Huge response

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Under giant resonance is understood in the nuclear physics a vibrational excitation of atomic nuclei . The name comes from the relatively large cross sections that were observed in the scattering experiments that stimulated them. Giant electric dipole resonance (GDR) was discovered in 1947 by G. C. Baldwin and G. S. Klaiber while studying photodisintegration and later in ( , n) reactions on uranium nuclei.

In a simple macroscopic picture by Maurice Goldhaber and Edward Teller , the giant resonance is described as the collective oscillation of the protons against the neutrons . In 1950 it was described by Helmut Steinwedel and J. Hans D. Jensen using a two-liquid model (proton and neutron liquid). The microscopic interpretation sees giant resonances as a coherent stimulation of single-particle-single-hole transitions in the shell model .

Huge resonances can e.g. B. arise through excitation of the nucleus with photons , electrons or heavy ions and decay through nucleon / nuclear emission (photon, neutron, α-particles, ...). The GDR is shown in the excitation energies of heavy nuclei above the separation energy of a nucleon at around 8 M eV and varies with the third root of the mass number  A , which was also predicted in the model by Jensen and Steinwedel. In the case of heavy nuclei with mass numbers above 60, the width of the resonance is typically a few MeV; with light nuclei, the resonance typically splits into several peaks . With deformed cores there are typically two peaks, depending on the oscillation along the axis of symmetry or perpendicular to it.

One can the resonances of angular momentum - eigenstates develop and then speaks of monopole, dipole, quadrupole - or generally multipole -Riesenresonanzen.

In addition to electrical, there are also magnetic dipole resonances in nuclei , see scissors mode . Typically, the giant resonances exhaust the sum rules for the respective (electrical / magnetic) multipole transitions.

The share that neutrons and protons have in the oscillation is expressed in the isospin character. The GDR has an isovector character (protons vibrate against neutrons), there are also giant isoscalar resonances (protons and neutrons vibrate in the same direction).

In addition to the GDR giant resonance, a smaller resonance peak in neutron-rich nuclei, the pygmy resonance (PDR), came into the focus of research from the 1990s , which clearly stood out from the “tail” of the giant resonance.

See also

literature

  • BL Berman, FC Fultz. In: Reviews of Modern Physics , Volume 47, 1975, p. 713 (review article on GDR)

Individual evidence

  1. ↑ Baldwin, Klaiber. In: Physical Review , Volume 71, 1947, p. 3
  2. ↑ In the core photo effect there were already indications from Walther Bothe and Wolfgang Gentner . In: Zeitschrift für Physik , Volume 71, 1936, p. 236. The more precise investigation by Baldwin and Klaiber was achieved using high-energy photons in the bremsstrahlung from a betatron .
  3. ↑ Goldhaber, Teller. In: Physical Review , Volume 74, 1948, p. 1048
  4. ↑ Steinwedel, Jensen. In: Physical Review , Volume 79, 1950, p. 1019
  5. ↑ Below the neutron threshold (detachment energy for neutrons), the GDR was investigated using nuclear resonance fluorescence and bremsstrahlung from accelerators