Reassigning the shapes of the 0⁺ states in the ¹⁸⁶Pb nucleus

Across the physics disciplines, the ¹⁸⁶Pb nucleus is the only known system, where the two first excited states, together with the ground state, form a triplet of zero-spin states assigned with prolate, oblate and spherical shapes. Here we report on a precision measurement where the properties of collective transitions in ¹⁸⁶Pb were determined in a simultaneous in-beam γ-ray and electron spectroscopy experiment employing the recoil-decay tagging technique. The feeding of the 02+ state and the interband 22+→21+ transition have been observed. We also present direct measurement of the energies of the electric monopole transitions from the excited 0⁺ states to the 0⁺ ground state. In contrast to the earlier understanding, the obtained reduced transition probability B(E2;21+→02+) value of 190(80) W.u., the transitional quadrupole moment ∣Qt(21+→02+)∣=7.7(33) eb and intensity balance arguments provide evidence to reassign the 02+ and 03+ states with predominantly prolate and oblate shape, respectively. Our work demonstrates a step-up in experimental sensitivity and paves the way for systematic studies of electric monopole transitions in this region. These electric monopole transitions probe the nuclear volume in a unique manner and provide unexploited input for development of the next-generation energy density functional models.