Splitting of the pygmy dipole resonance in 138Ba and 140Ce observed in the (α,α'γ) reaction

The N=82 nuclei 140Ce and 138Ba have been investigated by means of the (α,α'γ) coincidence method to study the pygmy dipole resonance (PDR). The experiments have been performed at the AGOR cyclotron at KVI, Groningen, at a primary beam energy of Eα=136 MeV. The Big-Bite Spectrometer and seven large-volume high-purity germanium detectors were used in coincidence to perform a simultaneous spectroscopy of the scattered α particles and the γ decay. The comparison with results of nuclear resonance fluorescence experiments reveals a splitting of the PDR into two components. Up to about 6 MeV the same states that could be observed in (γ,γ') are also excited in α-scattering experiments, whereas the higher-lying states are missing in the (α,α'γ) reaction. This indicates a structural splitting of the PDR into two modes with different underlying structure.