Prompt $\gamma$ rays have been observed for the first time from the neutron-deficient nucleus ${}^{191}\mathrm{Po}$ using the recoil-decay tagging technique at the RITU gas-filled separator. In addition improved $\alpha$ decay data have been measured for ${}^{191}\mathrm{Po}$ and its daughter product ${}^{187}\mathrm{Pb}.$ The complementary $\gamma$- and $\alpha$-decay data point to the onset of oblate deformation in the light odd-mass Po nuclei by approaching the neutron midshell at $N=104.\mathrm{}$ The pattern of the favored and unfavored states observed on top of the ${13/2}^{+}$ isomer in ${}^{191}\mathrm{Po}$ indicates a change from the weak-coupling towards the strong-coupling scheme in ${}^{191m}\mathrm{Po}.$ In the daughter nucleus ${}^{187}\mathrm{Pb},$ the ${13/2}^{+}$ and ${3/2}^{-}$ isomeric states become degenerate within the experimental accuracy. Evidence for triple shape coexistence at low energy has been found in the high-spin isomer in ${}^{187}\mathrm{Pb}.$ The results are supported by potential-energy surface calculations and by particle-plus-rotor calculations.

• Received 12 March 2001

DOI:https://doi.org/10.1103/PhysRevC.66.014313