The hot GDR revisited

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 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.

Transport equations for driven many-body systems

“…( 25). However, expression (27) holds only for t → ∞. (ii) For values of t relevant for the transport equation ( 24), condition (13) states that the level density ρ β can be considered to be independent of energy and can, thus, be taken out from under the integration.…”

“…At large excitation energies the GDR is primarily observed experimentally via gamma decay of compound nuclei formed in heavy-ion collisions. It is found that the GDR is substantially broadened or disappears altogether [25][26][27]. That is ascribed either to the large angular-momentum values involved in a heavy-ion induced reaction, to neutron evaporation, or to a significant intrinsic dynamical broadening of the GDR.…”

Expanding Nuclear Physics Horizons with the Gamma Factory

“…This isovector GDR was initially found as a spectral feature corresponding to the excitation of the nuclear ground state, but soon Brink and Axel concluded that giant resonances could be associated with any nuclear state, independent of the microscopic structure of this state [116,117]. This led to intense studies of GDR properties of excited states, the so-called GDR in hot nuclei [118]. Further prospects of GF studies of the GDR and potential multiphoton excitation are presented in Sec.…”

“…According to the Brink-Axel hypothesis, the GDR is a mode of excitation not only of the nuclear ground state, but of any excited state as well [116,117]. GDRs built on excited states are termed "hot GDRs" and have so far been produced in heavy-ion collisions leading to fusion or incomplete fusion [118]. As a result of the collision, a compound nucleus is formed, i.e., a highly excited nucleus in which the excitation energy is distributed over several or many particle-hole excitations [173].…”

“…The identification of /Γ ↓ with the equilibration time becomes questionable at high excitation energy well above particle threshold. For example, hot GDRs [118] are characterized by high neutron-evaporation rates and have correspondingly short lifetimes [175]. Neutron evaporation may happen prior to complete equilibration.…”