Giant Dipole Resonances in Excited Nuclei

Abstract

Since 1981 [1], many experiments have demonstrated the universal nature of the giant dipole resonance (GDR) in the high energy γ-decay of highly excited, equilibrated nuclei formed in heavy ion reactions. Typically the GDR is excited along with other degrees of freedom, so that its decay leaves the nucleus in an excited state. The signature of GDR decay is high energy (E_γ ~ 10–30 MeV) γ-emission. As in photoabsorption in the giant resonance region, other multipoles contribute only weakly, while non3tatistical emission processes such as pre-equilibrium giant resonance excitation and/or nuclear bremstrahlung are appreciable only at higher bombarding energies. The effect of GDR decay shows up as a “bump” for E_γ ~ E_D (~80A^-1/3 MeV in medium-heavy nuclei) on an otherwise exponentially decaying spectrum, and the shape of the bump reflects the average shape of the GDR built on excited states, which may be extracted by a statistical model analysis of the measured spectrum shape. Statistical evaporation or high energy γ rays with E_γ ~ E_D > E_n ≅ B_n + 2T (B_n=neutron binding energy, T = temperature) is favored to occur at the earliest stage of the evaporation process; as a result, the spectrum shape is characteristic of the properties of the highly excited nucleus.