Study Of Level Structure In Nearly Spherical Nuclei ⁹³Mo And ⁹⁰Zr

The angular momentum of nuclei can be generated by intrinsic single-particle excitation or collective excitation modes.The single-particle excitation is dominant in nuclei which proton or neutron numbers near the magic number,and these nuclei are affected by the shell effect,whereas the collective excitation mainly occurs in nuclei far from the magic number(deformed nuclei).The study on level structure is of great significance to the development of the shell model theory,so the study of level structure in weakly deformed and nearly spherical nuclei is the focus of experimental and theoretical research.In particular,the nuclei in the A?90 mass region provide a suitable laboratory to study the mechanism of particle-hole excitations,due to the existence of Z=38,40 subshells and N=50 core.A lot of interesting phenomena have been found and reported in this mass region due to the existence of the N=50 core,such as single-particle excitation,core breaking,magnetic rotation and isomer states.Based on above topics,we studied the nearly spherical nuclei ⁹³Mo and ⁹⁰Zr,and a detailed discussion on level structure is presented in later chapters.The main results are as follows:(1)The N=51 ⁹³Mo nucleus is investigated in terms of large scale shell model calculations.The SNE model space and SNET interactions were adopted in this code,and reasonable agreement is obtained between the experimental and calculated values.The calculated results show that the low-lying states are mainly dominated by proton excitations from(1f_5/2,2p_3/2,2p_1/2)across the Z=38 or Z=40 closed subshell to the 1g_9/2 orbital.For the higher-spin states,multi-particle excitations,involving the excitation of neutrons across the N=56 closed subshell into the high-j intruder 1h_11/2orbital,are crucial.The shell model calculations predict that the core breaking may begin to appear at 43/2?.(2)The semimagic nucleus ⁹⁰Zr with Z=40 and N=50,is studied using large scale shell model calculations with the NUSHELLX code.The GWB model space and GWBXG interaction are adopted in the code.A logical agreement is obtained between the available experimental data and predicted values.The calculated results indicate that the low-lying states are generated by the proton excitations across the Z=38 or Z=40 subshell closure into the higher orbital,whereas the N=50 core excitation plays a significant role in the higher-spin states of ⁹⁰Zr.Meanwhile,the evolution in lower-spin states from the neutron core excitations to proton excitations was systematically investigated along the neighboring N=50 isotones,and ⁹⁰Zr may be the critical nucleus for this evolution.Furthermore,the strong?I=1 sequence demonstrates an abrupt backbend to the alignment of the valence nucleons in fp proton orbitals and is proposed to have a?(fp)^-2(1g_9/2)~2??(1g_9/2)^-1(2d_5/2/1g_7/2)~1 configuration before the backbend,based on the shell model calculations.The properties of this sequence before the backbend indicate a general agreement with the fingerprints of magnetic rotation;hence,the sequence with the?(fp)^-2(1g_9/2)~2??(1g_9/2)^-1(2d_5/2/1g_7/2)~1 configuration is suggested as a magnetic rotational band arising from shear mechanism.