利用中-质微分流探测对称能的高密行为

基于同位旋和动量依赖的强子输运模型IBUU04，研究了¹³²Sn+¹²⁴Sn在三种不同碰撞能量、两种不同对称能作用下，中-质微分流随快度的变化关系. 发现中-质微分流的强度随碰撞能量的增大而增强. 在碰撞能量为400 MeV每核子时，中-质微分流对对称能最为敏感. 关键词： 重离子碰撞 中-质微分流 对称能     

Effect of the momentum dependence of nuclear symmetry potential on the transverse and elliptic flows

In the framework of the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model, the effect of the momentum dependence of nuclear symmetry potential on nuclear transverse and elliptic flows in the neutron-rich reaction ¹³²Sn+¹²⁴Sn at a beam energy of 400MeV/nucleon is studied. We find that the momentum dependence of nuclear symmetry potential affects the rapidity distribution of the free neutron to proton ratio, the neutron and the proton transverse flows as a function of rapidity. The momentum dependence of nuclear symmetry potential affects the neutron-proton differential transverse flow more evidently than the difference of neutron and proton transverse flows as well as the difference of proton and neutron elliptic flows. It is thus better to probe the symmetry energy by using the difference of neutron and proton flows since the momentum dependence of nuclear symmetry potential is still an open question. And it is better to probe the momentum dependence of nuclear symmetry potential by using the neutron-proton differential transverse flow the rapidity distribution of the free neutron to proton ratio.     

重离子碰撞中的同位旋效应

根据近年来利用同位旋相关输运理论在中能重离子碰撞同位旋效应研究方面的成果 ,并结合国际上在该问题上的进展 ,综合介绍和分析了中能重离子碰撞中的多重碎裂、原子核阻止和前平衡发射中子-质子比的同位旋效应及其随入射道条件:入射能量、碰撞参数、碰撞系统的质量 ,特别是随中子-质子比的演化过程.讨论了利用以上物理观测量在提取同位旋非对称核物质状态方程知识方面的可能性 ,并对其发展进行了展望. Based on the achievements for the intermediate energy heavy ion collision in our recent work and the progresses in the world, the isospin effects and the dependence of the entrance channel conditions on them in the intermediate energy heavy ion collisions were introduced, analysed and commended. From the calculation results by using isospin dependence quantum molecular dynamics, it is clear to see that the nuclear stopping power strongly depends on the in medium isospin dependence...     

Probing the high density behavior of the nuclear symmetry energy with high energy heavy-ion collisions

High energy heavy-ion collisions are proposed as a novel means to constrain stringently the high density (HD) behavior of nuclear symmetry energy. Within an isospin-dependent hadronic transport model, it is shown for the first time that the isospin asymmetry of the HD nuclear matter formed in high energy heavy-ion collisions is uniquely determined by the HD behavior of the nuclear symmetry energy. Experimental signatures in two sensitive probes, i.e., pi(-) to pi(+) ratio and neutron-proton differential collective flow, are also investigated.     

重离子碰撞中的对称势翻转

利用同位旋和动量相关的输运模型IBUU，以Sn,Sn同位素在低能与高能碰撞为例，在两种不同的对称势作用下，研究了重离子碰撞中的对称势翻转现象.我们发现从低能到高能在同位旋相分化、发射核子的中-质比、发射核子的双n/p比，中-质微分横向流观测量中均存在对称势翻转现象.对称势翻转效应的研究有利于确定对称能的密度依赖性. 关键词： 重离子碰撞 对称势翻转 对称能     

利用自由中-质比研究对称能的高密行为

基于同位旋和动量依赖的强子输运模型IBUU04, 采用两种具有不同密度依赖的对称能研究了¹³²Sn+¹²⁴Sn反应系统在400MeV/A, 600MeV/A, 800MeV/A三种不同碰撞能量下中快度发射核子中-质比随横动量的变化关系. 发现发射核子中--质比在低横动量端非常敏感于对称能, 发射核子中--质比的强度随碰撞能量的降低而增强. 并且发射核子中-质比对对称能的敏感性随着碰撞能量的增大而降低, 在高横动量端尤为明显, 在碰撞能量为400MeV/A时, 自由核子中-质比对对称能的敏感性最为明显.     

中能重离子反应中对称势对熵的影响

基于同位旋相关的量子分子动力学模型,研究了对称势、束流能量、对称能强度系数对中能重离子碰撞反应系统的熵的影响。研究发现,对称势对熵有一定影响,且对熵的影响大小与反应系统中质比有关。另外发现在40Ca+40Ca和40Si+40Si反应系统中对称能强度系数对熵也有影响,当对称能强度系数c=32 MeV时,对这两个系统来说其对熵的影响最大。通过实验上对熵的测量可以得到对称势的形式和大小。Based on the isospin-dependent quantum molecular dynamics model,effects of the symmetry potential,incident beam energy and different symmetry energy strength coefficients on the entropy of reaction system are studied.The results show that the symmetry potential affects the entropy,and the effect changes with the neutron-proton ratio of the reaction system.Besides,in 40Ca+40Ca and 40Si+40Sireaction systems,the symmetry energy strength coefficient also has effect on the entropy when the symmetry energy strength coefficient c=32 MeV,the effect on the entropy is largest for the two systems.Therefore the form and size of symmetry potential could be obtained by comparison of the model calculation and the experiments on the measurement of entropy.     

