Large effects of orientation fluctuations in the angular distribution of the GDR photons

Measurements of the angular distribution of high energy γ rays emitted in the decay of compound nuclei 165-167Er, 175,176Hf, 110Sn at E*=50-90 MeV and I≤40 are presented. In all cases the measured anisotropies as a function of the gamma transition energy are small. The compound nuclei were formed with heavy ion fusion reactions and the high energy γ rays were detected by the HECTOR array. A comparison between the data and model predictions shows that both shape and orientation fluctuations are very important and must be taken into account to reproduce the experimental results. The dominant role played by orientation fluctuations in the GDR lineshape and the effective induced nuclear deformations are discussed. (orig.)