An extended interacting boson model for the germanium nuclei

The low-lying states and especially the extraordinary behaviour of the first excited O+ state as a function of neutron number in the sequence of nuclei, 68Ge, 70Ge, 74Ge and 76Ge, are described (except for 76Ge) by introducing an s'-boson into the interacting boson model which usually involves only s- and d-bosons. This introduction can be understood by considering results from BCS-RPA calculations in the neutron configuration only. The BCS-RPA theory is, however, shown to be a poor approximation in the case under consideration. Results from the exact diagonalization of the Hamiltonian for the charge-independent pairing interaction in both the proton and neutron configuration are discussed