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A New Island of Superdeformation
in the Mass 80 Region
|The discovery of superdeformation in the A 80 (and A 60) mass region was
hampered for many years by experimental difficulties arising from the large number of exit
channels, the low detection efficiencies due to the high g-ray energies involved, and the
poor energy resolution resulting from Doppler broadening due to the large recoil energies
present. The breakthrough came when GAMMASPHERE was coupled with the Microball, which
together eliminated these difficulties (upper right spectra) and led to the establishment
of a new island of superdeformation where 34 rotational cascades in 14 nuclei are now
Unusual behavior among the many superdeformed bands in the mass 80 region is not rare. In 87Nb two superdeformed interacting bands with multiple crossings were found (middle left). The unusual moments of inertia, J(2) (middle right), provide evidence for a band crossing near 900 keV and another near 1100 keV in rotational frequency. From these, the gains in alignment and the interaction strengths were calculated, which in turn explain the observed interband decays. The higher frequency (1100 keV) crossing may signal the first observation in A 80 nuclei of the N = 6 i13/2 orbital, which originates from above the N = 82 shell gap! It is the combination of large deformation and rapid rotation that brings this super intruder orbital close to the Fermi surface. Locating the position and character of the active high - j orbitals is a major goal of superdeformed studies since this is an issue of great importance for theoretical models of fast nuclear rotation.
Another case of unusual behavior is found in 86Zr where the detailed trends (bottom left) of the bands 1,2 and 4 were only explained by theory if a triaxial superdeformed shape is assumed. This uncommon shape, which has each of its three axes of different lengths, is depicted in the potential energy surface shown and also pictorially illustrated (bottom right). These, and the bands in 80Sr are the first triaxial superdeformed structures found below mass 160. Despite this successful interpretation a mystery remains with band 3, for which the J(2) is about twice as large.
Other unusual behavior was found in 81Sr where decay out of the middle of a superdeformed band and decay back in was seen, and in 83Y and 82Sr where a pair of twinned identical superdeformed bands were revealed.
It was in the nucleus 84Zr that the first direct confirmation of the large quadrupole deformation and existence of the N = 44 deformed shell gap was made. Further interpretation of superdeformation near A80 will come from other detailed and accurate measurements of the lifetimes, and thus transition quadrupole moments, of the bands as a function of spin and neutron and proton number. Such studies are in progress. Independently, detailed theoretical calculations for this mass region are on the way.