Spectroscopy of₈₂¹⁸⁸Pb₁₀₆: Evidence for shape coexistence

In-beam γ-ray spectroscopy of ¹⁸⁸Pb has been carried out using Gammasphere. Time-correlated γ-γ coincidence methods have allowed the identification of new structures above and below the two-particle isomeric states. The detailed decay of the proposed K^π=8 ^-, 1 μs isomer has been established, together with a rotational band based on the isomer. Both decay and band properties confirm the association with a prolate deformation and the two-quasineutron 9/2⁺[624] ⊗7/2^-[514] configuration. The band structure identified above the 11^- isomer from the two-proton configuration 9/2 ^-[505]⊗ 13/2⁺[606] has a moment of inertia similar to those of the bands known in heavier isotopes and to the one-quasiproton components, but the perturbations and in-band properties are not as expected for a simple, symmetric oblate deformation. This structure is fed by a (19 ^-) isomer. Possible configurations for this and other multiquasiparticle states are discussed in the context of multi-quasiparticle calculations for coexisting deformations. Low-spin structures populated partly from the decay of the 8^- isomer have also been identified. Several of these may be associated with proposed excited 0⁺ states. Their properties, including yrare-yrast E0 decays and gamma-ray branching ratios, are analyzed using band-mixing models. These and other analyses support a shape coexistence scenario, with some qualifications.