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

Expérience AGORThe 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