Covariant Density Functional Description Of Octupole Deformation And Low-lying Spectrum In Neutron-rich Actinides

The nuclear spectroscopic properties of low-lying states are important physics quantities that reveal rich structure information of atomic nuclei,including the evolution of the shell structure,shape coexistence,shape phase transition,cluster structure and so on.Most of nuclei can be reasonably described by considering quadrupole deformation.However,there exists alternative double-band structure with low-energy positive-and negative-parity states in some specific mass regions and its spectrum is similar to diatomic molecular structure,which indicates possible existence of ground state reflection-asymmetric octupole deformation.In addition,octupole deformation plays a crucial role in nuclear cluster structure,cluster decay,fission process,and so on.Covariant density functional theory?CDFT?has achieved great success in describing properties of the nuclei in the whole nuclide chart.However,to describe excitation spectrum and electromagnetic transitions,one has to go beyond the mean-field approximation.Recently,our group has developed quadrupole-octupole collective Hamiltonian?QOCH?based on CDFT,which includes rotational symmetry,parity symmetry,and quadrupole and octupole shape fluctuations.QOCH model has been applied to a systematic analysis of medium-heavy and heavy nuclei.It has been shown to accurately describe the empirical trend of low-energy quadrupole and octupole collective states,and predicted spectroscopic properties are consistent with recent microscopic calculations based on both relativistic and non-relativistic energy density functionals.In this thesis,we further apply the successful QOCH model to study ocutpole shape transitions in neutron-rich nuclei: Ra,Th,U,Pu,Cm,Cf,Fm,and No with 190?N?212.Firstly,the physical parameters such as collective potentials,moment of inertia and mass parameters of nuclei are calculated using reflection-asymmetric CDFT.Secondly,the low-lying spectrum and collective wave functions are calculated by QOCH established by using the collective parameters as inputs,and thus observables can also be obtained.Finally,the theoretical results of low-lying negative-parity bands,odd-even staggering,average octupole deformations ??₃? and ?E3:3₁⁺?0₁⁺? show evidence of a shape transition from nearly spherical to stable octupole-deformed,and finally octupole-soft equilibrium shapes in the neutron-rich actinides.A microscopic mechanism for the onset of stable octupole deformation is also discussed in terms of the evolution of single-nucleon orbitals with deformation.