Fragment Production And Distribution In Low And Intermediate Energy Heavy Ion Collisions

Heavy-ion collisions are the only way for terrestrial laboratories to directly produce dense nuclear matter and serve as an important means for studying the properties of nuclear matter.The equation of state of nuclear matter is at the center of numerous theoretical and experimental works in nuclear physics,playing a vital role in the study of nuclear structure,nuclear reactions,and nuclear astrophysics.With the development of microscopic theories and statistical theories describing the interactions of nuclear matter,the kinetic information of various particles detected in heavy-ion reactions is regarded as the messenger for studying the equation of state of nuclear matter.Since light particles carry information about the dense nuclear matter in the early stages of collisions,their yield distribution,collective flow,and nuclear stopping power are commonly used to extract information about the equation of state of nuclear matter.In this master’s thesis,the Ultra-relativistic Quantum Molecular Dynamics（UrQMD）model is utilized to first study the stability and distribution characteristics of light particles produced in heavy-ion collisions in the low and intermediate energy region,as well as the role of the mean field in nucleon transport processes.The results show that the yields of light particles in heavy-ion reactions are significantly influenced by the impact parameters and mean field,reflecting the intensity of the collision to a certain extent.The production of light particles can be divided into three stages:pre-equilibrium emission,multifragmentation,and saturation.Subsequently,by incorporating the statistical decay model GEMINI,the collisions of gold and gold（Au+Au）at different impact parameters within the incident energy range of E_lab=40～400 MeV/nucleon and calcium and calcium（Ca+Ca）at E_lab=35 MeV/nucleon are simulated.The sequential decay effects on the yield,collective flow,nuclear stopping power,and symmetry energy probes n/p（neutron number/proton number）and ³H/³He（³H number/³He number）ratios of light particles produced in heavy-ion reactions are analyzed.In the energy range studied,the sequential decay of low-excited primary fragments produced in the dynamical process of heavy-ion collisions affects the yield and distribution of final-state p,²H,³H,³He,and ⁴He particles,thereby influencing their collective flow and nuclear stopping power.By comparing with the experimental data from the international INDRA and FOPI collaborations,the impact of sequential decay effects on the dynamic evolution process of light particles is assessed.Due to the isotropic nature of statistical decay,the sequential decay-produced light particles retain some of the dynamical information of the parent nucleus,thereby affecting the observables of these particles.This is manifested as a suppression of the direct flow slope parameter,an enhancement of the elliptic flow parameter,a increase in the penetration of light particles within nuclear matter,and a weakening of this effect with increasing incident energy.The influence of sequential decay on the symmetry energy probes n/p single ratio and double ratio is relatively small at low heavy-ion collisions.Using the ³H/³He double ratio as a probe for the stiffness of symmetry energy can eliminate the impact of sequential decay of fragments in the later stage of heavy-ion collisions,improving the accuracy of the measurement results,and better studying the properties and behavior of symmetry energy.