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 E_α=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.