Fragmentation of the two-phonon octupole vibrational states in 208Pb

An experiment designed to populate two-phonon vibrational states in Pb-208 by Coulomb excitation was performed with a Xe-136 beam at a bombarding energy of 650 MeV. The gamma rays from the decay of the excited states were measured with Gammasphere and scattered particles were detected in the compact heavy-ion counter CHICO. We have not been able to observe any state close to the expected harmonic energy of 5.2 MeV. However, we were able to extract the B(E3,3(1)(-)-->6(1)(+)) value for the lowest known 6(+) state at 4.424 MeV based on measured gamma-ray intensities. About 20% of the expected total E3 strength can be found in this state, suggesting a large fragmentation of this second octupole phonon state in Pb-208. Upper limits for the B(E3) strength were determined for higher-lying, but unseen, 6+ states ranging from 15% of the harmonic value at 5.2 MeV to 100% at 6.0 MeV. [S0556-2813(98)50411-6].