Münster 2002 – scientific programme

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HK: Physik der Hadronen und Kerne

HK 15: Plenary Session

HK 15.3: Plenary Talk

Tuesday, March 12, 2002, 15:15–15:45, Plenarsaal

Electric dipole strength in atomic nuclei – a key to the breaking of isospin symmetry — •Andreas Zilges — Institut für Kernphysik, TU Darmstadt, Schlossgartenstrasse 9, D-64289 Darmstadt, Germany

A global or local breaking of the symmetry of proton and neutron distributions in nuclei leads to electric dipole excitations. The most prominent feature is the Giant Dipole Resonance (GDR) at energies of about 15 MeV. At the other end of the energetic scale a bound two phonon octupole–quadrupole excitation has been established as a fundamental E1 mode around 4 MeV in all medium and heavy mass nuclei.

In recent photon scattering experiments at the Superconducting Darmstadt Linear Accelerator S–DALINAC we investigated several nuclei near shell closures in the energy range between the two phonon state and the neutron threshold. Collective electric dipole strength exhausting up to one percent of the energy weighted sum rule has been observed. These excitations are clearly separated from the GDR. Possible interpretations in terms of a local breaking of isospin symmetry and the influence of a neutron skin will be discussed.
^* supported by the DFG (contracts Zi 510/2-1 and FOR 272/2-2).