The Study Of Fast Fission Induced By Heavy Ion Collisions In The Fermi Energy Region And Its Symmetry Energy Effects

Symmetry energy plays a crucial role for understanding not only nuclear structures and reactions but also astrophysical phenomena.After decades of efforts and development by physicists,many efforts have been devoted to probe and constrain the symmetry energy,and a consensus on the constraints of symmetry energy has been obtained.Nevertheless,differences among the constraints of symmetry energy obtained from different models or approaches still exist,especially for the slope of symmetry energy.Understanding these differences is a challenge in this field and it stimulates nuclear physicists to purse new physical mechanisms and to propose new probes that are sensitive enough to further discriminate the stiffness of symmetry energy.The main content of this thesis is to study the symmetry energy effects by the fast fission mechanism.First of all,there is no suitable model on the quantitative description of fast fission induced by heavy ion collisions in the Fermi energy region since the discovery of fast fission in the 1980s.Considering that the Improved Quantum Molecular Dynamics(Im QMD-v2)model can successfully describe the collective properties of nuclear reactions at low and medium energy,we attempt to use this model to analyze the characteristics of fast fission.The analysis results show that the transport model can describe the properties of fission fragments well,including the two-step fission mechanism process,mass number distribution,fission time scale,the relative velocity,and the distribution of in-plane angle and out-plane angle,etc.It is also shown that the fission axis angular distribution of the forward angular region is sensitive to the surface energy coefficient,which may be regarded as a sensitive observable on the constrain of surface energy coefficient in future.Then,the symmetry energy effect of light particle emission in fast fission is further studied by Im QMD05 model.The results show that fast fission can make more coalescence invariant neutron(CIN)emitted in the backward angular region with a long timescale,but it is not for the coalescence invariant proton(CIP).The analysis also shows that the double ratio of CIN and CIP,DR₂₁,is sensitive to the stiffness of the symmetry energy,but insensitive to the coefficient of the symmetry energy.These suggest that DR₂₁ may be an effective observable on the constraint of the stiffness of symmetry energy.Finally,considering that the transport model cannot accurately describe the yield of light particles exactly and neutrons are not measured in nuclear physics experiments,₁₂ cannot be used to constrain the symmetry energy directly.Besides,the symmetry energy effect of the fission timescale is proposed to study by the angular distribution of fission axis.The results show that the survival time of the fission neck region is sensitive to the symmetry energy.The analysis also shows that the slope of fission axis angular distribution is sensitive to the symmetry energy stiffness,but insensitive to the symmetry energy coefficient.It indicates that the slope of the fission axis angular distribution may be a more sensitive observable on the constrain of the stiffness of symmetry energy.