Octupole vibrational states in the lead region

Summary

Collective properties of²⁰⁶Pb,²⁰⁷Pb,²⁰⁸Pb and²⁰⁹Bi have been investigated with gamma-ray spectroscopy using electromagnetic excitation by²⁰⁸Pb 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 for²⁰⁸Pb and²⁰⁹Bi are consistent with earlier publications, much smaller values have been found for²⁰⁶Pb and²⁰⁷Pb. Only for the doubly magic nucleus²⁰⁸Pb one observes the largest octupole collectivity of 34Weisskopf units (W.u.), which is reduced to 60–70% by the particle-vibration coupling in nuclei which consist of the²⁰⁸Pb 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, i.e. around 5.23 MeV. A first scattering experiment suggesting the observation of at least one member of the 2-phonon multiplet was performed via the reaction²⁰⁸Pb+²⁰⁸Pb at an incident energy well above the Coulomb barrier. In the present experiment the system²⁰⁸Pb+²⁰⁸Pb has been reinvestigated via Coulomb excitation at a safe bombarding energy of 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.