Splitting of the pygmy dipole resonance inBa138andCe140observed in the(α,α′γ)reaction

Abstract

The N = 82 nuclei Ce-140 and Ba-138 have been investigated by means of the (alpha,alpha',gamma) 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-alpha = 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 alpha particles and the gamma 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 (gamma,gamma') are also excited in alpha-scattering experiments, whereas the higher-lying states are missing in the (alpha,alpha',gamma) reaction. This indicates a structural splitting of the PDR into two modes with different underlying structure.