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.

The National Weather Service issued a red flag warning (warm temperatures, strong winds and low humidity) for Southern California that spans from Tuesday to Wednesday in the Santa Barbara, Los Angeles and Ventura counties, and from Tuesday to Thursday in the San Bernadino, Orange, Riverside and San Diego counties.

Sporadic power outages have materialized in the San Fernando Valley, a highly populated area north of the Hollywood Hills, with the Los Angeles Department of Water and Power reporting a few thousand customers without power as of 5 p.m. PST.