The two-photon decay of the (11-)/(2) isomer of 137Ba and mixed-symmetry states of 92,94Zr and 94Mo

The first part of this thesis covers the two-photon decay of the (11-)/(2) isomer in 137Ba. Clear evidence of this exotic decay mode is found, which opens up the possibility to use the two-photon decay probability as a general tool to investigate the structure of atomic nuclei. Details on the experiment are given, and the major challenges to successfully perform this difficult experiment are outlined. The two-photon decay experiment was conducted with the new LaBr3-detector ball which was setup from scratch in the framework of this thesis. The properties and performance of this powerful detector system - consisting of 18 large LaBr3-detectors - are discussed. Furthermore, proton scattering at 200 MeV has been performed on the nuclei 92,94Zr, 94,96Mo and 70Zn.The measured cross sections are compared to calculations in the framework of the Quasi-Particle Phonon Model (QPM). Conclusions about the validity of the QPM results are drawn. A comparison of the excitation cross sections of the 2+1 states and the one-phonon quadrupole states with mixed neutron and proton symmetry (2+ms) allows to prove the special character of the 2+ms states independently of electromagnetic transition strengths. The possible existence of 3- and 4+ states with mixed proton and neutron symmetry is discussed. Furthermore the origin of collectivity of low-lying states in spherical nuclei is investigated in the QPM.