Amazing stuff!
"In a surprising turn of events, an international team of scientists has found that lead-208 (208Pb), the heaviest known "doubly magic" nucleus, exhibits unexpected shape characteristics that current nuclear models fail to predict accurately. ...
Doubly magic refers to nuclei that have complete shells of both protons (82) and neutrons (126), a configuration that should theoretically favor a perfectly spherical shape. ...
The researchers conclusively demonstrated that both excited states they studied exhibit large, negative spectroscopic quadrupole moments, indicating the nucleus prefers an elongated (prolate) rather than flattened shape.
When compared to predictions from three different theoretical approaches—the nuclear shell model, density functional theory, and Hartree-Fock calculations—none could reproduce the sign and magnitude of the observed deformation. ...
lead-208 plays a crucial role in understanding how heavy elements are formed in cosmic events like neutron star mergers, i.e., the r-process. Therefore, the impact extends from nuclear studies to stellar evolution. ..."
"... Lead-208 is exceptionally stable due to being a "doubly magic" nucleus – and is the heaviest that we know of. However, a new study published in Physical Review Letters used a high-precision experimental probe to examine its shape and found that rather than being perfectly spherical, the nucleus of lead-208 is slightly elongated, resembling a rugby ball (prolate spheroid). ..."
From the abstract:
"Lead-208 is the heaviest known doubly magic nucleus and its structure is therefore of special interest. Despite this magicity, which acts to provide a strong restorative force toward sphericity, it is known to exhibit both strong octupole correlations and some of the strongest quadrupole collectivity observed in doubly magic systems.
In this Letter, we employ state-of-the-art experimental equipment to conclusively demonstrate, through four Coulomb-excitation measurements, the presence of a large, negative, spectroscopic quadrupole moment for both the vibrational octupole 3−1 and quadrupole 2+1 state, indicative of a preference for prolate deformation of the states. The observed quadrupole moment is discussed in the context of the expected splitting of the 3−⊗3− two-phonon states, due to the coupling of the quadrupole and octupole motion. These results are compared with theoretical values from three different methods, which are unable to reproduce both the sign and magnitude of this deformation. Thus, in spite of its well-studied nature, 208 Pb remains a puzzle for our understanding of nuclear structure."
Breakthrough study challenges long-held beliefs about the shape of atomic nuclei (original news release) "An international research collaboration led by the University of Surrey’s Nuclear Physics Group has overturned the long-standing belief that the atomic nucleus of lead-208 (²⁰⁸Pb) is perfectly spherical. The discovery challenges fundamental assumptions about nuclear structure and has far-reaching implications for our understanding of how the heaviest elements are formed in the universe."
Deformation and Collectivity in Doubly Magic 208 Pb (open access)
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