中能重离子碰撞中同位旋效应的灵敏探针

在重离子碰撞过程中, 基于重质量的弹核轰击轻质量靶核有大量中等质量碎片向前发射的逆运动学效应, 利用同位旋相关的量子分子动力学观察和研究了重离子碰撞过程中同位旋效应的灵敏性． 计算结果表明在所选能区, 中等质量碎片的多重性与带电粒子总数之间的关联是提取介质中核子 核子碰撞截面的灵敏观测量, 而此时对称势的同位旋效应却不明显． Based on the revers kinetic intermediate energy heavy ion collision in which much more intermediate mass fragments are emitted towards forward angles, a sensitive observable on isospin effects in heavy ion collisions is investigated by means of using isospin dependent quantum molecular dynamics (IQMD). The calculation results show that the correlation between the multiplicity of intermediate mass fragments N IMF and total number of charged particles N c is...     

α发射数可作为提取同位旋相关平均场的灵敏探针

利用同位旋相关的量子分子动力学, 对中能重离子碰撞过程中的α发射数的同位旋效应进行了分析。 研究表明, α发射数强烈地依赖于同位旋相关平均场的影响, 而与同位旋相关核子 核子碰撞截面的依赖较弱, 可以作为提取同位旋相关平均场信息的灵敏探针。 由于实验上α粒子发射数的观测比较容易, 而且在很宽地能量范围内, 无论丰中子系统还是缺中子系统都较好地满足上述规律, 这样就可以通过实验上对α发射数的探测, 来提取介质中同位旋相关平均场的信息。 同时建议由实验对缺中子系统的质子发射数进行探测, 从中提取同位旋相关核子 核子碰撞截面的信息。 The α emissions in heavy ion collisions at intermediate energies are studied by using isospin dependent quantum molecular dynamics model. The number of α emitted is found to be strongly dependent on the isospin dependent mean field and weakly on the nucleon nucleon cross sections. The number of α emitted is easy to be measured in experiment and the above conclusion is still available no matter in neutron rich systems or in neutron deficient systems in a wide energy region, so it can be used as an excellent probe for extracting information of the isospin dependent mean field. After several years searching, some information of the isospin asymmetry equation of state has been obtained in theoretically. We suggest to investigate the number of α emitted in neutron rich systems and the number of proton emitted in neutron deficient systems experimentally to obtain more information of the isospin dependent mean field and the isospin dependent nucleon nucleon cross section.     

中能重离子碰撞过程中同位旋分馏的特征和机制

利用同位旋依赖的QMD模型主要对中能重离子碰撞中同位旋分馏机制和主要特征进行了讨论和分析， 得到了一些有趣的结果， 并建议将同位旋分馏强度作为提取同位旋相关平均场和建立同位旋不对称核物质状态方程的探针. The degree of isospin fractionation is measured by (N/Z)gas / (N/Z)liq，where (N/Z)gas and (N/Z)liq are the saturated neutron proton ratio of nucleon emissions ( gas phase) and that of fragment emitted (liquid phase) in heavy ion collision at intermediate energy. The calculated results by using the isospin dependent quantum molecular dynamics model show that the degree of isospin fractionation is sensitive to the neutron proton ratio of colliding system. In particular， the degree of isospin fractionation sensitively depends on the symmetry potential and weakly on the in medium nucleon nucleon cross section for the neutron rich system. In this case， we propose that the degree of isospin fractionation can be directly compared with the experimental data so that the information about symmetry potential can be obtained.     

中能重离子碰撞动力学理论的新进展

简要评述当前几种主要的中能重离子碰撞动力学理论优缺点的基础上，介绍本小组初步建立起来的两体关联输运理论以及中能重离子碰撞中同位旋相关输运理论的进展情况． Based on the analyses of successes and shortcomings of Boltzmann type models and quantum molecular dynamics for heavy ion collisions, the new progress of two body correlation transport theory and isospin dependent transport models for heavy ion collision are proposed.      

