Search for the two-phonon octupole vibrational state in 208Pb

Abstract We have performed an experiment to search for the two-phonon octupole vibrational state in 208 Pb. Thick targets of 208 Pb, 209 Bi, 58,64 Ni, and 160 Gd were bombarded with 1305 MeV beams of 208 Pb supplied by the ATLAS accelerator facility at Argonne. Gamma rays were measured using the Argonne-Notre Dame BGO gamma ray facility, consisting of 12 Compton suppressed Ge detectors surrounding an array of 50 BGO scintillators. We have identified some 30 known γ rays from 208 Pb in coincidence spectra gated on low-lying transitions in 208 Pb. In addition, after unfolding these spectra for Compton response, we observe broad coincident structures in the energy region expected for the decay of 2-phonon states. Furthermore, we have confirmed the placement of a previously observed 2485 keV γ ray in 207 Pb and find no evidence consistent with the placement of this line in 208 Pb.