In-beam and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>α</mml:mi></mml:math>-decay spectroscopy of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mn>191</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">Po</mml:mi></mml:math>and evidence for triple shape coexistence at low energy in the daughter nucleus<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mn>187</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">Pb</mml:mi></mml:math>

Prompt $\ensuremath{\gamma}$ 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 $\ensuremath{\alpha}$ decay data have been measured for ${}^{191}\mathrm{Po}$ and its daughter product ${}^{187}\mathrm{Pb}.$ The complementary $\ensuremath{\gamma}$- and $\ensuremath{\alpha}$-decay data point to the onset of oblate deformation in the light odd-mass Po nuclei by approaching the neutron midshell at $N=104.$ 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 ${3/2}^{\ensuremath{-}}$ 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.

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