Limiting temperatures for collective motion: the giant dipole resonance in very hot nuclei

The study of the Giant Dipole Resonance (GDR) excited in hot nuclei allows to follow the evolution of collective motion with increasing nuclear temperature. A brief review of the characteristics of the GDR excited in nuclei with excitation energies up to ∼ 500 MeV is given. The results of recent experiments in which very hot nuclei have been studied with a nearly 4π detector are presented. Gamma-rays, light charged particles and evaporation residues emitted from hot nuclei of mass around 115 and excitation energies above 300 MeV, formed in the 36Ar + 90Zr at 27 MeV/u and 36Ar + 98Mo at 37 MeV/u reactions, have been measured with the MEDEA multidetector. The γ-ray yield from the decay of the Giant Dipole Resonance in these nuclei has been found to be independent of excitation energy and bombarding energy. The measured γ-ray spectra are compared with statistical calculations encompassing several recent theoretical models for the quenching of gamma-ray emission from the dipole resonance at very high temperatures. The best agreement with the data is obtained by assuming a cut-off of the resonance γ-emission above an excitation energy of 250 MeV.