Theoretical investigations led by scientists at RIKEN's Nishina Center for Accelerator-Based Science propose a fundamental revision to the understanding of heavy atomic nuclei, suggesting a significant population exhibits triaxial, almond-like shapes. This finding directly challenges the long-standing nuclear structure model established in the 1950s by Aage Bohr and Ben Mottelson, which posited that deformed heavy nuclei are elongated along a single axis, resembling rugby balls.
Captions: Illustrations of atoms often depict the nucleus as a round blob made up of neutrons and protons
Takaharu Otsuka, a visiting scientist at RIKEN and Professor Emeritus at the University of Tokyo, initiated this conceptual shift, proposing that an almond-like configuration with oval cross-sections would be a more intrinsically natural state for these nuclei. This hypothesis initially encountered considerable skepticism within the physics community. The recent theoretical study, utilizing the computational resources of the Fugaku supercomputer, has now provided validation for Otsuka's proposition.
The complex calculations demonstrated that virtually all heavy ellipsoidal deformed nuclei possess these triaxial geometries, marking a substantial revision to the established description of nuclear architecture that has been in place for nearly seven decades. The research team, which included Yusuke Tsunoda from the University of Tokyo and Noritaka Shimizu from the University of Tsukuba, theoretically showed that two novel mechanisms contribute to stabilizing the triaxial asymmetry over axially symmetric deformation by increasing binding energy. These mechanisms involve the quantum theoretical restoration of rotational symmetry and tensor forces within nuclear interactions.
This finding carries significant ramifications for nuclear physics, particularly concerning nuclear dynamics and the search for new superheavy elements. The triaxial structure implies that these nuclei can rotate around two distinct axes, departing from the previously modeled restriction to rotation around a single axis. The established Bohr-Mottelson model, for which Aage Bohr shared the 1975 Nobel Prize in Physics, was foundational for understanding collective motion, but the new results indicate its limitations in describing certain complex deformations.
Furthermore, the theoretical results align with predictions from the proxy-SU(3) symmetry framework, an algebraic approach developed to describe heavy nuclei. The research, formally documented in the journal Physical Review C, explicitly links the nuclear tensor force—a direct result of pi meson exchange between nucleons—to the determination of nuclear shapes. This robust new picture is anticipated to influence future theoretical and experimental explorations into the characteristics of exotic nuclei and the fundamental forces governing atomic structure.