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.     

中能重离子碰撞中同位旋相关核子-核子碰撞截面和同位旋相关平均场的探针

在最近几年我们通过使用同位旋相关的量子分子动力学模型（IQMD）系统的研究了同位旋相关的平均场和介质中核子－核子（N－N）碰撞截面对中能重离子碰撞（HIC）中碎裂和耗散的同位旋效应。我们发现原子核阻R和Qzz，中等质量碎片多重性Nimf和质子（中子）发射数Np(Nn）敏感的依赖于介质中N－N碰撞截面的同位旋效应，而弱的依赖于同位旋相关的平均场（对称势），这些物理量作为提取相对高能范围缺中子系统的同位旋相关介质中N－N碰撞截面的探针。我们也可以通过相对低能区到150MeV/u的前平衡核子发射中质比来提取关于对称势的知识和讨论它的同位旋依赖性。     

中能重离子碰撞中同位旋相关核子-核子碰撞截面和同位旋相关平均场的探针

在最近几年我们通过使用同位旋相关的量子分子动力学模型 (IQMD)系统的研究了同位旋相关的平均场和介质中核子 核子 (N N)碰撞截面对中能重离子碰撞 (HIC)中碎裂和耗散的同位旋效应。我们发现原子核阻止R和Qzz,中等质量碎片多重性Nimf和质子(中子 )发射数Np(Nn)敏感的依赖于介质中N N碰撞截面的同位旋效应 ,而弱的依赖于同位旋相关的平均场 (对称势 ) ,这些物理量作为提取相对高能范围缺中子系统的同位旋相关介质中N N碰撞截面的探针。我们也可以通过相对低能区到 1 5 0MeV/u的前平衡核子发射中质比来提取关于对称势的知识和讨论它的同位旋依赖性。     

Isospin and symmetry energy study in nuclear EOS

This paper summarizes the isoscaling and isospin related studies in asymmetry nuclear reactions by different dynamic and statistical models. Isospin dependent quantum molecular dynamics model (IQMD) and lattice gas model (LGM) are used to study the isoscaling properties and isoscaling parameters dependence on incident energies, impact parameters, temperature and other parameters. In the LGM model, the signal of phase transition has been found in free neutron (proton) chemical potential difference Δµ_n or Δµ_p as a function of temperature, or in free neutron and proton chemical potential difference Δµ_n−Δµ_p. Density dependence of symmetry energy coefficient C _sym(ρ/ρ ₀) is also studied in the frame of LGM, with the potential parameters which can reproduce the nuclear ground state property, soft density dependence of symmetry energy is deduced from the simulation results. Giant dipole resonance (GDR) induced by isospin asymmetry in entrance channel is also studied via IQMD model, and the dynamic dipole resonance shows isospin sensitivity on the isospin asymmetry of entrance channel and symmetry energy of the nuclear equation of state (EOS). GDR can also be regarded as a possible isospin sensitive signature.

     

Microscopic three-body force for asymmetric nuclear matter

Brueckner calculations including a microscopic three-body force have been extended to isospin-asymmetric nuclear matter. The effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs. isospin asymmetry β = (N - Z)/A is fulfilled in the whole asymmetry range 0≤β≤1 up to high densities. The three-body force provides a strong enhancement of the symmetry energy which increases with density in good agreement with the predictions of relativistic approaches. The Lane's assumption that proton and neutron mean fields linearly vary vs. the isospin parameter is violated at high density due to the three-body force, while the momentum dependence of the mean fields turns out to be only weakly affected. Consequently, a linear isospin split of the neutron and proton effective masses is found for both cases with and without the three-body force. The isospin effects on multifragmentation events and collective flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron star cooling. Received: 18 February 2002 / Accepted: 16 May 2002     

Neutron–proton elliptic flow difference as a probe for the high density dependence of the symmetry energy

