Electric dipole strength below the giant dipole resonance

Abstract

Electric dipole strength in nuclei is concentrated in the well-known Giant Dipole Resonance but in nuclei with neutron excess about one per cent of the total E1 strength may also be found at lower excitation energies. We report on a measurement of elecic dipole strength below the GDR in several semi- and doubly-magic nuclei. The found E1 strength can be understood as a collective oscillation of excess neutrons against a N≈Z core. Such an oscillation is often called “Pygmy Dipole Resonance.”