Research on excitation mechanism for medium and heavy nuclei via approaches beyond mean field approximations

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We have studied the nuclear structure of medium and heavy nuclei in terms of various models and have obtained the following results. (1) We investigated the doublet bands in the doubly-odd nuclei around the mass 130 in terms of the quadrupole coupling model. By analyzing wave functions, we have found that the level scheme of these states arises from chopsticks configurations of two angular momenta of the unpaired neutron and the unpaired proton, weakly coupled with the quadrupole excitations of the even-even core. (2) We performed shell-model calculations for neutron-rich Se, As, Ge, and Ga isotopes, and compared the calculated energy levels and electromagnetic transitions with the experimental data. Through the analysis of the shell model results, it is confirmed that the spin alignment of two neutrons in the 0g_<9/2> orbital becomes apparent at high spins. (3) We have developed a new algorithm for generator coordinate method calculations in order to investigate the structure in transitional nuclei. An application of this model to the even-even Se and Ge nuclei indicated the importance of triaxial deformation in the mass 80 region.