A triplet of differently shaped spin-zero states in the atomic nucleus 186Pb

Abstract

Understanding the fundamental excitations of many-fermion systems is of significant current interest. In atomic nuclei with even numbers of neutrons and protons, the low-lying excitation spectrum is generally formed by nucleon pair breaking and nuclear vibrations or rotations. However, for certain numbers of protons and neutrons, a subtle rearrangement of only a few nucleons among the orbitals at the Fermi surface can result in a different elementary mode: a macroscopic shape change^1,2,3. The first experimental evidence for this phenomenon came from the observation of shape coexistence in ¹⁶O (ref. 4). Other unexpected examples came with the discovery of fission isomers⁵ and superdeformed nuclei⁶. Here we find experimentally that the lowest three states in the energy spectrum of the neutron deficient nucleus ¹⁸⁶Pb are spherical, oblate and prolate. The states are populated by the α-decay of a parent nucleus; to identify them, we combine knowledge of the particular features of this decay⁷ with sensitive measurement techniques (a highly efficient velocity filter⁸ with strong background reduction, and an extremely selective recoil-α-electron coincidence tagging method^8,9,10). The existence of this apparently unique shape triplet is permitted only by the specific conditions that are met around this particular nucleus.