Rotational Structure Of Triaxial Octupole Deformed Nucleus

Symmetry is one of the most fundamental knowledge and important research topics in physics.In atomic nuclei,spontaneous symmetry breaking of the density distribution results in various shapes which are associated with different spatial symmetries.To further deepen the understanding of nuclear deformation and symmetry,by using the rotational model and group theory,this thesis discusses the symmetry of triaxial deformation and octupole deformation in even-even nuclei and odd-odd nuclei,and the corresponding rotational band structures.The main contents include:First,the excited alternating-parity bands in octupole deformed even-even nucleiBoth of the ground(denoted by G+/G-for positive/negative parity sequence)and excited(Ex+ and Ex-)alternating-parity bands had been observed in a few octupole deformed even-even nuclei.Taking 148Ce as an example,the restored symmetries for the observed bands are investigated by constructing a symmetry group {E,R1,R2,R3,S1,S2,S3,P}.All possible wave functions belonging to 8 group representations have been analyzed.It is found that to agree with the observed transitions in 148Ce,G+and Ex+should belong to the same group representation(+1,+1,+1,+1,+1,+1,+1,+1),while G_and Ex_belong to representation(+1,-1,-1,+1,+1,+1,-1,-1).A new understanding of the symmetries for the observed excited band is provided,and a different,value of simplex quantum number is proposed.From another perspective,the wave functions of the observed ground and excited bands should be symmetrical and antisymmetrical,respectively,which exactly correspond to two modes of symmetry restoration.Second,the chiral-parity quartet bands in octupole deformed odd-odd nucleiThe chiral-parity quartet bands is another important experimental for the triaxial deformation.In the odd-odd nuclei with triaxial deformation,if the octupole deformation is also presen,the chiral-parity quartet bands may appear.The appearance of chiral-parity quartet bands requires the existence of unpaired protons and neutrons in the triaxial deformation nuclei,thus,the triaxial-octupole particle rotor model is developed in this thesis,and the progress of the model is shown,including the theoretical derivation of the triaxial-octupole Nilsson potential,the rotor Hamiltonian of the odd-odd nuclei,the derivation of the rotor Hamiltonian in the particle number representation,and the calculation method of the matrix elements of these Hamiltonian.This thesis explores the rotational structure of triaxial-octupole nuclei,and deepens the understanding of the mechanism of symmetry breaking and restoration.