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 ${}^{109,110}$Sn nuclei have been measured at temperature $T=1.8\mathrm{}\mathrm{MeV}$ and at four values of the angular momentum $I$. The GDR width is $\approx 2$ times larger than at $T=0\mathrm{}$ and increases by $\approx 20\%$ as $I$ goes from 40 to $54ħ$. The $a_2\left(E_{\gamma }\right)$ increases by a factor of $\approx 2$. 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 $T=0\mathrm{}$ case.