Superdeformation
The large deformation of the fission isomers stems from the electrostatic
repulsion arising from the large numbers of protons in these nuclei. However,
superdeformation can occur in other nuclei, where it arises from shell-correction
terms, stabilised by high rotational frequencies. It turns out that these
extra regions of stability occur for nuclei with approximately integer
axis ratios (i.e. 3:2, 2:1 etc.). This scale of deformation can be brought
about by roation, when the centrifugal force arising from the high-spin
collective rotation stretches the nucleus, producing a prolate (rugby-ball)
shape. At these deformations, new magic numbers arise similar to those
found for spherical nuclei. Superdeformation has now been found in several
regions of the nuclear chart, in nuclei around A=60, A=80, A=130, A=150
and A=190.
The first rotational bands associated with superdeformed nuclei were observed
in the mid-eighties in data taken with the TESSA array at Daresubry
Laboratory, UK by P.J. Nolan and P.J. Twin, both of Liverpool
University. Because of the high degree of deformation, one would expect
the SD nucleus to be highly collective - that is, almost all of the nucleons
should participate in the collective rotational motion. This leads to a
moment of inertia which approaches that of a rigid rotor. Thus a rotational
band in a SD nucleus is characterised by a sequence of highly collective
E2 transitions, with a regular and small energy-spacing (see below). This
is how the SD shape was first recognised: subsequent lifetime measurements
for states in the bands showed that they were indeed associated with large
positive quadrupole deformation parameters. A "typical" example of the
gamma-ray spectrum associated with a superdeformed band is shown below.
Over the past decade, a great deal of effort (both experimental and theoretical) has gone into the study of superdeformed structures. The extremes of angular momentum and deformation represented by SD nuclei have provided a new testing-ground for the standard nuclear models, which had been well examined for lower spins and ``normal'' (lower) nuclear deformations. We now have an extensive knowledge of the general properties of SD nuclei and the discovery of excited SD bands has made it possible to study particle-like excitations across the new semi-magic shell-gaps. However, there are still some unsolved problems associated with superdeformation in general.
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