In the project area "Nuclear Structure", the working group deals with the elementary properties of atomic nuclei. The symmetry between the building blocks of the atomic nucleus, the protons and neutrons, is a fundamental assumption of nuclear physics. Any breaking of this symmetry has important consequences for our picture of this quantum system. The study of electric and magnetic dipole transitions and related structures is one way of gaining a deeper understanding. Examples include pygmy dipole resonance (which in the collective picture is often interpreted as an oscillation of a neutron skin against a residual nucleus of protons and neutrons), octupole excitations or mixed-symmetry states. We carry out the following investigations, among others:

• Experiments with fusion-evaporation reactions and direct reactions with ions at the tandem accelerator of the University of Cologne and at other accelerators to elucidate the structure of low-spin excitations in medium and heavy atomic nuclei. The HORUS array with 14 high-resolution HP(Ge) detectors for gamma radiation and the complementary SONIC array developed by our group for particle identification and spectroscopy are available to us at the tandem accelerator. Further experiments of our group take place at the Horia Hulubei National Institute of Physics and Nuclear Engineering in Bucharest/Romania.
• Experiments with quasi-monoenergetic real photons in the MeV energy range from laser Compton backscattering at the Triangle Universities Nuclear Laboratory in Durham, NC, USA.
• Experiments with photons in the MeV energy range from bremsstrahlung at the Helmholtz Center Dresden-Rossendorf.
• Accompanying calculations with various nuclear structure models.

The projects are funded by the German Research Foundation among others. The University of Cologne is a partner in the BMBF research focus NuSTAR.