Nuclear Superdeformation

The Regions of Superdeformation

Superdeformed nuclei are beautiful examples of quantum rotors. They have been referred to as "nuclear pulsars" and indeed close analogies have been found between the fast rotation of the nucleus and that of a neutron star! Additionally we have made the stunning discovery that superdeformed nuclei are the best example of single-nucleonic motion in a deformed potential. Thus the physics of superdeformation involves a fascinating interplay between the microscopic (shell structure) and the macroscopic (e.g. surface and Coulomb energies) properties of the nucleus. Consequently, they provide a unique laboratory for testing nuclear models. Many superdeformed rotational bands have been observed throughout the nuclear chart (see accompanying figure), and they cover the full range of possible spins, from I = 0h right up to I = 70h close to the fission limit. GAMMASPHERE has played a leading role in the discovery and investigation of these extreme and unusual nuclear "shape isomers". For example, the recent observation of superdeformation in Zn nuclei using GAMMASPHERE has opened up a new region in which to investigate the properties of highly collective superdeformed states.

The high sensitivity of GAMMASPHERE has enabled for the first time extremely precise measurements of transition energies and decay probabilities. A full understanding of the decay of superdeformed bands, in terms of the mixing of superdeformed with normal deformed states, requires accurate knowledge of the transition rates (state lifetimes) at the point of decay. A Recoil Distance Method (RDM) experiment on ¹⁹⁴Pb is shown opposite, the state lifetime is deduced from the ratio of the stopped (u) to moving (s) component of the gamma ray peak, requiring spectra with very high statistics and low background.

At higher spins and transition energies the Doppler Shift Attenuation Method (DSAM) is used. GAMMASPHERE allows a simultaneous measurement of the lifetimes of states in multiple superdeformed bands, which greatly reduces the systematic errors which have plagued this type of measurement up until now. It is now possible to extract very accurate relative quadrupole moments (deformations) of different structures. The gamma-ray spectra of superdeformed bands in ¹⁹²Hg and ¹⁹⁴Hg (middle portion of the figure) are clearly identical. The question is, are other properties, such as level spins, or deformations, also identical? GAMMASPHERE data have recently shown that the states in ¹⁹²Hg and ¹⁹⁴Hg which decay with the same transition energies do not have the same spin values. However, they do have the same lifetimes (as seen by the identical fractional Doppler shift curves, right figure), and hence we conclude that these superdeformed nuclei have the same deformation. Knowing which properties of "identical" bands are the same (apart from transition energies) imposes strong constraints on possible explanations of this unexpected and as yet unexplained phenomenon.