superdeformed

In nuclear physics, superdeformed refers to highly elongated intrinsic shapes of atomic nuclei. In these states the nucleus has an ellipsoidal deformation with a major-to-minor axis ratio close to 2:1, corresponding to a large quadrupole deformation (beta2 around 0.6). This shape creates a local minimum in the potential energy surface at high spin, allowing nuclei to adopt a markedly elongated configuration.

Superdeformed structures are observed as rotational bands, sequences of gamma rays with very regular energy spacings. The bands arise from the rapid rotation of an elongated nucleus, producing characteristic intraband transitions with large moments of inertia and enhanced transition probabilities for quadrupole radiation.

Discovery and regions where observed: Superdeformed rotational bands were first observed in the 1980s using gamma-ray spectroscopy in medium-heavy nuclei, notably in the A ~ 150 region such as 152Dy. Since then, superdeformed bands have been identified in other mass regions, including A ~ 190, and in several isotopes across the nuclear chart.

Theoretical framework and implications: The phenomenon is understood as the emergence of a second, highly deformed minimum in the nuclear potential energy surface at large deformation and high angular momentum. The cranked shell model and other mean-field approaches explain the occupation and alignment of high-j orbitals that favor such shapes. Superdeformed states provide stringent tests of nuclear structure models, including shell effects, pairing, and the behavior of nuclei under extreme rotation.

Decay and transition to normal states: At lower spins, superdeformed bands typically decay toward normally deformed configurations via gamma emission, a process called decay out. The study of these transitions illuminates the coupling between different deformation regimes and the evolution of nuclear shapes with spin.