Long standing problem of 210Bi and the realistic proton-neutron effective interaction

The odd-odd nucleus Bi-210 is studied within the framework of the shell model using effective two-body matrix elements derived from the CD-Bonn nucleon-nucleon potential. The experimental energies of the proton-neutron multiplet pi h(9/2)nu g(9/2) are remarkably well reproduced by the theory, which accounts for the 1(-) state being the ground state instead of the 0(-) predicted by the Nordheim strong coupling rule. It is shown that the core-polarization effects are crucial to produce this inversion. The similarity between proton-neutron multiplets in the Sn-132 and Pb-208 regions is discussed in connection with the effective interaction.