INTRUDER ANALOG STATES - NEW CLASSIFICATION OF PARTICLE-HOLE EXCITATIONS NEAR CLOSED SHELLS

Shell-model particle-hole intruder excitations and the collective bands built on them near closed shells are studied and classified, together with low-lying collective bands in adjacent nuclei with the same number of "valence" protons and neutrons, into multiplets. A detailed numerical study within the interacting boson model gives support to such a classification scheme. We introduce the concepts of intruder spin and intruder analog states and show how such a symmetry can be useful to understand nuclear collective structures at and near to closed shells.