We employ an isospin dependent version of the QMD transport model to study the influence of the isospin dependent part of the nuclear matter equation of state and in-medium nucleon–nucleon cross-sections on the dynamics of heavy-ion collisions at intermediate energies. We find that the extraction of useful information on the isospin-dependent part of the equation of state of nuclear matter from proton or neutron elliptic flows is obstructed by their sensitivity to model parameters and in-medium values of nucleon–nucleon cross-sections. Opposite to that, neutron–proton elliptic flow difference shows little dependence on those variables while its dependence on the isospin asymmetric EoS is enhanced, making it more suitable for a model independent constraining of the high-density behaviour of asy-EoS. Comparison with existing experimental FOPI-LAND neutron–hydrogen data can be used to set an upper limit to the softness of asy-EoS. Successful constraining of the asy-EoS via neutron–proton elliptic flow difference will require experimental data of higher accuracy than presently available.     

Isospin dynamics in heavy ion collisions: From Coulomb barrier to quark gluon plasma

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this report, we present a selection of new reaction observables in dissipative collisions particularly sensitive to the symmetry term of the nuclear Equation of State (Iso-EoS). We will first discuss the Isospin Equilibration Dynamics. At low energies, this manifests via the recently observed Dynamical Dipole Radiation, due to a collective neutron-proton oscillation, with the symmetry term acting as a restoring force. At higher beam energies, Iso-EoS effects will be seen in Imbalance Ratio Measurements, in particular from the correlations with the total kinetic energy loss. For fragmentation reactions in central events, we suggest to look at the coupling between isospin distillation and radial flow. In Neck Fragmentation reactions, important Iso-EoS information can be obtained from the correlation between isospin content and alignment. The high density symmetry term can be probed from isospin effects on heavy ion reactions at relativistic energies (few AGeV range). Rather, isospin sensitive observables are proposed from nucleon/cluster emissions, collective flows and meson production. The possibility to shed light on the controversial neutron/proton effective mass splitting in asymmetric matter is also suggested. A large symmetry repulsion at high baryon density will also lead to an “earlier” hadron-deconfinement transition in n-rich matter. A suitable treatment of the isovector interaction in the partonic EoS appears very relevant.     

Neck fragmentation reaction mechanism

Based on a microscopic transport model, we study the origin of nonstatistical intermediate mass fragment (IMF) production in semicentral heavy ion collisions at the Fermi energies. We show that a fast, dynamical IMF formation process, the neck fragmentation mechanism, can explain the experimentally observed features: deviations from Viola systematics and anisotropic, narrow angular distributions. It may be regarded as the continuation of the multifragmentation mechanism towards intermediate impact parameters. Its relation to other dynamical mechanisms, the induced fission and the abrasion of the spectator zones, that can also contribute to mid-rapidity IMF production, is discussed. The dependence on beam energy and centrality of the collision is carefully analysed. The competition between volume and surface instabilities makes this mechanism very sensitive to the in-medium nucleon-nucleon interactions, from the cross sections for hard collisions to the compressibility and other equation of state (EOS) properties.

For charge asymmetric collisions the sensitivity of various observables to the symmetry energy is investigated. Of particular interest appears the isospin diffusion dynamics with no signal of isospin equilibration. However, in spite of the short time scales and of the dynamical aspects, we notice isoscaling features of the neck mechanism. We observe that isospin enrichment of the neck zone as well as the isoscaling parameters are sensitive to the density dependence of asymmetry term of EOS around and below saturation value.     

Explosion-evaporation model for fragment production in medium-energy nuclear collisions

Statistical considerations are applied to the multifragment disassembly of a piece of hot nuclear matter created in medium-energy nuclear collisions. A two-stage model, consisting of a quick explosion and a slower evaporation, is presented. Results are compared to those of previous simpler calculations and to recent experimental data. The agreement is encouraging for a realistic range of the main parameters of the model: the available energy per nucleon, the isospin asymmetry, and the extension of the primary explosion in space and time.     

