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

An experiment designed to populate two-phonon vibrational states in {sup 208}Pb by Coulomb excitation was performed with a {sup 136}Xe 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{sub 1}{sup {minus}}{r_arrow}6{sub 1}{sup +}) value for the lowest known 6{sup +} state at 4.424 MeV based on measured {gamma}-ray intensities. About 20{percent} of the expected total E3 strength can be found in this state, suggesting a large fragmentation of this second octupole phonon state in {sup 208}Pb. Upper limits for the B(E3) strength were determined for higher-lying, but unseen, 6{sup +} states ranging from 15{percent} of the harmonic value at 5.2 MeV to 100{percent} at 6.0 MeV. {copyright} {ital 1998} {ital The American Physical Society}