The hot GDR revisited

The properties of the Isovector Giant Dipole Resonance are reviewed as a function of the temperature of the state on which it is built. The experimental methods, based on scintillation detectors efficient for the detection of high energy gamma-rays, are described. Methods for determining the excitation energy and temperature from the measurement of light charged particle energy spectra taking pre-equilibrium emission into account are presented. The resonance properties, energy, width and strength, are followed as a function of increasing temperature. The data are analyzed in the framework of the statistical model, which is briefly presented, by using the codes CASCADE and DCASCADE. Various prescriptions for the characteristics of the resonance as well as theoretical models are incorporated into these statistical codes in view of a direct comparison with the data. The successes and deficiencies of the Thermal Shape Fluctuation model at low temperatures are discussed. A salient feature is the surprisingly abrupt disappearance of dipole strength above a limiting temperature which depends on the nuclear mass. Several models taking into account the competition between the time scales of collective degrees of freedom and nuclear lifetime only roughly reproduce the trend of the data. This disappearance of strength is tentatively linked to the nuclear liquid–gas phase transition.