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Like in earlier work by Schiffer et al, the effective interaction is derived from experimental two-body muttiplets. However, now the assumption is that a muttiplet state is formed by two unpaired fermions relative to a core of correlated J=0 pairs. Then the need for two ranges, as proposed by Schiffer, disappears for the force between identical nucleons in a model space which is large enough to include pairing correlations, A form with a single attractive medium range is preferred for the identical nucleon interaction in order to reproduce collective 2 + states in even-even nuclei. In contrast, the proton-neutron force requires a very short range or two ranges to reproduce the empirical values of multipole coefficients, observed in odd-odd nuclei. Therefore we discuss the fact that the effective interaction is not always isospin invariant. As a typical case broken-pair calculations in the N=50 region are considered. But the conclusions drawn, will also apply to other regions of the periodic table.

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