Nuclear shell model and phase transitions

The usual nuclear shell model defines nuclear properties through the two-body interaction Hamiltonian. It is not clear, however, how the individual matrix elements contribute to the physical quantities. It is the purpose of this study to understand how various groups of interaction matrix elements affect the nuclear observables and the many-body properties, such us collectivity and the nuclear level density. By changing the values of specific matrix elements, we find a transition between a collective and a non-collective phases, accompanied by changes in the level density which become more pronounced for even-odd and odd-odd nuclei. We also find an enhancement of the level density in the collective state, present for all nuclei.