At very high spins an oblate-to-triaxial transition in the equilibrium shape of hot medium mass nuclei is expected, with superdeformed major to minor axis ratios of 2:1 and larger. To search for this shape transition, we measured γ ray production cross sections and angular distributions for the decay of ${}_{}{}^{59,63}{}_{}{}^{}\mathrm{Cu}$ compound nuclei populated in the fusion of ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$${+}^{27}$Al and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$${+}^{45}$Sc over a wide range of spin (J=0-47ħ) and excitation energy (${\mathit{E}}^{\mathrm{*}}$=55–130 MeV). Very broad giant dipole resonance (GDR) strength functions are deduced at high bombarding energy (spin), implying the existence of large deformation in the ensemble of decaying states. Thermal shape and orientation fluctuation calculations based on the rotating liquid drop model provide a good description of the GDR strength functions for all cases. The calculations fail to reproduce the GDR strength functions at high bombarding energy (spin) cases when the oblate-to-triaxial shape transition and the associated softness in the PES are removed, indicating the observed broadening of the GDR strength function is due mostly to spin-driven deformation. Threshold bremsstrahlung production is inferred from measured angular distribution anisotropies in ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$${+}^{45}$Sc collisions at 125 and 149 MeV.

• Received 16 March 1995

DOI:https://doi.org/10.1103/PhysRevC.52.578