Increase in Width of the Giant Dipole Resonance in Hot Nuclei: Shape Change or Collisional Damping?

Abstract

The strength function and the angular distribution of the high energy \ensuremath{\gamma} rays emitted by the giant dipole resonance (GDR) in hot rotating ${}^{109,110}$Sn nuclei have been measured at temperature $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.8\mathrm{MeV}$ and at four values of the angular momentum $I$. The GDR width is $\ensuremath{\approx}2$ times larger than at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ and increases by $\ensuremath{\approx}20%$ as $I$ goes from 40 to $54\ensuremath{\Elzxh}$. The ${a}_{2}({E}_{\ensuremath{\gamma}})$ increases by a factor of $\ensuremath{\approx}2$. 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 $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ case.