Splitting of the pygmy dipole resonance inBa138andCe140observed in the(α,α′γ)reaction
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Abstract
The $N=82$ nuclei $^{140}\mathrm{Ce}$ and $^{138}\mathrm{Ba}$ have been investigated by means of the $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})$ 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 ${E}_{\ensuremath{\alpha}}=136$ 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 $\ensuremath{\alpha}$ particles and the $\ensuremath{\gamma}$ 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 $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ are also excited in $\ensuremath{\alpha}$-scattering experiments, whereas the higher-lying states are missing in the $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction. This indicates a structural splitting of the PDR into two modes with different underlying structure.
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