Identification of excited states in $^{188}\mathrm{Bi}$ and $^{188}\mathrm{Po}$

International audience; The neutron-deficient Bi188 and Po188 isotopes have been studied by γ-ray spectroscopy using the recoil-decay tagging technique with the Argonne Gas-Filled Analyzer. A new 0.25(5)-μs isomeric state and a prompt cascade formed by 319-, 366-, and 462-keV γ rays have been established on top of the (10−) α-decaying isomer in Bi188. The first excited (2+) state in Po188 was identified, its excitation energy of 242(2) keV continues the nearly constant trend for the first 2+ states in Po190,192,194. The state is most likely a member of a prolate rotational band built on the ground state, albeit mixing with other coexisting configurations cannot be excluded. The new results obtained in the present work provide new information to shape coexistence in bismuth and polonium isotopes near the neutron midshell at N=104. In this mass region, a reduction in the prompt γ-ray yield obtained with recoil decay tagging was observed for a few nuclides, and the possible reasons are presented.