A New Island of Superdeformation

in the Mass 80 Region

Improved y-ray energy resolution applying Doppler shift corrections using the recoil direction of the nucleus from the charged-particle momenta determined by the Microball.  The reaction was 28Si + 58Ni ­­> 80Sr + Alpha2p at 130 MeV.  The superdeformed band is in 80Sr. Download the image to the right as a Post Script file.

Interband transitions between superdeformed bands in 87Nb (left). The peaks in the dynamic moments of inertia for these two bands indicate two crossings (upper right). The interaction at hw=900 keV has been removed and now a clear crossing at hw=1100 keV is seen (lower right). The latter implies the first presence of the N = 6 i13/2 orbital in A 80 nuclei! Download the image to the far left as a Post Script file. Download the image to the near left as a Post Script file.

Moments of inertia for the four superdeformed bands in 86Zr (left). The detailed variations with rotational frequency for bands 1, 2 and 4 are only understood assuming large triaxial deformations, as shown by the potential energy surface and schematic shape diagram (right). Download this image as a Post Script file. Download the image to the upper right as a Post Script file. Download the image to the lower right as a Post Script file.

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.