中能重离子碰撞过程中的同位旋分馏

本文主要在我们近几年来对中能重离子碰撞中同位旋分馏研究工作的基础上,结合国际上对这个问题研究的进展。对同位旋分馏动力学,产生同位旋分馏的机制,入射道效应,特别是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可能性进行了讨论和分析。并进一步对该问题的深入研究进行了讨论。     

非对称核物质中同位旋对称势的动量和密度依赖性

在Brueckner-Hartree-Fock理论框架内, 研究了同位旋非对称核物质中质子和中子单粒子势的动量相关性及其随同位旋非对称度的变化, 在此基础上计算了同位旋对称势, 并讨论了三体核力的影响. 结果表明同位旋对称势对于同位旋非对称度的依赖性很弱, 但对于动量和密度均有较强的依赖性. 当密度固定时, 同位旋对称势随动量增加而减小. 尽管三体核力对于质子和中子单粒子势的动量相关性有较大影响, 但对同位旋对称势的影响很小. 还与目前重离子碰撞输运理论模型中所使用的各种参数化的唯象对称势进行了比较.     

Neutron and proton transverse emission ratio measurements and the density dependence of the asymmetry term of the nuclear equation of state

Recent measurements of preequilibrium neutron and proton transverse emission from (112,124)Sn+(112,124)Sn reactions at 50 MeV/A have been completed at the National Superconducting Cyclotron Laboratory. Free nucleon transverse emission ratios are compared to those of A=3 mirror nuclei. Comparisons are made to Boltzmann-Uehling-Uhlenbeck (BUU) transport calculations and conclusions concerning the density dependence of the asymmetry term of the nuclear equation of state at subnuclear densities are made. Comparison to BUU model predictions indicate a density dependence of the asymmetry energy that is closer to a form in which the asymmetry energy increases as the square root of the density for the density region studied. A coalescent-invariant analysis is introduced as a means of reducing suggested difficulties with cluster emission in total nucleon emission.     

利用非对称核物质状态方程对中子星的质量和半径的研究

在温度、密度及同位旋相关的核物质状态方程的基础上,通过求解Tol-man-Oppenheimer?Volkoff方程得到了中子星的质量与中心密度的关系,发现随着中心密度的变化,中子星存在一个最大质量.同时计算结果表明,中子星的最大质量与核物质状态方程的不可压缩系数、有效质量及对称能强度系数等密切相关.对中子星半径的研究表明,较硬的核物质状态方程给出的中子星半径较大,而且较大的对称能强度系数和较大的核子有效质量也会给出较大的中子星半径.     

Decomposition of the equation of state of asymmetric nuclear matter into different spin-isospin channels

We investigate the equation of state of asymmetric nuclear matter and its isospin dependence in various spin-isospin ST channels within the framework of the Brueckner-Hartree-Fock approach extended to include a microscopic three-body force (TBF). It is shown that the potential energy per nucleon in the isospin-singlet T=0 channel is mainly determined by the contribution from the tensor SD coupled channel. At high densities, the TBF effect on the lsospin-triplet T=1 channel contribution turns out to be much larger than that on the T=0 channel contribution. At low densities around and below the normal nuclear matter density, the isospin dependence is found to come essentially from the isospin-singlet SD channel and the isospin-triplet T=1 component is almost independent of isospin asymmetry. As the density increases, the T=1 channel contribution becomes sensitive to the isospin asymmetry and at high enough densities its isospin dependence may even become more pronounced than that of the T=0 contribution. The present results may provide some microscopic constraints for improving effective nucleon-nucleon interactions in a nuclear medium and for constructing new functionals of effective nucleon-nucleon interaction based on microscopic many-body theories.     