核物质热力学性质的同位旋和动量相关性

利用3个具有不同的同位旋和动量相关性的热力学模型研究了非对称核物质的热力学性质, 它们是重离子碰撞中同位旋弥散数据约束下的、 同位旋和动量相关的MDI模型, 完全动量无关的MID模型, 以及同位旋标量动量相关的extended MDYI(eMDYI)模型。 主要研究了同位旋非对称热核物质的对称能和系统力、 化学不稳定性以及液气相变的温度效应。 MDI模型对称能的温度效应来源于动能和势能两部分贡献, 而MID和eMDYI模型只有势能部分对对称能的温度效应有贡献。 研究结果还表明, 力学不稳定性区域、 化学不稳定性区域和液气共存区都依赖于模型的同位旋和动量相关性, 以及对称能的密度依赖关系。In this article, three models with different isospin and momentum dependence are used to study the thermodynamical properties of asymmetric nuclear matter. They are isospin and momentum dependent MDI interaction constrained by the isospin diffusion data of heavy ion collision, the momentum independent MID interaction and the isoscalar momentum dependent eMDYI interaction. Temperature effects of symmetry energy, mechanical and chemical instability and liquid gas phase transition are analyzed. It is found that for MDI model the temperature effects of the symmetry energy attribute from both the kinetic and potential energy, while only potential part contributes to the decreasing of the symmetry energy for MID and eMDYI models. We also find that the mechanical instability, chemical instability and liquid gas phase transition are all sensitive to the isospin and momentum dependence and the density dependence of the symmetry energy.     

Isospin Effects of the Mean Field and Two—Body Collision on the Fragmentation Process

Isospin effects of the mean field and two-body collision on the fragmentation as well as their dependences on the momentum-dependent interaction at intermediate energy heavy ion collisions are studied by using an isospindependent quantum molecular dynamics model.We find the prominent isospin effects of the multiplicity of the intermediate mass fragments Ninf,where Nimf depends sensitively on the isospin effect of the in-medium nucleonnucleon cross section and weakly on the variation of symmetry potential in the intermediate energy region.The momentum dependence interaction enhances the sensitivity of Nimf on the isospin effect of two-body collision.     

Roles of Coulomb Interaction on Isospin Fractionation in Intermediate Energy Heavy Ion Collisions

As we know that in fact, nuclei offer an interesting isospin situation, where symmetry potential, Coulomb interaction and isospin dependent in-medium nucleon-nucleon collisions are simultaneously present. But, in general, the isospin effects of Coulomb interaction is not studied separately in more detail, even though the studies for the isospin effect of the isospin dependent mean field in the heavy ion collisions include automatically the role of Coulomb term. However the Coulomb interaction is an important asymmetry term, which can bring important isospin effect into the observable in the intermediate energy heavy ion collisions. The study for the role of Coulomb interaction on the isospin effects of observable in intermediate energy heavy ion collisions, is important for exploring isospin physics and setting up the equation of state for the isospin asymmetry nuclearmatter.     

高动量分布对核反应中原子核的阻止本领的影响

利用同位旋相关的量子分子动力学模型研究了中能重离子碰撞过程中,具有不同中质比的中等质量核碰撞系统的高动量分布对原子核的阻止本领的影响,以及这种影响随入射能量的演化。研究结果表明:对于76Kr+76Kr,88Kr+88Kr碰撞系统,当入射能量较小时高动量分布对原子核的阻止本领影响很小;但当反应能量较大的时候,高动量分布对原子核的阻止本领影响较大,且这种影响随着入射能量和碰撞核质量的增大而增大。对于Ca同位素链的反应系统,高动量分布增大了反应系统的原子核的阻止本领。因此,高动量分布对中等质量核碰撞系统的原子核的阻止本领有重要影响,且与入射能量和碰撞核质量密切相关。Based on the isospin-dependent quantum molecular dynamics model, the effect of high momentum distribution on nuclear stopping in medium mass nuclear collision system is studied with different neutron-proton ratio in intermediate energy heavy ion collisions, and the evolution of this effect with the incident energy. The results show that when the incident energy is small, the high momentum distribution has little effect on the nuclear stopping. However, when the reaction energy is large, the high momentum distribution has a great influence on the nuclear stopping and the influence increases with the increase of incident energy and the mass of collision nucleus. For the Ca isotopes reaction system, high momentum distribution increases the nuclear stopping of the reaction system. Therefore, high momentum distribution has an important influence on nuclear stopping in medium mass nuclear collision system, and closely related to the incident energy and the mass of collision nucleus.     

