Study Of Symmetry Energy Effect On Stagger Modality Of Heavy-Ion Reactions

The equation of state of nuclear matter is of great importance to both nuclear physics and astrophysics.The equation of state for symmetric nuclear matter has been well determined,while the form of the equation of state for asymmetric nuclear matter still remains widely controversial.The density dependent of the symmetry energy is a pressing issue nowadays.Heavy-ion reaction,as the only way to produce nuclear matter under extreme conditions in terrestrial laboratories,is an important tool for studying the equation of state of nuclear matter.The symmetry energy affects the different transport behaviour of protons and neutrons during heavy-ion collisions,and different forms of density dependence of the symmetry energy lead to different isospin relaxation processes,thus studying the mechanism of isospin relaxation helps people to gain more insight on the symmetry energy.The experiment of 25 MeV/u 86Kr+208Pb reaction has been performed by Zhigang Xiao from Tsinghua University.The energy spectrum and the yield ratio of t/3He in coincidence with various heavy fragments with 7 ≤A ≤14 emitted in each collision.The fine features of thermodynamic relaxation and isospin degree of freedom relaxation in nuclear reaction process are investigate.It is found that the spectra of the velocities of triton t and 3He in coincidence with various heavy fragments exhibit scaling behavior,and the yield ratio R(t/3He)exhibits anti-correlation with δIH.It suggests that if more(less)neutrons are carried away by the heavy fragment at an earlier stage,the following light one carries less(more)neutrons.The two particles staggerly emit to the neutron-rich and neutron-deficient sides like ping-pong balls.This "ping-pong" modality of emitted particles suggests that the neutron richness of the emitted particles is balanced near a certain value.Based on this experiment,a theoretical analysis was carried out by using the improved quantum molecular dynamics(ImQMD)model combined with GEMINI statistical decay model.The stagger modality of the isospin degree of freedom in particles emission and the isospin anti-correlation behaviour are verified theoretically.The symmetry energy effect on the isospin anti-correlation behaviour are investigated,with various density dependence of symmetry energy adopted as power function.i.e.Esym=Cs,k/2(ρ/ρ0)2/3+Cs,p/2(ρ/ρ0)γ.By examining the R(t/3He)ratio as a function of δIH at different impact parameters with various γ,it is found that the isospin anti-correlation behaviour is less sensitive on the impact parameter,but more sensitive on the γ.The larger γ gives the lower R(t/3He)and the weaker isospin anti-correlation.A comparison between theory and experiment reveals that the theoretical results with γ=0.6-0.8 can largely cover the experimental results,and the super soft and super stiff symmetry energy with γ=0.4 and 2.0 can be eliminated rather certainly.This conclusion is consistent with the now recognized reasonable range of symmetry energy.The observable R(t/3He)-δIH is expected to provide a new way to finely constrain the symmetry energy parameter.Finally,the shortcoming of the ImQMD model in describing velocities and yields of light particles is discussed.The accurate quantitative extraction of symmetry energy based on the observable R(t/3He)ratio still requires further model improvement and development.