Shape coexistence and particle-core coupling in ¹⁸⁹Pb and ¹⁹¹Pb nuclei

Competing structures in neutron-deficient even-even lead isotopes have been extensively studied, with low-lying excited states assigned with different shapes. However, the structural properties of the odd-mass lead isotopes remain comparatively less explored. Here, we report on the first lifetime measurement of low-lying yrast states above the 13/2⁺₁ state in 191Pb, and the first direct in-beam measurement of internal conversion-electron branches between ΔJ = 0 states in ¹⁸⁹Pb. These experiments were conducted at the University of Jyväskylä using the APPA and SAGE setups at the RITU and MARA separators, respectively. Contrary to earlier interpretations, we propose that these low-lying yrast states in ¹⁹¹Pb and ¹⁸⁹Pb nuclei arise from the coupling of an unpaired neutron in the i_13/2 orbital to the members of the oblate band based on the first excited proton (2p - 2h) intruder 0⁺ states in ¹⁹²Pb and ¹⁹⁰Pb nuclei, respectively. This interpretation is supported by the measured lifetimes in ¹⁹¹Pb, which yield an average transition quadrupole moment of |Q_t|=4.81 (16) eb comparable to predominantly oblate bands in the region. Additional support comes from the particle-core coupling scheme observed in ¹⁸⁹Pb. Our results confirm the triple shape-coexistence in ¹⁸⁹Pb, provide new insights into the evolution of nuclear shapes in neutron-deficient lead isotopes and establish a benchmark for future theoretical models.