THE GIANT-DIPOLE RESONANCE IN HOT SN NUCLEI

We have studied the Giant Dipole Resonance (GDR) built on the excited states of Sn-112 formed in the fusion of F-19 with Nb-93. The excitati on energy of the compound nucleus Sn-112 was 130 and 152 MeV. High-ene rgy gamma-rays were detected in a 7-element BaF2 cluster and 4 NaI(Tl) scintillators. Coincidences with the evaporation residues detected in two parallel-plate avalanche-counters ensured that the gamma-rays wer e associated with fusion events. In order to study the characteristics of the GDR, we have performed analyses of the gamma-ray spectra withi n the statistical model, using a continuous evolution of the GDR centr oid energy with the mass and the temperature of the emitting nucleus, and a nuclear temperature dependent level density parameter. The theor etical prescriptions concerning the evolution of the GDR width with ex citation energy have been tested. We show that, in order to reproduce the gamma-spectra, the increase of the width must be more rapid than t he two theoretical predictions. The width has to reach large values, o f the order of 15 to 20 MeV above 100 MeV of excitation energy. These values are greater than the saturation value of about 12 MeV which has been published following the analyses using constant parameters. On t his condition, the shape of the spectra can be reproduced with or with out a saturation of the width. It is therefore not possible to say if the increase of the width is due to angular momentum effects or to col lisional effects.