A Photonuclear Reaction Model Based on IQMD in Intermediate-Energy Region

A photonuclear reaction transport model based on an isospin-dependent quantum molecular dynamics model （IQMD） is presented in the intermediate energy region, which is named as GiQMD in this study. Methodology to simulate the course of the photonuclear reaction within the IQMD frame is described to study the photo- absorption cross section and π meson production, and the simulation results are compared with some available experimental data as well as the Giessen Boltzmann-Uehling-Uhlenbeck model.

Isotopic dependence of the yield ratios of light fragments from different projectiles and their unified neutron skin thicknesses

The yield ratios of neutron-proton(R(n/p))and^(3)H-^(3)He(R(^(3)H∕^(3)He))with reduced rapidity from 0 to 0.5 were simulated at 50 MeV/u even-even ^(36−56)Ca+^(40)Ca,even-even ^(48−78)Ni+^(58)Ni,and ^(100−139)Sn(every third isotopes)+112 Sn for full reduced impact parameters using the isospin-dependent quantum molecular dynamics(IQMD)model.The neutron and proton density distributions and root-mean-square radii of the reaction systems were obtained using the Skyrme-Hartree-Fock model,which was used for the phase space initialization of the projectile and target in IQMD.We defined the unified neutron skin thickness asΔRnp=<r^(2)>^(1∕2) n−<r^(2)>^(1∕2)p,which was negative for neutron-deficient nuclei.The unifiedΔRnp values for nuclei with the same relative neutron excess from different isotopic chains were nearly equal,except for extreme neutron-rich isotopes,which is a type of scaling behavior.The yield ratios of the three isotopic chain-induced reactions,which depended on the reduced impact parameter and unified neutron skin thickness,were studied.The results showed that both R(n/p)and R(^(3)H∕^(3)He)decreased with a reduced impact parameter for extreme neutron-deficient isotopes;however,they increased with reduced impact parameters for extreme neutron-rich isotopes,and increased with theΔRnp of the projectiles for all reduced impact parameters.In addition,a scaling phenomenon was observed betweenΔR np and the yield ratios in peripheral colli-sions from different isotopic chain projectiles(except for extreme neutron-rich isotopes).Thus,R(n/p)and R(^(3)H∕^(3)He)from peripheral collisions were suggested as experimental probes for extracting the neutron or proton skin thicknesses of non-extreme neutron-rich nuclei from different isotopic chains.

输运理论中原子核初始化研究

基于同位旋相关的量子分子动力学(IQMD)输运理论模型,研究中能重离子碰撞的初始化问题。成功的初始化可以很好地减少碰撞过程中出现的非物理涨落和虚粒子发射,有利于两体碰撞过程顺利进行。计算发现,不同的对称势对原子核的稳定性有一定的影响。选取不同质量数的Fe,Kr,Sn元素及其同位素,通过其在自身平均场中随时间的演化来验证原子核的稳定性,统计出初始化稳定原子核的数量,选取出最为稳定的对称势。发现平方项的对称势U2最为稳定,其对应的斜率L、不可压缩系数Ksym都在已知的约束范围内,表现出同位旋相关平均场取硬势时,原子核更为稳定的特征。

Classifier for centrality determination with zero-degree calorimeter at the cooling-storage-ring external-target experiment

The zero-degree calorimeter(ZDC)plays a crucial role toward determining the centrality in the Cooling-Storage-Ring External-target Experiment(CEE)at the Heavy Ion Research Facility in Lanzhou.A boosted decision tree(BDT)multi-classification algorithm was employed to classify the centrality of the collision events based on the raw features from ZDC such as the number of fired channels and deposited energy.The data from simulated^(238)U+^(238)U collisions at 500 MeV∕u,generated by the IQMD event generator and subsequently modeled using the GEANT4 package,were employed to train and test the BDT model.The results showed the high accuracy of the multi-classification model adopted in ZDC for centrality determination,which is robust against variations in different factors of detector geometry and response.This study demon-strates the good performance of CEE-ZDC in determining the centrality in nucleus-nucleus collisions.

