Direct measurement of three different deformations near the ground state in an atomic nucleus.

Abstract

Atomic nuclei serve as prime laboratories for investigations of complex quantum phenomena, where minor nucleon rearrangements cause significant structural changes. ¹⁹⁰Pb is the heaviest known neutron-deficient Pb isotope that can exhibit three distinct shapes: prolate, oblate, and spherical, with nearly degenerate excitation energies. Here we report on the combined results from three state-of-the-art measurements to directly observe these deformations in ¹⁹⁰Pb. Contrary to earlier interpretations, we associate the collective yrast band as predominantly oblate, while the non-yrast band with higher collectivity follows characteristics of more deformed, predominantly prolate bands. Direct measurement of the $E0\left(0_2^{+}\to 0_1^{+}\right)$ transition and γ-e ^- coincidence relations allowed us to locate and firmly assign the $0_2^{+}$ state in the level scheme and to discover a spherical $2_3^{+}$ state at 1281(1) keV with $B\left(E2;2_3^{+}\to 0_1^{+}\right)=1.2\left(3\right)$ W.u. These assignments are based purely on observed transition probabilities and monopole strength values, and do not rely on model calculations for their interpretation.