Study of the giant dipole resonance in hot nuclei

We have studied the gamma decay of the Giant Dipole Resonance (GDR) built on excited states of ¹¹²Sn obtained in the fusion reaction ¹⁹F + ⁹³Nb at 8 and 10 MeV/nucleon. The corresponding excitation energy of the compound nucleus ¹¹²Sn is 130 and 152 MeV respectively. These experiments have been performed at the Tandem post-accelerator of Saclay. The high-energy gamma-rays were measured with a 19-element BaF₂ cluster and 4 NaI(Tl). Coincidences with evaporation residues in two parallel-plates avalanche counters assured that the gamma rays were associated with fusion events. To simulate the experimental set-up response we used the Monte Carlo shower code GEANT3. In order to study the characteristics of the GDR (energy, width, strength), we analysed the gamma spectra within the statistical model using recent theoretical prescriptions. Our data are compatible with the theoretical prescription which assumes that the GDR energy depends on the mass and the temperature of the nuclei in the compound nucleus decay chain and that the GDR width depends on the mass, the charge, the temperature and the angular momentum of these nuclei. We also used a level density parameter which depends on the nuclear temperature. These results suggest that the saturation of the GDR width at an excitation energy of around 130 MeV is due only to the saturation of the angular momentum leading to fusion. (author)