"Nuclei interact with the external environment through a number of different fields—electromagnetic, weak and hadronic. The collective excitations induced by these interactions are known as giant resonances. The best known example is the giant dipole resonance, which is stimulated when the electric field of an incident gamma ray exerts a force on the positively charged protons in a nucleus, moving them relative to the uncharged neutrons ... Other giant resonances that have been studied are the monopole, quadrupole and spin-isospin modes of oscillation. The spin-isospin mode involves charge-changing processes, in particular beta decay. The quadrupole and monopole giant resonances are most easily seen with fields that act equally on neutrons and protons, because in these modes the neutrons and protons oscillate in the same mode. The giant resonances are collective oscillations and the various modes of oscillation depend on specific aspects on the nuclear force to sustain them. In the monopole mode, the motion is radial and the frequency depends on the compressibility of the nucleus. In the dipole and spin-isospin resonances, the protons and neutrons are excited out of phase, and the proton-neutron interaction provide the restoring force."
In nuclear physics, giant resonance is a high-frequency collective excitation of atomic nuclei, as a property of many-body quantum systems. In the macroscopic interpretation of such an excitation in terms of an oscillation, the most prominent giant resonance is a collective oscillation of all protons against all neutrons in a nucleus.
"Perhaps the best-known giant resonance in nuclei is the giant dipole resonance (GDR). The GDR is described in classical hydrodynamics as a class of nuclear motion in which the neutrons and protons within a nucleus move collectively against one another, providing a separation between the centers of mass and charge, thus creating a dipole moment."
"Maurice Goldhaber has emphasized that the situation with respect to possible nuclear resonances in (γ,n) or (γ,fission) reactions was quite unclear at the time of George C. Baldwin and G. Stanley Klaiber’s papers on these reactions. ... This was because the rapid rise of their yield to a prominent peak with increasing energy, followed by a slower fall off was then thought to have been due to the competition between the rapidly rising density of nuclear states and the eventual domination of other reaction channels at higher energies. Goldhaber realized, however, that there could be an analogy between a possible collective nuclear resonance and the restrahl resonance (essentially the transverse optical phonon mode) in polar crystals. Goldhaber sought out Teller because of his paper with Russell Lyddane and Robert Sachs, ... relating the restrahl frequency to the asymptotic behavior of the crystal’s dielectric function. Goldhaber and Teller, in their paper together, went on to predict universal, giant photo-nuclear resonances. ..."
"The spectrum of gamma-radiation emitted by a highly excited nucleus can be calculated in two ways. In the first method the transition probability for gamma emission is related to the photon absorption cross-section by detailed balance. The second method relies on the fact that an excited hot nucleus has thermal fluctuations. In particular it has a fluctuating dipole moment which produces thermal radiation. The two methods are closely related and in both cases the spectrum of the radiation emitted is dominated by the giant dipole resonance. The equivalence of the detailed balance and thermal radiation theories can be demonstrated explicitly for a coupled oscillator model of the giant resonance."
"A powerful method to study the properties of a system is to subject it to a weak external perturbation and to examine its response. For the atomic nucleus subjected to the absorption of a photon or to the scattering of a particle (electron, proton, etc.) the response is ... a function of the energy and linear momentum transferred to the system. ... Up to about 10 MeV the nucleus responds through the excitation of relatively simple states often involving only one or a few particles. In the energy range between 10 and 30 MeV the system response exhibits broad resonances. These are the giant resonances ... Giant resonances correspond to a collective motion involving many if not all the particles in the nucleus. The occurrence of such a collective motion is a common feature of many-body quantum systems. In quantum-mechanical terms the resonance corresponds to a transition between the ground state and the collective state and its strength is described by a transition amplitude. Intuitively it is clear that the strength of the transition will depend on the basic properties of the system such as the number of particles participating in the response and the size of the system. This implies that the total transition strength should be limited by a sum rule which depends only on ground-state properties. If the transition strength of an observed resonance exhausts a major part, say greater than 50%, of the corresponding sum rule we call it a giant resonance."