Dipole Excitation of Soft and Giant Resonances in 132Sn and neighboring unstable nuclei

Abstract

The evolution of dipole‐strength distributions above the one‐neutron threshold was investigated for exotic neutron‐rich nuclei in a series of experiments using the electromagnetic projectile excitation at beam energies around 500 MeV/u. For halo nuclei, the large observed dipole strength (shown here for 11Be) is explained within the direct‐breakup model to be of non‐collective character. For neutron‐rich oxygen isotopes, the origin of the observed low‐lying strength is concluded to be due to single‐particle transitions on theoretical grounds. The dipole strength spectra for 130,132Sn exhibit resonance‐like structures observed at energies around 10 MeV exhausting a few percent of the Thomas‐Reiche‐Kuhn (TRK) sum rule, separated clearly from the dominant Giant Dipole Resonance (GDR). The data agree with predictions for a new dipole mode related to the oscillation of excess neutrons versus the core nucleons (“pygmy resonance”).