中能重离子核反应中椭圆流与横向速度的关联

用同位旋相关的量子分子动力学(IQMD)模型模拟了碰撞能量为25 MeV/A、碰撞参数在3～5 fm范围的64Cu+64Cu核反应,通过对中快度区间、质量数A≤2的轻碎片物理信息的分析,发现通过引入在平面横向动量和出平面横向动量的差值,能够定性地理解核碎片在不同横向半径处的受力变化,并能较好的解释椭圆流随横向半径的变化关系。该差值也可以作为研究核反应动力学的一个探针。

Investigation of the symmetry energy of nuclear matter using isospin-dependent quantum molecular dynamics

Simulations of infinite nuclear matter at different densities,isospin asymmetries and temperatures are performed using the isospin-dependent quantum molecular dynamics(IQMD)model to study the equation of state and symmetry energy.A rigorous periodic boundary condition is used in the simulations.Symmetry energies are extracted from the binding energies under different conditions and compared to the classical molecular dynamics(CMD)model using the same method.The results show that both models can reproduce the experimental results for the symmetry energies at low densities,but IQMD is more appropriate than CMD for nuclear matter above the saturation density.This indicates that IQMD may be a reliable model for the study of the properties of infinite nuclear matter.

Density fluctuations in intermediate-energy heavy-ion collisions

Within the framework of the isospin-dependent quantum molecular dynamics model,we simulate^(129)Xe+^(119)Sn collisions in an incident energy range of 20 to 190MeV/nucleon and discuss the liquid-gas phase transition with density fluctuations.For comparison,we also extract the effective Fisher parameter τ_(eff),multiplicity of intermediate-mass fragments(IMFs),and information entropy.It is found that the Fisher parameter and maximum information entropy of collisions have peak values at E_(beam)=50-80 MeV/nucleon.In addition,the maximum multiplicity of IMFs has a plateau around 70 MeV/nucleon.For higher-order density moments in a larger central region of[-5,5]^(3) fm^(3) in the collision system,a maximum plateau also appears as function of beam energy at energies exceeding 70 MeV/nucleon.These observables,which are consistent with each other,indicate a liquid-gas phase transition around 70 MeV/nucleon for the^(129)Xe+^(119)Sn system.

Symmetry energy dependence of the pygmy and giant dipole resonances in an isospin dependent quantum molecular dynamics model

Isospin-dependent Quantum Molecular Dynamics model (IQMD) has been applied to investigate the Pygmy Dipole Resonance (PDR) and Giant Dipole Resonance (GDR) in Ni isotopes by Coulomb excitation. By Gaussian fitting to the photon emission spectra, the peak energies and strengths of PDR and GDR are extracted. Their sensitivities to impact parameter, incident energy and the symmetry energy are discussed. By the comparison of energy-weighted sum rule (EWSR) with the data and other calculations for 68 Ni, the parameters of density-dependence of symmetry energy in the IQMD are constrained. In addition, the N/Z dependence of PDR and GDR parameters of Ni isotopes are investigated, and the results that the EWSR increases linearly with the N/Z are obtained.

Decay modes of highly excited nuclei

The deexcitation of single excited ^(112) Sn nuclei at T = 1–30 MeV is simulated using the isospin-dependent quantum molecular dynamics(IQMD) model and GEMINI model. The fragmentation mechanism, critical behavior,and kinematic characteristics are investigated within these two models. The results show that the IQMD model can be applied to the analysis of fragmentation processes, critical points, and slope temperature extraction. The results of IQMD are generally consistent with experimental <M_(IMF)>- Z_(bound) data. However, GEMINI can reproduce the experimental data better than IQMD for isotopic distributions.

The Time Evolution of Radial Collective Flow in Reaction of ^(197)Au with ^(197)Au at 150 MeV/u

Dynamical time evolution for central collision of 197 Au on 197Au at 150 MeV/u is in-vestigated by using Isospin-dependent Quantum Molecular Dynamics （IQMD） calculation. Thepreequilibrium emission, its influence and the time evolution of radial collective flow are dis-cussed in detail.

