Angular momentum dependence of giant dipole vibrations in hot dysprosium nuclei

The spin dependence of the giant dipole resonance built on excited states in ¹⁵⁶Dy has been studied with a new technique using Ge-BGO spectrometers. The observed structure of the GDR and the measured γ-ray anisotropies are consistent with a change in nuclear shape from prolate to oblate with increasing spin. No evidence is found for centroid shifts nor for superdeformed shapes at high temperature.