Search for the Jacobi shape transition in rapidly rotating nuclei

Abstract

We have studied quasicontinuous gamma radiation from reactions of ${}^{48}\mathrm{Ca}$ beams on targets of ${}^{50}\mathrm{Ti},$ ${}^{64}\mathrm{Ni},$ ${}^{96}\mathrm{Zr},$ and ${}^{124}\mathrm{Sn}.$ The bombarding energies were in the range 195--215 MeV, and were chosen so as to bring in a high angular momentum without severe fragmentation of the cross sections for fusion residues. Experiments were performed both with the 8PI spectrometer at the LBNL 88-Inch Cyclotron, and with Gammasphere at the ANL ATLAS accelerator. The results have been analyzed in a variety of ways, with the consistent result that in all but the heaviest target, the nuclear moments of inertia tend to increase with increasing angular momentum up to the highest values observed. This could come from a transition towards a highly deformed Jacobi-like shape, or possibly be due to the appearance of high-$j$ intruder orbitals at the Fermi surface. We will argue that these effects can be equivalent.