The hot GDR revisited
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Review
The hot GDR revisited Domenico Santonocito1,a , Yorick Blumenfeld2 1 2
INFN - Laboratori Nazionali del Sud, Via S. Sofia 62, 95123 Catania, Italy IJCLab, CNRS/IN2P3, Université Paris-Saclay, 91405 Orsay, France
Received: 17 July 2020 / Accepted: 24 September 2020 © The Author(s) 2020 Communicated by Nicolas Alamanos
Abstract 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 preequilibrium 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.
1 Introduction Giant Resonances (GR) are a general property of nuclei which consist in a collective excitation of nucleons. In a hydrodynamic model they are viewed as a high frequency vibration around the equilibrium density or shape of the nucleus. Microscopically, they can be described as a coherent superposition of particle-hole excitations. Numerous types of GR excitations exist which can be categorized according to their multipolarity L, their isospin T and their spin s. A GR a e-mail:
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is described by three observables, its centroid energy E, its width Γ and its strength S expressed as a percentage of the corresponding sum rule. A detailed presentation and discussion of GRs can be found in [1,2]. The first GR was discovered in 1947 through photo-fission experiments [3] and was assigned to the Isovector Giant Dipole resonance (IVGDR) (ΔL = 1, ΔT = 1, ΔS = 0) which corresponds to an out-of-phase oscillation of the protons against the neutrons. Subsequently, the other types of GRs were discovered, often through light hadron (proton, deuteron, alpha-particle...) scattering. A comprehensive picture of the GR landscape is now available which has led to unique knowledge of the bulk properties of the nucleus such as incompressibility or symmetry en
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