Evidence for a two-phonon octupole vibrational state in²⁰⁸Pb

Collective properties of^206,208Pb have been investigated with gamma-ray spectroscopy using nuclear and electromagnetic excitation by²⁰⁸Pb projectiles. From the measurement of particle-particle-γ and particle-particle-γ γ coincidences a new energy level in²⁰⁸Pb at 5.683 MeV was observed and the data suggest an interpretation as a 2-phonon octupole vibrational state with spin 4⁺ or 6⁺. For this state one finds the same dynamical deformation parameter β₃ as for the first-excited collective 3^- state. The results are compared to predictions of models which consider, in particular, the aspects of the quadrupole interaction and the interaction of the 2-particle pairing vibration with the 2-phonon octupole vibration. For the²⁰⁶Pb+²⁰⁸Pb system at 6.2 MeV/u the nuclear potential could be determined from the measured inelastic cross sections.