Collective transitions in nuclei with 'rotations' around the symmetry axis

Abstract

Nuclei near doubly closed shells form a large total spin by alignment of the single-particle angular momenta. One often says they 'rotate' around a symmetry axis. Such nuclei should not rotate collectively and should show isomeric (yrast traps) and retarded transitions. But recent measurements in these nuclei show that most transitions are collectively enhanced and not retarded. It is shown that this enhancement of several single-particle units (2-10) can be understood qualitatively by gamma vibrations around the axially symmetric equilibrium deformation. The authors also study the effect of particle number projection before variation on the pairing properties at high spins.