Giant nuclear resonances
Giant nuclear resonances
Elementary modes of oscillation of the whole nucleus, closely related to the normal modes of oscillation of coupled mechanical systems. Giant nuclear resonances occur systematically in most, if not all, nuclei, with oscillation energies typically in the range of 10–30 MeV. Among the best-known examples is the giant electric dipole (E1) resonance, in which all the protons and all the neutrons oscillate with opposite phase, producing a large time-varying electric dipole moment which acts as an effective antenna for radiating gamma rays. See Gamma rays
Giant resonances are usually classified in terms of three characteristic quantum numbers: L, S, and T, where L is the orbital angular momentum, S is the (intrinsic) spin, and T is the isospin carried by the resonance oscillation. The number L is also the multipole order, with possible values L = 0 (monopole), L = 1 (dipole), L = 2 (quadrupole), L = 3 (octupole), and so on. The spin quantum number S is either 0 or 1. The S = 0 resonances are often called electric, and the S = 1 ones magnetic (EL or ML, where L is the multipole order), stemming from the fact that these giant resonances have strong decay modes involving the emission of either electric (for EL resonances) or magnetic (for ML resonances) multipole photons of the same multipole order as the resonance. A giant resonance with S = 0 corresponds to a purely spatial oscillation of the nuclear mass (or charge density), while one with S = 1 corresponds to a spin oscillation. The isospin quantum number T, which is also either 0 or 1, determines the relative behavior of neutrons versus protons; in a T = 0 or isoscalar giant resonance, the neutrons and protons oscillate in phase, whereas in a T = 1 or isovector resonance the neutrons and protons oscillate with opposite phase. See Multipole radiation, Nuclear moments
These resonances are called giant because of their great strength, 50–100% of the theoretical limit, concentrated in a compact energy region. The oscillation energy is characteristic of the type of giant resonance and is determined by the restoring force and the nuclear mass; the force is due to the nuclear attraction between nucleons, the most important part being the component of the same multipole order as the giant resonance.
The giant electric dipole (E1) resonance is the oldest and best known of the nuclear giant resonances. It is the dominant feature in reactions initiated by gamma rays. The absorption of a gamma ray induces the giant E1 oscillation, which breaks up, in this case, by emitting neutrons. This resonance is also the dominant feature in the reverse process, in which gamma rays are produced by proton and neutron bombardments of nuclei. The resonance is isovector (L = 1, S = 0, T = 1).
The isoscalar giant E0 (electric monopole; L = 0, S = 0, T = 0) resonance lies very close in energy to the giant E1 resonance, whereas the isoscalar giant E2 (electric quadrupole; L = 2, S = 0, T = 0) resonance lies somewhat lower. Both are strongly excited in forward-angle inelastic scattering of energetic alpha particles.
The isoscalar E0 resonance is called the breathing mode, as the whole nucleus undergoes a purely radial oscillation, alternately expanding and contracting. The isoscalar E0 resonance energy is important in determining the nuclear compressibility.
In ordinary nuclear beta decay, a neutron inside a nucleus is transformed into a proton, and an electron and an antineutrino are produced. In one of the simplest types of beta decay, called Gamow-Teller decay, the transformed neutron is otherwise undisturbed, except that its spin may be reversed. As a result, the nucleus usually gains a small amount of energy. If beta decay involved a higher energy transfer to the nucleus, it would drive the giant Gamow-Teller resonance, which is a pure spin oscillation where the neutron spin and the proton spin oscillate out of phase (L = 0, S = 1, T = 1). A giant Gamow-Teller resonance is a strong feature in the (p, n) reaction in which neutrons emerge at 0° from nuclei struck by energetic protons. This reaction substitutes a proton for a neutron in the nucleus via a spin-dependent interaction, in a manner analogous to beta decay but with a much larger energy transfer.
The properties of the giant Gamow-Teller resonance are important in certain problems in nuclear astrophysics.
Studies of the giant electric dipole resonance have been extended to highly excited hot nuclei. These studies provide unique information about the properties of such nuclei, in particular their shape. The shape sensitivity arises from the resonance splitting in a deformed nucleus. The size of the splitting gives the magnitude of the deformation, whereas the relative strength of the components determines the sense of the deformation: prolate (football-shaped) or oblate (doorknob-shaped).
Giant resonances play an important role in energetic nuclear reactions occurring in nature. Among the best examples are supernovae explosions. The rate of electron capture reactions, which cool the core of the massive star involved in the explosion and accelerate its gravitational collapse, depends on the properties of the giant Gamow-Teller resonance. The strength of the shock wave created by the collapse is directly related to the nuclear compressibility discussed above in the context of the giant isoscalar E0 resonance. Higher-energy neutrinos from the central region of the star travel outward and heat the nuclei in the mantle via inelastic scattering reactions which excite various giant resonances. Certain elements found in nature may have been produced primarily as giant resonance decay products in these reactions. See Neutrino, Nuclear reaction, Nuclear spectra, Nuclear structure, Resonance (quantum mechanics)