Increase in width of the giant dipole resonance in hot nuclei: Shape change or collisional damping?

The strength function and the angular distribution of the high energy γ rays emitted by the giant dipole resonance (GDR) in hot rotating 109,110Sn nuclei have been measured at temperature T=1.8 MeV and at four values of the angular momentum I. The GDR width is 2 times larger than at T=0 and increases by 20% as I goes from 40 to 54Latin small letter h with stroke. The a2(Eγ) increases by a factor of 2. Based on these two facts and on the comparison with theory we conclude that large deformations driven by I and combined with shape and orientation fluctuations are responsible for the measured increases. Collisional damping is constant and practically equal to the T=0 case.