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On the Synthesis of the Superheavy Nuclei and Asymmetric Nuclear Matter Equation of State Based on the Transport Theory

Author: LiWenFei
Tutor: XuHuShan;ZhangFengShou;LiJunQing
School: Institute of Modern Physics
Course: Particle Physics and Nuclear Physics
Keywords: transport theory heavy ion fusion superheavy nuclei (SHN) asymmetric nuclear matter EOS HIRFL-CSR
CLC: O571.6
Type: PhD thesis
Year: 2004
Downloads: 129
Quote: 1
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Abstract


HIRFL-CSR accelerator system can/will provide heavy-ion beams with energies from several MeV/u up to 1.1 GeV/u. The synthesis of the superheavy nuclei (SHN) and the study of the asymmetric nuclear matter equation of state (EOS) are two of the proposed physics programs at it. Before pursuing the experimental research, detailed theoretical study which aimed at the related physics is important. This thesis is devoted to the theoretical study on the two proposed physics programs at HIRFL-CSR. In this thesis, for SHN study, a theoretical model to describe the synthesis of the SHN is founded. Based on this model, the optimal excitation energy and optimal project-target combination to synthesize the SHN are calculated. For exploring the asymmetric nuclear matter EOS, using the isospin dependent quantum molecular dynamics (IQMD) model and Skyrme Hartree-Fock theory, the properties of the asymmetric nuclear matter are studied, and several possible ways to extract the information of the asymmetric nuclear matter EOS at HIRFL-CSR are provided.A model for describing the synthesis of the SHN is developed based on the di-nuclear system (DNS) conception. The advantages of this model are that the dynamical effects are included in the fusion process reasonably and the structure effects of the driving potential can be well maintained. By using this code, the following contents are investigated emphatically based on the requirements from experimental research:1) The optimal excitation energies are predicted for synthesizing the elements of 114,116 and 118 by cold fusion reactions after reproducing the existing experimental data of optimal excitation energies for synthesizing element 104-112. As the excitation function for producing the SHN is very narrow, the result can be treated as a valuable reference for selecting the optimal beam energies for experimental study.2) The projectile or target dependence of the evaporation residue cross-section is studied, and the optimal projectile-target combination for producing a certain SHN can be predicted. The result shows that there exists an optimal projectile-target combination which gives the highest evaporation residue cross-section. This is different from the idea that the cross-section increases with the neutron enrichment of the projectile-target combination in general. For an example, in the case of synthesis of the 112th element through Zn isotope induced cold fusion, the optimal projectile-target combination is 67Zn+208Pb, not the combination of very expensive isotope 70Zn on 208Pb.3) The spin populations of the SHN are studied. The calculations show that the spin population of the SHN depends strongly on the fission barrier. Higher fission barrier gives wider spin population. And the spin population of the SHN is not very sensitive to the deformation of the SHN at saddle point. This indicates that it is possible to extract the fission barrier by measuring the spin populations of SHN in experiment.In addition, the survival probability and its dependences on the mass, excitation energy and angular momentum are investigated, and the pairing effect of the survival probability is studied. Moreover, in order to explore the new mechanism of synthesizing SHN, the possibility for synthesizing SHN by a few nucleons transfer reaction has also been discussed preliminarily. Present results show that it is difficult to synthesize the SHN with charge number Z larger than 108 by a few nucleons transfer reaction.The asymmetric nuclear matter EOS is very important in astrophysics and in understanding the structure of exotic nuclei. However, up to now very limited knowledge is known to people, especially for the EOS at high density. Based on the Skyrme-Hartree-Fock theory and the IQMD model, the properties of the asymmetric nuclear matter are studied and some possible ways to extract the information of the asymmetric nuclear matter EOS to be carried out at the HIRFL-CSR accelerator system are provided. The related contents are:1) Based on the IQMD model, the chemical instabilities in heavy-ion collisions are investigated. Calculations show that the chemical instability can happen in the process of heavy ion collisions at intermediate energies. With the incident energy getting higher and higher, the chemical instability events become fewer and fewer. This result is important for understanding the observed isospin effects in nuclear multifragmentation. In addition, the chemical instability at high density is investigated for the first time based on the Skyrme-Hartree-Fock theory.2) It is difficult to produce the nuclear matter with very high density in laboratory. As the neutron stars are composed of high density neutron rich nuclear matter, they provide the nature laboratories for studying the asymmetric nuclear matter EOS at high density. Therefore, it is very useful to investigate the neutron star properties using different asymmetric nuclear matter EOS. Based on the Skyrme-Hartree-Fock theory, the proton fractions of the neutron stars are investigated for two typical asymmetric nuclear matter EOS, namely the soft symmetry potential and stiff symmetry potential. It is found that the pure neutron mater core can be formed in the center of the neutron star with the soft symmetry potential nuclear matter EOS.3) Using the IQMD model, the isospin fractionation in the nuclear multifragmentation is studied, and the production mechanism of the isospin fractionation is investigated from the static and dynamic point of view. And the calculations show that the isospin fractionation can be used as a probe to study the isospin dependence of the asymmetric nuclear matter EOS. By comparing the results with the experimental data of MSU group, it is concluded that the asymmetric nuclear matter should have soft symmetry potential. This provides the constraint for the density dependence of the asymmetric nuclear matter EOS.4) The energy threshold for producing the radial flow and its dependences on the isospin of the incident systems are calculated. It is found that the asymmetric nuclear matter EOS with soft symmetry potential and stiff symmetry potential gives opposite isospin dependence of the threshold for producing the radial flow. This indicates that one can extract the information of the isospin dependent part of the nuclear matter EOS by investigating the isospin dependence of the radial flow in experiment.The results in this thesis provide the valuable theory references for studying the SHN and the asymmetric nuclear matter EOS at the HIRFL-CSR accelerator system.

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CLC: > Mathematical sciences and chemical > Physics > Nuclear physics,high energy physics > Nuclear physics > Heavy ion nuclear physics
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