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

Abstract

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 $0_2^{+}$ state and the interband $2_2^{+}\to 2_1^{+}$ 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 $B\left(E2;2_1^{+}\to 0_2^{+}\right)$ value of 190(80) W.u., the transitional quadrupole moment $\mid Q_t\left(2_1^{+}\to 0_2^{+}\right)\mid =7.7$(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.

The authors study an interesting phenomena of shape coexistence in ¹⁸⁶Pb. In an elegant and well-documented experiment, they confirm the coexistence of the three 0⁺ states in the ¹⁸⁶Pb nucleus and reassign the shapes associated with the excited 0⁺ states.