Studies On Symmetry Energy At Sub-saturation Densities And Nucleon Effective Mass Splitting

Within an isospin and momentum dependent transport model,symmetry energy at sub-saturation densities and isospin splitting of nucleon effective mass are investigated in intermediate-energy heavy-ion reactions.Heavy-ion reac-tion mainly studies the dynamical processes of collisions between two nuclei.Studies on mechanisms of nuclear reactions induced by radioactive beams are hot topics in nuclear physics.There are many radioactive beam facilities that already exist,or being constructed in the world.At these facilities,nuclear reactions can be done from keV of low-energy to GeV of relativistic energy.The Fermi energies?10-100 MeV/nucleon?heavy-ion reaction is affected by the nucleon-nucleon interaction potential,Pauli effects and the nucleon-nucleon collisions.People use transport models to simulate the heavy-ion collisions to explore the isospin dependence of in-medium nuclear effective interactions and the equation of state of the neutron-rich nuclear matter,particularly the isospin-dependent term in the equation,i.e.,the density dependence of the symmetry energy.Firstly the neck dynamics of isospin particles?nucleons and light clusters?are thoroughly investigated in the isotopic nuclear reactions of¹¹²Sn+¹¹²Sn and¹²⁴Sn+¹²⁴Sn.Studies on the single and double neutron to proton ratios of free nucleons and gas-phase nucleons?nucleons,hydrogen and helium isotopes?at incident energies of 50 and 120 MeV/nucleon are carried out,respectively.With these isospin sensitive observables,one can constrain the splitting of neutron and proton effective masses and the symmetry energy at subsaturation densi-ties.It is found that both the effective mass splitting and the symmetry energy affect the kinetic energy spectra of the single ratios,in particular at the high en-ergies.The effect of symmetry energy on the double ratios is more larger than the effective mass splitting.A soft symmetry energy with stiffness coefficient of ?_s=0.5 is constrained from the new experimental data from the National Super-conducting Cyclotron Laboratory of Michigan State University?NSCL/MSU?.The splitting of m_n^*>m_p^*is roughly consistent with the experimental data with the soft symmetry energy.Secondly the isospin effect in the quasi-fission reactions has been inves-tigated with the LQMD transport model.It is found that the projectile-like and target-like fragments formed are influenced by reaction system,incident energy and impact parameter from the the fragments distributions as a func-tion of the parallel velocity and total kinetic energy.The yields of the primary fragments in the quasi-fission reactions almost exhibit a symmetric distribu-tion in the projectile-like and target-like domain.The average n/p ratios of the projectile-like and target-like fragments are sensitive to the stiffness of the sym-metry energy.A hard symmetry energy is available for creating the neutron-rich fragments in neutron-rich nuclear reactions.The primary fragments have high excitation energies,so de-excitation process of primary fragments with the GEMINI statistical decay code is calculated.Owing to larger excitation energy,the isospin effect is weakened via the decay process,in particular in the heavy mass domain.