Shell-model study of octupole collectivity near 208Pb

We show that the collectivity of the particle-hole wave function of low-lying octupole 3(-) states in doubly magic nuclei is mainly due to the neutron-proton interaction. Both the enhanced reduced transition probability to the ground state, B(E3;3(1)(-) -> 0(1)(+)), and the coupling of the octupole excitation to a nucleon result from the coherent action of all the components of the collective state. The results obtained with a realistic shell-model interaction both for Pb-208 and for Pb-209 agree with the geometric collective model of Bohr and Mottelson, where octupole excitations are associated with phonons corresponding to collective shape oscillations of the surface of the nucleus.