Relativistic equation of state of nuclear matter for supernova and neutron star

We construct the equation of state (EOS) of nuclear matter using the relativistic mean field (RMF) theory in the wide density, temperature range with various proton fractions for the use of supernova simulation and the neutron star calculations. We first construct the EOS of homogeneous nuclear matter. We use then the Thomas-Fermi approximation to describe inhomogeneous matter, where heavy nuclei are formed together with free nucleon gas. We discuss the results on free energy, pressure and entropy in the wide range of astrophysical interest. As an example, we apply the resulting EOS on the neutron star properties by using the Oppenheimer-Volkoff equation.     

Three-Body Force Effects on EOS of Asymmetric Nuclear Matter and Proton Fraction in Neutron Star Matter

The three-body force effects on the equation of state and its iso-spin dependence of asymmetric nuclearmatter and on the proton fraction in neutron star matter have been investigated within Brueckner-Hartree-Fock approachby using a microscopic three-body force. It is shown that, even in the presence of the three-body force, the empiricalparabolic law of the energy per nucleon vs. isospin asymmetry β＝ ( N - Z) /A is fulfilled in the whole asymmetry range0≤β≤1 and also up to high density. The three-body force provides a strong enhancement of symmetry energy at highdensity in agreement with relativistic approaches. It also shows that the three-body force leads to a much more rapidincreasing of symmetry energy with density in relatively high density region and to a much lower threshold density forthe direct URCA process to occur in a neutron star as compared to the predictions adopting only pure two-body force.     

用中子晕弹核探寻同位旋非对称核物质状态方程

总结和评述了用中子晕弹核探寻同位旋非对称核物质状态方程。 在具有同位旋和动量依赖的同位旋相关量子分子动力学框架内, 采用对比中子晕弹核和相等质量稳定弹核在完全相同入射道条件下物理观测量的差别, 来突出中子晕核明显的同位旋效应和加强物理观测量对于同位旋的灵敏性, 从而提取核物质状态方程。 例如, 与稳定弹核碰撞系统相比, 中子晕弹核明显提高了发射核子的中子\|质子比和增加了它对于对称势的灵敏性, 这两点特征非常有利于提取对称势。 同样, 由于中子晕弹核碰撞系统在低能区（E<60 MeV）由于内部松散结构减弱了核子碰撞力度和动量耗散, 与稳定核碰撞系统相比, 明显提高了原子核阻止; 而在高能区由于两体碰撞同位旋效应的增加, 从而明显提高了原子核阻止。 利用这些特征可以提取核子\|核子碰撞截面的介质效应和同位旋依赖性。 Based on the isospin and momentum dependent quantum molecular dynamics, we use the comparison for the differences between observables in the reactions induced by the halo\|neutron projectile and stable projectile with the same mass under complete same incident channel condition to protrude the isospin effect of halo\|neutron projectile and strengthen the sensitivity of observable on the isospin for extracting the information for the equation of state. For example, the halo\|neutron projectile increases the emitted neutron\|proton ratio and then enhances its sensitivity on the symmetrical potential. Two points above are more favorable than the normal neutron\|rich and neutron\|poor projectiles for extracting the symmetry potential. We also found that the neutron\|halo projectile induces the decrease of nuclear stopping in lower beam energy region and the increase in higher beam energy region, compared to corresponding same mass stable projectile under the same incident channel condition, so that we can use these properties to extract the information for the medium effect and isospin effect of in\|medium nucleon\|nucleon cross section.     

中能重离子碰撞中前平衡核子发射的同位旋效应

利用同位旋相关的输运理论,研究了中能重离子碰撞中丰中子和缺中子碰撞系统在较宽能区范围内前平衡发射核子的同位旋效应.结果表明除低能区外,在100MeV/u以上核子–核子碰撞动力学效应起主要作用的能区,前平衡发射的中子–质子比仍然对介质中核子–核子碰撞截面的同位旋效应不灵敏,然而对于对称势的改变非常灵敏.故从低能量到较高能量的较大能区内,前平衡发射的中子–质子比的理论值与实验值的比较是提取对称势形式和强度的灵敏探针.