Thermal and angular momentum effects in the giant-dipole-resonance decay of hot rare-earth nuclei

Recoil-γ coincidences have been measured in the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{159}$Tb at beam energies of 300 and 400 MeV. In the γ-ray spectra corresponding to excitation energies above 140 MeV a split giant dipole resonance is observed with widths substantially smaller than found at lower excitation energy. The possible influence of excitation energy and angular momentum on this effect is discussed. The intensity ratio of the two dipole resonance components is found to change with excitation energy, indicating a transition of the nuclear shape from prolate towards oblate.