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. 
R. Bijker, A. Frank and S. Pittel (ICN/UNAM, Mexico and Bartol   Research Institute, U. of Delaware)