High-spin negative parity states in doubly even Hg, Pt and Os nuclei

A systematic study of the level structure of ²⁰⁰Hg, ¹⁹⁸Hg, ¹⁹⁶Hg, ¹⁹⁴Pt, ¹⁹²Pt, ¹⁹⁰Pt, ¹⁸⁸Os, ¹⁸⁶Os and ¹⁸⁴Os has been performed by means of (α, 2nγ) and (p, 2nγ) in-beam γ-ray spectroscopy. Much new information has been obtained about the individual level spectra, and arguments based on level energy and population intensity systematics have been used to trace the behaviour of interesting spectral features through this series of transitional nuclei. Of particular interest is a 5^-, 7^-, 9^- ... sequence of levels which appears to be a recurring feature in the spectra of the doubly even Pt and Hg nuclei in the mass range A = 190-200. These negative parity levels are connected by enhanced E2 transitions and they are very strongly populated in the de-excitation of the residual nuclei formed in (α, 2n) reactions. It is proposed that the 5^-, 7^-, 9^- ... sequences constitute rotation-aligned bands analogous to the decoupled bands identified by Stephens and co-workers in odd-A transitional nuclei. In the doubly even nuclei, the bands may arise from the coupling of an i 13 2 neutron with a low-j neutron partner (from the p _{1 2}, p _{3 2} and f _{5 2} orbitals). Negative parity levels in the Os nuclei are populated much less strongly in the (α, 2nγ) reactions. While 5^- levels corresponding to those in the Pt and Hg nuclei appear to be fairly well established in ¹⁸⁶Os, ¹⁸⁸Os (and ¹⁹⁰Os), it is not yet clear whether the 5^-, 7^-, 9^- ... band sequence extends into the Os nuclei.