title: Evidence for pygmy and giant dipole resonances in 130Sn and 132Sn.
abstract: The dipole strength distribution above the one-neutron separation energy was measured in the unstable 130Sn and the double-magic 132Sn isotopes. The results were deduced from Coulomb dissociation of secondary Sn beams with energies around 500 MeV/nucleon, produced by in-flight fission of a primary 238U beam. In addition to the giant dipole resonance, a resonancelike structure ("pygmy resonance") is observed at a lower excitation energy around 10 MeV exhausting a few percent of the isovector E1 energy-weighted sum rule. The results are discussed in the context of a predicted new dipole mode of excess neutrons oscillating out of phase with the core nucleons.
authors.affiliation: Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany.
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authors.lastname: Simon
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authors.lastname: Waluś