Pygmy response
The Pygmy resonance (PDR, pygmy dipole resonance) is a dipole -Schwingungsanregung of neutron-rich nuclei . It owes its name (roughly translated to dwarf dipole resonance ) to the considerably lower strength of the giant dipole resonance (GDR), in which the protons as a whole oscillate against the neutrons in the nucleus. It is also sometimes referred to as soft dipole resonance (SDR). The resonance is mainly found in nuclei close to the neutron threshold. The pygmy resonance has attracted the attention of research especially since the 1990s, when it became possible to investigate the structure of unstable nuclei with new accelerators. It is often interpreted qualitatively as the oscillation of a neutron “skin” against a residual nucleus consisting of the same number of protons and neutrons. The restoring force for this oscillation is the "nuclear force", a residual interaction of the strong interaction . The electric charge of the nucleus mediates the coupling with external electromagnetic fields - scattering of photons is used to study these resonances.
As a rule, the Pygmy resonance is found between the 2- phonon state (e.g. two quadrupoles or octupole quadrupole) and the giant dipole resonance in an energy range of 4–10 MeV . It covers about 1% of the energy-weighted sum rule (EWSR) for electric dipole strength, almost the entire rest is taken up by the electric dipole giant resonance, which explains the name.
In even-even nuclei with a neutron number of 82, indications of the Pygmy resonance below the neutron threshold were found. The E1 strength fragments very strongly here into numerous states and can e.g. B. can be investigated in photon scattering experiments. Due to the strong fragmentation, high-purity germanium detectors are used for the spectral analysis of the scattered photons. With the help of Coulomb excitation , a resonance-like structure was found in neutron-rich radioactive Sn isotopes above the threshold at the GSI .
The structure and nature of PDR are completely unexplained, such as the question of the isospin character. Possible collective models of PDR are vibrations of a neutron skin against the residual nucleus or excitation of alpha clusters in the nucleus.
The PDR may play a role in the investigation of the formation of the elements in astrophysics, as it could influence the nuclear reaction rate for neutron capture from extremely neutron-rich nuclei in stars and stellar explosions (the so-called R-process ) if they are in the appropriate energy range in these nuclei is below the neutron threshold. The pygmy resonance, interpreted as the oscillation of a thin neutron shell, also serves as a model for the behavior of asymmetric neutron-rich nuclear matter, for example in neutron stars or heavy nuclei.
Web links
- Zilges: E1 excitations in atomic nuclei: From Giants, Pygmies and Octupoles . (PDF) Presentation slides 2009
- Paar: Explanation of theoretical work on Pygmy resonance by Paar, with relativistic RPA
Individual evidence
- ↑ Near the so-called neutron threshold, only a small amount of excitation energy is needed to free a neutron from the nucleus
- ↑ D. h. Nuclei with an even number of neutrons and an even number of protons
- ↑ E1 stands for electric dipole
- ↑ Excitation of the nucleus through scattering of a positively charged particle (ion), caused by the rapidly changing Coulomb field of the scattered particle
- ↑ Piekarewicz: Pygmy dipole resonance as a constraint on the neutron skin of heavy nuclei . In: Physical Review C , 73, 1986, arxiv : nucl-th / 0602036