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

Across the physics disciplines, the Pb-186 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 Pb-186 were determined in a simultaneous in-beam gamma-ray and electron spectroscopy experiment employing the recoil-decay tagging technique. The feeding of the 0(2)(+) state and the interband 2(2)(+) -> 2(1)(+) 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; 2(1)(+) -> 0(2)(+)) value of 190(80) W.u., the transitional quadrupole moment vertical bar Q(t)(2(1)(+) -> 0(2)(+))vertical bar = 7.7(33) eb and intensity balance arguments provide evidence to reassign the 0(2)(+) and 0(3)(+) 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.

Automated summary

The Pb-186 nucleus has been found to have three excited states with different shapes, and precision measurements have been conducted. These measurements are of great importance for the development of next-generation energy density functional models.