Octupole vibrational states in the lead region

Collective properties of 206Pb, 207Pb, 208Pb and 209Bi have been investigated with gamma-ray spectroscopy using electromagnetic excitation by 208Pb projectiles. By measuring the absolute yields of the de-excitation γ-rays, B(E3) values for the 1-phonon octupole vibrational states were obtained. While the present data for 208Pb and 209Bi are consistent with earlier publications, much smaller values have been found for 206Pb abd 207Pb. Only for the doubly magic nucleus 208Pb one observes the largest octupole collectivity of 34 Weisskopf units (W.u.), which is reduced to 60-70% by the particle-vibration coupling in nuclei which consist of the 208Pb core plus (or minus) a few nucleons. As a consequence of the phonon vibration character of the 3- state, one expects a quadruplet of 2-phonon octupole states with spins and parities 0+, 2+, 4+ and 6+ at about twice the energy of the 1-phonon state. A first scattering experiment suggesting the observation of at least one member of the 2-phonon multiplet was performed via the reaction 208Pb+208Pb at an incident energy well above the Coulomb barrier. In the present experiment the system 208Pb+208Pb has been reinvestigated via Coulomb excitation at a safe bombarding energy at 5.0 MeV/u. The reduced cross-section for the excitation of the octupole states has been compensated by the higher full energy efficiency of five EUROBALL CLUSTER detectors. With the present experimental set-up transition probabilities down to 10-5-10-6 can be measured