The Soft Rotor Model Description Of The ?-unstable Nuclei

The interacting boson model is an algebraic model for the description of the collec-tive motions in nuclei,which has been proven to be successful in describing nuclear structure and related properties of nuclei in the low-energy regime.There are three analytically solvable limiting cases,called the U(5)(vibrational)limit,the O(6)(?-unstable)limit,and the SU(3)(ro-tational)limit of the model,of which the corresponding regions in the nuclear chart have been experimentally identified.Therefore,like the shell model code,the interacting boson model code has been adopted in nuclear laboratories worldwide as a standard tool in analyzing the nuclear spectroscopy.However,there are still some substantial deviations of the model results from the experimental data in each limiting case.Especially,most observed electric quadrupole moments of excited states of the typical ?-unstable nuclei are obviously nonzero,while they are zero in the original O(6)limit theory description,which,therefore,should be improved.The purpose of this thesis is to introduce a ?-soft rotor term into the model Hamiltonian,which has a geometric origin,so as to improve the results of the original O(6)limit theory with the vanishing of electric quadrupole moments of the yrast states and some E2 forbid-den transitions in even-even nuclei.The model Hamiltonian is diagonalized under the U(6)(?)O(6)(?)O(5)(?)SO(3)(?)SO(2)basis.Moreover,an exponential form of the O(6)quadrupole-quadrupole interaction originated from the shell model is adopted,with which the level energies with large seniority number can be effectively improved in accordance with the experimental data.The soft rotor model is proposed and diagonalized in this algebraic approach,which is applied to describe the spectra,electric quadrupole moment,and B(E2)values among low-lying states of the ?-unstable even-even nuclei.The results are compared with the corresponding experimental data and those of the original O(6)limit theory.The results show that the mean square deviations of the level energies of the typical O(6)limit theory even-even nuclei,194,196Pt,128,130Ba,122,124Xe,described in the soft rotor model are less than those of the original O(6)limit theory,which indicates that the soft rotor model is more appropriate.Especially,the inconsistencies of the vanishing of electric quadrupole moments of the yrast states and some E2 forbidden transitions in the original O(6)limit theory is overcome in the soft rotor model,which are obvious improvements to the O(6)limit theory.