The time evolution of protons and 3He fragments from Au＋Au/Pb＋Pb reactions at 0.25, 2, and 20 GeV/nucleon is investigated with the potential version of the Ultrarelativistic Quantum Molecular Dynamics （UrQMD） model combined with the traditional coalescence afterburner. In the coalescence process, the relative distance R0 and relative momentum P0 are surveyed in the range of 3-4 fm and 0.25-0.35 GeV/c, respectively. For both clusters, a strong reversed correlation between R0 and Po is seen and it is time-dependent as well. For protons, the accepted （R0, P0） bands lie in the time interval 30-60 fm/c, while for 3He, a longer time evolution （at about 60-90 fm/c） is needed. Otherwise, much smaller R0 and P0 values should be chosen. If we further look at the rapidity distributions from both central and semi-central collisions, it is found that the accepted [tout, （R0, P0）] assemble can provide consistent results for proton yield and collective flows especially at mid-rapdities, while for 3He, the consistency is destroyed at both middle and projectile-target rapidities.
The in-medium NN→N△ cross sections and its differential cross sections in isospin asymmetric nuclear medium are investigated in the framework of the one-boson exchange model by including isovector mesons, i.e., δand p mesons. Our results show that the in-medium NN→N△ cross sections are suppressed when the density increases, and the differential cross sections become isotropic with an increase in the density around the △ threshold energy. The isospin splitting on the medium correction factor, R =σ_(NN→N△)~*/σ_(NN→N△)^(free) is observed for different channels of NN→N△, especially around the threshold energy for all the effective Lagrangian parameters. By analyzing the selected effective Lagrangian parameters, our results show that the larger effective mass is, the weaker medium correction R is.
Fragment mass distributions of quasifission induced by 238U on 160, 26Mg, 32S, 35C1, 40Ca and 65Zn targets at several bombarding energies have been calculated with the two-step model. The experimental data are reproduced consistently, and the mechanism of the distribution shape is also analyzed by investigating the different incident channels.
By considering different values of the time-step for the potential updates in the ultra-relativistic quantum molecular dynamics(UrQMD) model, we examine its influence on observables, such as the yield and collective flow of nucleons and pions from heavyion collisions around 1 GeV/nucleon. It is found that these observables are affected to some extent by the choice of the time-step,and the impact of the time-step on the pion-related observables is more visible than that on the nucleon-related ones. However,its effect on the π-/π+yield ratio and elliptic flow difference between neutrons and protons, which have been taken as sensitive observables for probing the density-dependent nuclear symmetry energy at high densities, is fairly weak.
LiYan ZouMiao LiChenChen GuoYongJia WangQingFeng LiLing Liu
A potential version of the UrQMD （UrQMD/M） transport model and a traditional coalescence model are combined to calculate the production of 3He fragments in central Pb＋Pb collisions at SPS energies 20-80 GeV/nucleon. It is found that the Lorentz transformation in the afterburner influences visibly the 3He yield and should be considered in calculations. The rapidity distribution of 3He multiplicities （including the concave shape） can be described well with UrQMD/M when it stops during tout=（100＋25） fm/c and the coalescence afterburner with one parameter set of （R0,P0）=（3.8 fm, 0.3 GeV/c） is taken into use afterwards.
Qing Feng LiYong Jia WangXiao Bao WangCai Wan Shen
Some nearly-symmetric fusion reactions are systematically investigated with the improved quantum molecular dynamics(Im QMD)model. By introducing two-body inelastic scattering in the Fermi constraint procedure, the stability of an individual nucleus and the description of fusion cross sections at energies near the Coulomb barrier can be further improved. Simultaneously, the quasifission process in154Sm+160Gd is also investigated with the microscopic dynamics model for the first time. We find that at energies above the Bass barrier, the fusion probability is smaller than 10-5for this reaction, and the nuclear contact time is generally smaller than 1500 fm/c. From the central collisions of Sm+Gd, the neutron-rich fragments such as164,165 Gd,192W can be produced in the Im QMD simulations, which implies that the quasi-fission reaction could be an alternative way to synthesize new neutron-rich heavy nuclei.