First-Order Symmetry Energy Induced by Neutron-Proton Mass Difference

The lst-order symmetry energy coefficient of nuclear matter induced merely by the neutron-proton(h-pj mass difference is derived analytically,which turns out to be completely model-independent.Based on this result,the lst-order symmetry energy E_(sym,1)~((npDM))(A) of heavy nuclei such as ~(208)Pb induced by the n-p mass difference is investigated with the help of a local density approximation combined with the Skyrme energy density functionals.Although E_(sym,1)~((npDM))(A) is small compared with the second-order symmetry energy,it cannot be dropped simply for an accurate estimation of nuclear masses as it is still larger than the rms deviation given by some accurate mass formulas.It is therefore suggested that one perhaps needs to distinguish the neutron mass from the proton one in the construction of nuclear density functionals.     

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.     

Covariance analysis of an isospin-dependent probe

Based on a modified quantum molecular dynamics model,we calculate the neutron-proton ratio and the nuclear stopping of reaction systems with different symmetry potentials and collision cross sections.We perform correlations of several probes using the covariance data processing method.It is shown that the correlation between the nuclear stopping and the isospin-dependent nucleon-nucleon cross sections is strong,but the nuclear stopping and symmetry potentials have a weak correlation.The correlation between neutron-proton ratio and symmetry potentials in the case of low energy is stronger.The correlation between neutron-proton ratio and isospin-dependent collision cross sections is enhanced with the increase of energy,but remains weak.In addition,the correlations of the emission numbers of the deuteron with the symmetry potentials and collision cross sections at different beam energies are not obvious compared to two prior physical quantities.In this paper,we define a parameter to quantitatively describe the sensitivity of isospin-dependent probes.By analyzing this parameter,one can extract more information about the isospin effects of the physical quantity.     

质子椭圆流与对称能的密度依赖性

基于同位旋和动量依赖的强子输运IBUU04模型, 研究了¹³²Sn+¹²⁴Sn碰撞系统中的质子椭圆流对对称能的敏感关系. 研究发现入射能量从每核子400MeV到800MeV时质子椭圆流在低横动量端对对称能的敏感性高于高横动量端, 同时发现随着入射能量的增大, 质子椭圆流对对称能的敏感性在降低. 在研究入射能量范围内, 当入射能量为每核子400MeV左右时质子椭圆流对对称能最为敏感.     

中能重离子碰撞中的同位旋分馏过程

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中同位旋分馏过程.研究结果表明自由粒子中质比与碎片中质比的比值即同位旋分馏强度灵敏地依赖于对称势,而对同位旋相关核子–核子碰撞截面的依赖很弱.同位旋分馏对对称势的灵敏主要来自于气相部分,而液相部分对对称势不够灵敏.气相部分灵敏地依赖于对称势是直接造成同位旋分馏强度对对称势灵敏的主要原因.同时还讨论了各种液相部分的取法,其结果表明不同取法对以上结论的影响不大.因此理论结果与实验数据可以直接比较从而提取对称势的知识.并对引发同位旋分馏的动力学的起因进行了分析和讨论.     

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     

The nuclear symmetry energy in relativistic Brueckner-Hartree-Fock calculations

We extend the relativistic Brueckner-Hartree-Fock approach for nuclear matter to dense neutron matter. Special attention is devoted to contributions arising from tensor interactions due to the exchange of π-and ϱ-mesons and the dependence upon neutron-proton asymmetry. The role of the symmetry energy in studies of beta-stable neutron stars is also discussed.     

探究重离子碰撞中密度和动量依赖的单核子势的影响

从Skyrme有效核子-核子相互作用出发,得到了单核子平均场、介质中的核子-核子散射截面以及核子的初始化密度分布,自洽地用于 Boltzmann-Uehling-Uhlenbeck(BUU) 输运模型中。使用对应不同软硬程度对称能、相反中子-质子有效质量劈裂的六组Skyrme参数(SkI2, Gs, KDE0v1, NRAPR, BSk9和SV-mas08),利用BUU输运模型对$^{124}{\rm{Sn}}$+$^{124}{\rm{Sn}}$和$^{112}{\rm{Sn}}$+$^{112}{\rm{Sn}}$进行了碰撞模拟。结果表明,由中子-质子有效质量劈裂效应引起的自由双中质比差异在较高的核子动能下明显。此外,与NSCL实验数据的比较表明,在用到的六种相互作用之中,KDE0v1相互作用所对应的双中质比结果似乎与实验更为符合。     

Isospin Effects of Threshold Energy of Radial Flow in Heavy Ion Collisions

The threshold energies of radial flow in reactions of ^40 Ca-^40Ca and ^48Ca＋ ^48Ca in central collisions are investigated within an isospin dependent quantum molecular dynamics model by using three different forms of symmetry energy. It is found that the neutron-rich system has smaller threshold energy of radial flow and this quantity depends on the form of symmetry potential. It is indicated that the threshold energy of radial flow can provide a new method to determine the symmetry energy of asymmetric nuclear matter.     

Neutron-proton differential flow as a probe of isospin-dependence of the nuclear equation of state

The neutron-proton differential flow is shown to be a very useful probe of the isospin-dependence of the nuclear equation of state (EOS). This novel approach utilizes constructively both the isospin fractionation and the nuclear collective flow as well as their sensitivities to the isospin-dependence of the nuclear EOS. It also avoids effectively uncertainties associated with other dynamical ingredients of heavy-ion reactions at intermediate energies.