Isospin Effect of Coulomb Interaction on Nuclear Stopping in Intermediate Energy Heavy Ion Collisions

Nuclei offer an interesting isospin situation where symmetry potential, Coulomb term and isopin dependent nucleon-nucleon collisions are simultaneously present. However, in general, Coulomb potential is not studied in the heavy ion collisions. As we know that the Coulomb potential is an important asymmetry term which can bring important isospin effect into the observable in the intermediate energy heavy ion collisions. Recently extensions by Colonna et al.[1] to finite nuclei show that Coulomb and surface effects reduce instability regions. So in this paper the isospin effects of Coulomb term and symmetry potential on the nuclear stopping at intermediate energy heavy ion collisions are studied by using isospin-dependent quantum molecular dynamics model (IQMD).     

Nuclear stopping as a probe for in-medium nucleon-nucleon cross sections in intermediate energy heavy ion collisions

Using an isospin-dependent quantum molecular dynamics, nuclear stopping in intermediate heavy ion collisions has been studied. The calculation has been done for colliding systems with different neutron-proton ratios in beam energy ranging from 15 MeV/ u to 150 MeV/ u. It is found that, in the energy region from above Fermi energy to 150 MeV/ u, nuclear stopping is very sensitive to the isospin dependence of in-medium nucleon-nucleon cross section, but insensitive to symmetry potential. From this investigation, we propose that nuclear stopping can be used as a new probe to extract the information on the isospin dependence of in-medium nucleon-nucleon cross section in intermediate energy heavy ion collisions.     

动量相关作用对两体耗散同位旋效应的重要性

利用同位旋相关的量子分子动力学,研究了中能重离子碰撞中动量相关的状态方程对原子核阻止基于两体耗散的同位旋效应的影响.计算结果表明原子核阻止对同位旋相关和同位旋无关的核子–核子碰撞截面(两体耗散)的差值强烈地依赖于动量相关势,即在有动量相关势的情况下原子核阻止对同位旋相关和同位旋无关的核子–核子碰撞截面的差值大于没有动量相关势的情况.这就意味着动量相关作用明显地提高了原子核阻止对于核子–核子碰撞截面的灵敏性.因此,在考虑动量相关势的情况下,原子核阻止可以更准确地作为提取同位旋相关的核子–核子碰撞截面的一个探针.     

中能重离子碰撞中同位旋分馏的同位旋效应

基于改进的同位旋相关量子分子动力学模型,研究了中能重离子碰撞中同位旋分馏强度(N/Z)_气(N/Z)_液随着碰撞系统中子–质子比和碰撞参数的变化所呈现出的同位旋效应,得到了一些有兴趣的结果.如在碰撞系统质量、入射能量和碰撞参数固定的条件下,(N/Z)_气(N/Z)_液随碰撞系统中子–质子比的增加而增加.对于丰中子系统而言,(N/Z)_气(N/Z)_液灵敏地依赖于对称势而较弱地依赖于核子–核子碰撞截面;而缺中子系统,(N/Z)_气(N/Z)_液对于对称势和介质中核子–核子碰撞截面都不灵敏.对于造成这种现象的物理机制进行了分析和讨论.一般核反应中,碰撞参数是各种物理观测量的灵敏函数,但计算结果表明同位旋分馏强度对于碰撞参数并不灵敏,故对于丰中子系统而言,同位旋分馏强度是提取对称势知识的灵敏物理观测量.     

重离子碰撞中原子核阻止的同位旋效应

利用含有3种对称势形式的同位旋相关的量子分子动力学,研究了中能重离子碰撞中原子核阻止的同位旋效应和随入射道条件的系统演化过程.计算结果表明,原子核阻止灵敏地依赖束流能量、碰撞参数、碰撞系统的质量和核子–核子碰撞截面的同位旋相关性,而3种对称势和碰撞系统的中质比对它的影响不很明显,但在大约费米能量以下能区,原子核阻止同时依赖于介质中核子–核子碰撞截面和对称势.故认为在费米能量以上能区直至150MeV/u,原子核阻止是提取介质中核子–核子碰撞截面的一个新的物理观测量.     

中能重离子碰撞中的中子(质子)发射的同位旋效应

利用同位旋相关的量子分子动力学,对中能重离子碰撞过程中的中子和质子发射的同位旋效应进行了分析.计算结果表明在有动量相关作用条件下,在很宽的能量和碰撞参数范围内,缺中子碰撞系统的中子(质子)发射数强烈地依赖于同位旋相关的核子–核子碰撞截面,而较弱地依赖于对称势.在对丰中子碰撞系统的研究中,上述规律减弱.这样就可以通过实验上对缺中子碰撞系统的中子(质子)发射数的探测,来提取介质中同位旋相关核子–核子碰撞截面的知识.     

