High-spin negative-parity states in ²⁰⁰Hg from ^200mAu decay and ¹⁹⁸Pt(α, 2nγ) reaction spectroscopy

Levels in ²⁰⁰Hg populated in the β^- decay of the 19 h isomer of ²⁰⁰Au and in the reaction ¹⁹⁸Pt(α, 2nγ)²⁰⁰Hg have been investigated. The β-decay populates an 11^- level in ²⁰⁰Hg at 2642 keV, which de-excites through levels at the following energies (spin-parities): 2144(9^-), 1963(7^-), 1852(5^-), 1707(6 ⁺), 947(4⁺) and 368(2⁺) keV. The relative γ-ray intensities measured in the in-beam experiment were invaluable in the construction of an unambiguous ²⁰⁰Hg level spectrum. The 9^- → 7^- and 7 ^- → 5^- E2 transitions are considerably enhanced suggesting that the negative-parity levels constitute a collective "band". It is proposed that the β^- decay involves a one-particle transition between the (h _{11 2}π, i _{13 2}ν)12^- isomer of ²⁰⁰Au and a (h _{9 2}ν, i _{13 2}ν) two-neutron admixture in the ²⁰⁰Hg 11^- state. Indications of the systematic occurrence of similar sequences of negative-parity levels in other A = 190-200 transitional nuclei are discussed.