Lifeboat Foundation

Since Morinaga proposed more than six decades ago that the excited \(0_2^+\) state in the ¹⁶ O nucleus was deformed¹, a large body of experimental evidence has been collected to demonstrate that atomic nuclei can possess different shapes². Apart from the lightest elements, shape coexistence has been suggested to be present in all nuclei³ and the competition of different configurations can result in several different shapes within the same nucleus⁴. Nevertheless, coexistence of three or more total energy minima near the ground state have been predicted to occur in only few regions in the chart of nuclei⁵, but direct experimental proof remained to be obtained. A notable example to date is the ¹⁸⁶ Pb₁₀₄ nucleus, where the three lowest-energy states are 0⁺ states, each assigned with a different shape – namely spherical, prolate and oblate^6,7. The ¹⁸⁶ Pb nucleus lies at the heart of the neutron-deficient Pb region, which has been a subject for numerous theoretical and experimental investigations^3,8,9,10,11. Within the mean-field picture, the total energy curve along the quadrupole deformation shows spherical, prolate and oblate minima close in energy. These minima are related to the spherical Z = 82 shell gap, and prolate and oblate deformed gaps in the proton and neutron Nilsson orbitals, respectively. From a shell model perspective, the deformed minima (noted as \(\pi (h_9/2⁴)\) for prolate and \(\pi (h_9/2²)\) for oblate in the present work) are expected to have a complex spherical multiparticle-multihole configuration both for protons and neutrons^10,11,12. Similar competition of different configurations is present in neighbouring isotopes around the N = 104 midshell¹³. In ¹⁸⁸ Pb, in addition to low-lying deformed bands associated with predominantly prolate and oblate shapes^14,15,16, three isomeric states assigned with different shapes^17,18 have been proposed.

Intruding structures built on different configurations have also been observed in nuclei in the region around ¹⁸⁶ Pb. In fact, the shape staggering of Hg isotopes observed in an isotopic shift experiment was a groundbreaking discovery in the 1970’s¹⁹ that triggered multiple investigations into shape coexistence. Laser spectroscopic measurements have examined the onset of ground-state deformation also in the even-mass Po and Pt isotopes^20,21. Since the neutron-deficient Pb isotopes are spherical in their ground states^22,23,24, the onset of deformation in the Pb isotopes can be assessed by investigating the \(2_1^+\) states. It is proposed that the heaviest Pb nucleus exhibiting collectivity associated with deformation is ¹⁹⁴ Pb²⁵, whereas in heavier Pb isotopes the underlying configurations of the lowest excited states arise from single-nucleon excitations in the seniority scheme leading to a spherical interpretation²⁶.