Average Property of Isospin Fractionation Ratio Induced by Halo-neutron Nucleus ^6He

We study the average property of isospin fractionation ratio (n/p)free/(n/p)frag induced by halo-neutron nucleus 6He in intermediate energy heavy ion collisions for different beam energies within IQMD. This study is based on the extended density distribution and larger neutron-proton ratio for 6 He which includes obvious effects of isospin dependent interaction and loosely inner structure of 6He on the average property of the isospin fractionation ratio (n/p)free/(n/p)frag as shown in Fig. 1. Fig. 1 shows (n/p)free/(n/

Improved Isospin Dependent Quantum Molecular Dynamics Model

Isospin dependent quantum molecular dynamics (IQMD) model is further developed by introducing switch function method which deals with correctly the surface interaction and shell effect in the process of projectile and target approaching. In the improved IQMD (ImIQMD), the time evolution of the nucleons in the system is governed by Hamiltonian equation of motion, and the Hamiltonian consists of the kinetic energy and the effective interaction potential energy. The effective interaction potential energy is composed of the

Isospin Effect of Multifragmentation and Critical Phenomena in Finite Nuclei

Isospin effect of multifragmentation for the finite nuclei 112Sn and 132Sn is studied by using a phenomeno-logical asymmetric nuclear equation of state and an isospin dependent quantum molecular dynamics (IQMD) model. The initial distribution of nucleons in phase space is randomly sampled by using temperature dependent Hartree-Fock method in coordinate and momentum space respectively, and dynamical evolution of nucleons is described by IQMD. Various initial density contribution for the intermediate mass fragment (IMF) production is analyzed at a certain temperature. It is found that the isospin effect gradually vanishes with the temperature increasing as shown in Fig.1.

Initial fluctuation effect on elliptic flow in Au+Au collision at 1 GeV/A

How the initial fluctuation affects on the elliptic flow is investigated by investigating the rapidity, transverse 4-velocity, centrality dependencies of elliptic flow for Au+Au at 1 GeV/A with the help of an Isospin Quantum Molecular Dynamics (IQMD). In addition, we compare the flow calculated with respect to participant plane created by the initial geometry in coordinate space with the flow reconstructed by the experimental event-plane method, and compare the flow with the experimental data of the FOPI collaboration. It shows that there exists some discrepancy between the flows reconstructed by the above two methods.

轻系统多重碎裂的同位旋效应

利用同位旋相关的量子分子动力学模型，对＾４０Ｃａ＋＾４０Ｃａ和＾４０Ａｒ＋＾４０Ａｒ系统在不同入射能量时，同位旋效应对多重碎裂的影响进行了讨论。低能区的多重碎裂由于多种同位旋相关的因素相互抵消，因此多重碎裂的同位旋效应不明显。随着能量的增加，同位旋相关的戴面造成多重碎裂的明显差别。

原子核阻止是核子-核子碰撞截面的可能探针

利用同位旋相关的输运理论 ,研究了不同中子 -质子比的碰撞系统在中能重离子碰撞过程中 ,原子核阻止的同位旋效应及其对束流能量和碰撞参数的依赖性 .计算表明对于所研究的 4个碰撞系统 ,在从费米能附近到大约1 50MeV/u的较宽入射能量范围内 ,近心碰撞的原子核阻止强烈地依赖于核子-核子碰撞截面的同位旋相关性 ,而对称势对它的影响并不明显 .故原子核阻止是提取介质中核子 -核子碰撞截面的灵敏探针 .

中能重离子反应中的在平面流和椭圆流

利用同位旋相关的量子分子动力学模型研究了40 Ca + 40 Ca系统在能量从3 0MeV/u到 1 50MeV/u的在平面流和椭圆流 .发现随着入射能量的增加 ,在平面流出现从负到正的跃迁 ,而椭圆流则随入射能量的增大而减少并且出现从正到负的跃迁 .椭圆流和在平面流对于入射能量和碰撞参数较为敏感 ,相反对于对称能和核态方程却不太敏感 .