Correlated-pair structure in nuclei and possible relation to IBM-2

Abstract

The broken-pair model is used to study the nature of collective pair structure in even nuclei and a possible connection with the interacting-boson model IBM-2. In our selfconsistent treatment there is no particle-hole ambiguity, such as has been reported in recent literature. We find that about four particles away from a closed shell, for both protons and neutrons, the collective 2 + (D-pair) structure becomes roughly independent of mass number due to the proton-neutron force. Only near the closed shell pronounced fermion configurations show up. The same is true for the 3-(F-pair). In lighter nuclei, the 20<N_<50 shell, no really collective 4 + (G-pair) is formed but in heavier nuclei, 50_<N_<82, it begins to develop. No other types of collective-pair structure appear, such as a collective 0~-(S'-pair). When microscopically computed quantities are compared with currently used parameters in IBM fits, we find not only a considerable renormalization of boson energies, but also of the proton-neutron interaction. One effect responsible for this is Pauli blocking which makes the interaction seniority dependent. The trend of the proton-neutron interaction parameter ~c agrees with single-shell formulas, but for the IBM parameters )~P single-shell predictions are of little value. With some rules of thumb for renormalization the calculated parameters come close to those found in empirical fits.