Comparission of E1 Response of 154Sm and 155Sm in the Pygmy Dipole Resonance (PDR) Region

Abstract

The dipole response associated with the pygmy dipole resonance (PDR) in 154Sm and 155Sm has been studied. In the 154Sm nucleus 1- phonons with K=0 and K=1 branches have been calculated using the Translation and Galilean Invariant Quasiparticle Random Phase Approximation (TGI-QRPA). The structure of the more pronounced electric dipole (E1) peaks in PDR region in 154Sm is composed of predominantly two-quasiproton or twoquasineutron states. The calculations in 155Sm has been performed in the framework of the Translation and Galilean Invariant Quasiparticle Phonon Nuclear Model (TGI-QPNM) based on the TGI-QRPA 1- phonons calculated for 154Sm. When going from 154Sm to neighbouring 155Sm, the fragmentation of the E1 strength is dramatically enhanced. The results emphasize the role of the quasiparticle⨂phonon interactions in enhancing the fragmentation of the strength in the PDR region in 155Sm. Even though the strong fragmentation of the E1 strength obtained for 155Sm, in 5-8 MeV energy region the summed E1 strength is comparable to that in 154Sm. The results indicate that one quasiparticle behaves solely as a spectator in 155Sm.