Isospin Effects in the Isospin Fractionation Process in Intermediate Energy HeavyIon Collisions

Based on the isospin dependent quantum molecular dynamics model we investigated the isospin effects of isospin fractionation (N/Z)_gas/(N/Z)_liq from the neutron-proton ratio of colliding system and impact parameter. Here (N/Z)_gasand (N/Z)_liq is the neutron-proton ratio of nucleon emission for gas phase and that of fragment emission for liquid phase, respectively. Our results show that the (N/Z)_gas/(N/Z)_liq increases with the neutron-proton ratio of colliding systems when the system mass, incident energy and impact parameters are fixed. For the neutron-rich systems, the (N/Z)_gas/(N/Z)_liq depends sensitively to the form of symmetry potential and weakly on the isospin effect of the in-medium nucleon-nucleon cross section; For the neutron-poor colliding system, however, the (N/Z)_gas/(N/Z)_liq is not sensitive to both the form of symmetry potential and the isospin effect of the in-medium nucleon-nucleon cross section. We also found that the dependence of (N/Z)_gas/(N/Z)_liq on the impact parameter is not obvious. Detailed discussion and explanation for leading to these results are presented in the paper. According to these results, we propose that (N/Z)_gas/(N/Z)_liq can be directly compared with the experimental data to get the information about symmetry potential.     

重离子碰撞中同位旋分馏对碰撞系统同位旋的依赖性

利用同位旋相关的量子分子动力学模型，研究了中能重离子碰撞过程中同位旋分馏现象及其对于碰撞系统同位旋的依赖性.计算结果表明:在碰撞系统膨胀的低密度区, 气相(发射核子)与液相(碎片)的中子-质子比出现不均等分配现象,即同位旋分馏. 同位旋分馏的强弱明显地依赖于碰撞系统的中子-质子比,其强度随着系统中子-质子比的增大而增大. 丰中子碰撞系统产生丰中子的气相和缺中子的液相,而缺中子碰撞系统产生缺中子的气相和丰中子的液相.在丰中子的碰撞系统中同位旋分馏强度敏感地依赖于对称势,而对于两体碰撞的同位旋效应并不敏感，但对于缺中子的碰撞系统,同位旋分馏强度对于对称势不敏感， 同时发现动量相关作用对于同位旋分馏过程的作用不明显. 关键词： 中能重离子碰撞 同位旋分馏 同位旋效应 对称势     

中子晕核引起核反应中的同位旋效应

利用同位旋相关的量子分子动力学(IQMD)对中子晕核，⁸He和¹⁰He引起核反应中重要的同位旋效应和松散的中子晕结构影响的平均特征进行了研究.因为IQMD中的互作用势和介质中核子-核子碰撞截面灵敏地依赖于碰撞系统的密度分布.而扩展的中子晕密度分布包含了中子晕核的同位旋效应和松散的中子晕结构的平均特征，从而将这些信息通过动力学碰撞带入到反应机理中. 为了清楚地鉴别中子晕核带入反应机理重要的同位旋效应和松散中子晕结构的影响，通过比较中子晕核和相等质量稳定弹核在相同入射道条件下，所得物理观测量之间的差别加以确定.计算结果确实发现具有初始晕核信息的中子扩展密度分布将重要的同位旋效应和松散中子晕结构带入到各种物理观测量中.例如与相等质量稳定相比，中子晕核的晕特征引起了原子核阻止的降低；并明显地增加了核子发射中子-质子比和同位旋分馏比. 关键词： 中子晕核 原子核阻止 核子发射中子-质子比 同位旋分馏比     

多重碎裂的同位旋效应和动量相关作用的影响

利用同位旋相关的量子分子动力学模型,对中能重离子碰撞过程中多重碎裂对于同位旋自由度和动量相关作用的依赖性进行了研究.结果表明:在相对较高能区,碎片的平均多重性敏感地依赖于介质中核子–核子碰撞截面的同位旋效应,但很弱地依赖于对称势;动量相关作用增强了中等质量碎片多重性对于核子–核子碰撞截面的同位旋依赖的敏感性.中等质量碎片的平均多重性可用作提取介质中同位旋相关的核子–核子碰撞截面的探针.