Adam Hecht

In triaxial deformed nuclei, the short, intermediate, and long deformation axes may be used to define a coordinate system. In 1997, Frauendorf and Meng [Fr97] proposed that the handedness of the nuclear system, the chirality, can be defined by the orientation of the total angular momentum vector with respect to these axes. With a counterclockwise rotation from the tip of the total angular momentum vector, an axis order of short-intermediate-long defines a right-handed coordinate system. An axis order of long-intermediate-short defines a left-handed system. In their model, for a nucleus to be chiral, not only must the nucleus be triaxial, but the total angular momentum must be aplanar. It cannot be along one of the deformation axes, nor can it be along a plane defined between the deformation axes. In odd-odd nuclei, the total angular momentum is the vector sum of a single proton angular momentum, a single neutron angular momentum, and a collective rotation. With the appropriate particle numbers, all three angular momentum vectors may be aligned orthogonally, leading to an aplanar total angular momentum vector and thus a chiral system. In reference [Fr97], Frauendorf and Meng suggest that a symmetry between the right- and left-handed chiral systems should be manifested by an energy degenerate pair of DeltaI = 1 rotational bands with the same microscopic configuration. With mixing of the intrinsic chiral solutions the energy degeneracy may be broken in the laboratory frame. In this case, a pair of DeltaI = 1 bands with a slight degeneracy breaking may be observed. They also suggested that a pair of rotational bands observed in 134Pr have the experimental signature expected of chiral band pairs. In that mass region, calculations predict a triaxial deformation due to the shape polarizing effects of the unpaired valence nucleons in this nucleus. In addition, the unpaired proton and neutron angular momentum vectors are predicted to have orthogonal angular momenta when the nucleus is near the ground state. If the arguments for chiral behavior in 134Pr are valid, then neighboring odd-odd nuclei, having similar particle numbers and deformations, should also show chiral behavior. That is, if chiral behavior exists, it should be a regional effect. To test this, experiments were performed to populate high spin states in the neighboring nuclei 136Pm and 138Eu. To test the limits of the region, experiments were also performed to populate high spin states in 140Eu. (Abstract shortened by UMI.)