Seminar Details

Fine Structure of the Pygmy Dipole Resonance

Anton Tonchev, Duke University
Wednesday, November 11, 4:10 PM - Nuclear Science Seminar
NSCL Seminar Room

In stable and weakly bound neutron-rich nuclei a resonance-like concentration of dipole states has been observed for excitation energies around the neutron separation energy. This clustering of strong dipole states has been named the pygmy dipole resonance (PDR) in comparison to the giant dipole resonance that dominates the E1 response. Understanding the PDR is presently of great interest in nuclear structure. Microscopic nuclear models predict the existence of the PDR arising from the formation of a neutron skin in very neutron-rich exotic nuclei. In addition, the dipole strength distributions at the particle separation energies might affect the reaction rates in astrophysical scenarios where photo-disintegration reactions are important, i.e., in hot stars and stellar explosions. This talk is giving an overview of the high-sensitivity studies of E1 and M1 transitions in N = 50 and 82 nuclei using the nearly monoenergetic and 100% linearly polarized photon beams from the High-Intensity-Gamma-Ray Source facility. The fine structure of the dipole-strength distribution of the PDR has been observed and novel information about the character of this mode of excitation has been obtained. The observations will be compared with calculations using statistical and quasiparticle random-phase approximation.