The ${}_{}{}^{206}{}_{}{}^{}\to _{}^{202}{}_{}{}^{}$At${\to }^{198}$Bi, ${}_{}{}^{204}{}_{}{}^{}\to _{}^{200}{}_{}{}^{}$At${\to }^{196}$Bi, and ${}_{}{}^{202}{}_{}{}^{}\to _{}^{198}{}_{}{}^{}$At${\to }^{194}$Bi a-decay chains have been studied by standard spectroscopic techniques using an on-line isotope separator. All the studied doubly odd isotopes have at least two isomers, which decay by a combination of the following decay modes: a emission, ${\mathrm{\beta }}^{+}$/EC (electron capture) decay, and internal transition (IT). The internal transition, a highly retarded E3, is the j-forbidden transition between the [π${\mathit{h}}_{9/2}$⊗ν${\mathit{i}}_{13/2}$${]}_{10}^{\mathrm{-}}$ and the [π${\mathit{h}}_{9/2}$⊗ν${\mathit{f}}_{5/2}$${]}_7^{+}$ states. The B(E3) values of these IT’s together with their energy behavior as a function of the neutron and proton number, compared to the energy difference between the 13/$2^{+}$(ν${\mathit{i}}_{13/2}$) and 5/$2^{\mathrm{-}}$(ν${\mathit{f}}_{5/2}$) states in the odd-mass Pb isotones, indicate that these proton-neutron-coupled states have a rather pure shell-model character.

• Received 28 May 1992

DOI:https://doi.org/10.1103/PhysRevC.46.1209