The Study Of The Isospin Relaxation Time In Intermediate-energy Heavy-ion Collisions

In intermediate-energy heavy-ion collisions,the isospin degree of freedom of the system goes through the relaxation process from the non-equilibrium state to the equilib-rium state,and complicates the dynamical process.Meanwhile,this process reflects the nature of the nuclear reaction kinematics.The isospin relaxation process is related to the nuclear symmetry energy and the neutron-proton effective mass splitting,both of which have important implications not only in nuclear physics but also in astrophysics.But the density dependence of the nuclear symmetry energy still has a lot of uncertainties,and it is not sure whether the neutron or the proton has the larger effective mass in the neutron-rich nuclear medium.The quantitative measurement of the isospin relaxation time may help us further constrain the nuclear symmetry energy and the neutron-proton effective mass splittingIn intermediate-energy heavy-ion nuclear reactions,the isospin relaxation process exists in a long time scale,so a stable transport model is needed to study it.We have im-proved the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU)transport model by using the lattice Hamiltonian method.The canonical equations of motion of the par-ticles are calculated by this method,which can keep the accuracy of calculation for a long time and meet the requirement of study of the isospin relaxation timeIsospin-relaxation times,characterizing isospin transport processes between the proj ectile and the target with different N/Z ratios and that between the neck and the spec-tator with different isospin asymmetries and densities in intermediate-energy heavy-ion collisions,are studied with in an improved isospin-and momentum dependent in-teraction(ImMDI)and the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU)transport model within the lattice Hamiltonian framework.The respective roles and timescales of the isospin diffusion and drift as the maj or mechanisms of isospin trans-port in intermediate-energy heavy-ion collisions are discussed.Effects of the nuclear symmetry energy and the neutron-proton effective mass splitting on the isospin relax-ation times are examinedOur research indicates that the isospin relaxation time from mn-p*<0 is longer than that from mn-p*>0 in both cases.In the isospin transport process between the proj ectile and the target with different N/Z ratios,the isospin-relaxation time and the slope parameter of the nuclear symmetry energy generally have a positive correlation The situation is different in the isospin transport process between the neck and the spec-tator in noncentral heavy-ion collisions.In this case,the isospin-relaxation time and the slope parameter of the nuclear symmetry energy have a negative correlation,because the initial gradient of the isospin asymmetry and the isovector drift coefficient are dif-ferent.The impact parameters also affect the isospin relaxation time between the neck region and the spectatorAt last,the extracted isospin relaxation time in 70Zn +70 Zn collisions from the present study is within the experimental uncertainty range.Significant improvement of the accuracy for measuring experimentally the isospin relaxation time and additional in-formation about the collision centrality are necessary to extract useful information about the nuclear symmetry energy and the neutron-proton effective mass splitting from com-paring quantitatively the model calculations with the experimental result.Meanwhile,our study may help us better comprehend the isospin diffusion and the isospin drift mechanisms.