SPIN-INDUCED SHAPE CHANGES IN LIGHT-MEDIUM MASS COMPOUND NUCLEI

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 tran sition, we measured gamma ray production cross sections and angular di stributions for the decay of Cu-59,Cu-63 compound nuclei populated in the fusion of S-32+Al-27 and O-18+Sc-45 over a wide range of spin (J = 0 - 47h) and excitation energy (E = 55-130 MeV). Very broad giant di pole resonance (GDR) strength functions are deduced at high bombarding energy (spin), implying the existence of large deformation in the ens emble of decaying states. Thermal shape and orientation fluctuation ca lculations based on the rotating liquid drop model provide a good desc ription of the GDR strength functions for all cases. The calculations fail to reproduce the GDR strength functions at high bombarding energy (spin) when the oblate-to-triaxial shape transition and the associate d softness in the potential energy surface are removed, indicating the observed broadening of the GDR strength function is due mostly to spi n-driven deformation. Threshold bremsstrahlung production is inferred from measured angular distribution anisotropies in O-18+Sc-45 collisio ns at 125 and 149 MeV.