Transition from the seniority to the anharmonic vibrator regime in nuclei

A recent analysis of experimental energy systematics suggests that all collective nuclei fall into one of three classes - seniority, anharmonic vibrational, or rotational - with sharp phase transitions between them. We investigate the transition from the seniority to the anharmonic vibrator regime within a shell model framework involving a single large j orbit. The calculations qualitatively reproduce the observed transitional behavior, both for U(5)-like and O(6)-like nuclei. They also confirm the preeminent role played by the neutron-proton interaction in producing the phase transition.