge is known to people, especially for the EOS at high density. Based on the Skyrme-Hartree-Fock theory and the IQMD model, the properties of the asymmetric nuclear matter are studied and some possible ways to extract the information of the asymmetric nuclear matter EOS to be carried out at the HIRFL-CSR accelerator system are provided. The related contents are:1) Based on the IQMD model, the chemical instabilities in heavy-ion collisions are investigated. Calculations show that the chemical instability can happen in the process of heavy ion collisions at intermediate energies. With the incident energy getting higher and higher, the chemical instability events become fewer and fewer. This result is important for understanding the observed isospin effects in nuclear multifragmentation. In addition, the chemical instability at high density is investigated for the first time based on the Skyrme-Hartree-Fock theory.2) It is difficult to produce the nuclear matter with very high density in laboratory. As the neutron stars are composed of high density neutron rich nuclear matter, they provide the nature laboratories for studying the asymmetric nuclear matter EOS at high density. Therefore, it is very useful to investigate the neutron star properties using different asymmetric nuclear matter EOS. Based on the Skyrme-Hartree-Fock theory, the proton fractions of the neutron stars are investigated for two typical asymmetric nuclear matter EOS, namely the soft symmetry potential and stiff symmetry potential. It is found that the pure neutron mater core can be formed in the center of the neutron star with the soft symmetry potential nuclear matter EOS.3) Using the IQMD model, the isospin fractionation in the nuclear multifragmentation is studied, and the production mechanism of the isospin fractionation is investigated from the static and dynamic point of view. And the calculations show that the isospin fractionation can be used as a probe to study the isospin dependence of the asymmetric nuclear matter EOS. By comparing the results with the experimental data of MSU group, it is concluded that the asymmetric nuclear matter should have soft symmetry potential. This provides the constraint for the density dependence of the asymmetric nuclear matter EOS.4) The energy threshold for producing the radial flow and its dependences on the isospin of the incident systems are calculated. It is found that the asymmetric nuclear matter EOS with soft symmetry potential and stiff symmetry potential gives opposite isospin dependence of the threshold for producing the radial flow. This indicates that one can extract the information of the isospin dependent part of the nuclear matter EOS by investigating the isospin dependence of the radial flow in experiment.The results in this thesis provide the valuable theory references for studying the SHN and the asymmetric nuclear matter EOS at the HIRFL-CSR accelerator system.更多还原