First high-resolution (α,α'γ) experiments around 35 MeV/nucleon and observation of a splitting of the pygmy dipole resonance in 140Ce

In the framework of this thesis a new setup has been designed, built and tested at the KVI Groningen which allows for the first time to perform (α, α'γ) coincidence experiments with high resolution at around 35 MeV/nucleon on nuclei with a high level density. The method combines the nuclear structure information from α-scattering experiments with the excellent energy resolution of γ-spectroscopy. A high selectivity to dipole excitations is achieved by selecting the ground-state decay channel. This allows to study the electric Pygmy Dipole Resonance (PDR) in heavy nuclei in great detail. The results of a first (α, α'γ) experiment on the semi-magic nucleus 140Ce are discussed in this work. Thirteen electric dipole states have been observed up to an excitation energy of 6 MeV. The comparison to data obtained in photon scattering experiments revealed an unexpected splitting of the PDR into a high and a low energy part with different underlying nuclear structure. (orig.)