Giant resonances in excited Sn isotopes

The giant dipole resonance built on excited states was studied in $^{110}$,112Sn nuclei at excitation energies of 62, 80, 110, and 130 MeV, by measuring \ensuremath{\gamma} rays following heavy ion fusion reactions $^{16}$O${+\mathrm{}}^{94}$Mo and $^{19}$F${+\mathrm{}}^{93}$Nb. The energy of the dipole resonance was found to be 14.8\ifmmode\pm\else\textpm\fi{}0.4 MeV independent of excitation energy. This value is approximately 1 MeV below the extrapolated ground state value. The width increases almost quadratically from 6.7 MeV at 62 MeV to 10.8 MeV at 130 MeV. This agrees qualitatively with a theoretical prediction that the increase in width for these nuclei is mainly due to an averaging of strength functions over various oblate shapes induced at high spin and temperature. The sensitivity to the isovector quadrupole vibration was found to be small even at the highest excitation energy. No convincing conclusion could be made for its existence in these Sn nuclei.