Article

# Spectral Structure of the Pygmy Dipole Resonance

[more]
Duke University, Durham, North Carolina 27708-0308, USA.
(Impact Factor: 7.51). 02/2010; 104(7):072501. DOI: 10.1103/PHYSREVLETT.104.072501
Source: PubMed

ABSTRACT

High-sensitivity studies of E1 and M1 transitions observed in the reaction (138)Ba((gamma) over right arrow, gamma') at energies below the one-neutron separation energy have been performed using the nearly monoenergetic and 100% linearly polarized photon beams of the HI (gamma) over right arrowS facility. The electric dipole character of the so-called "pygmy" dipole resonance was experimentally verified for excitations from 4.0 to 8.6 MeV. The fine structure of the M1 "spin-flip" mode was observed for the first time in N = 82 nuclei.

### Full-text

Available from: Anton P. Tonchev
• Source
##### Article: Spectroscopic features of low-energy excitations in skin nuclei
[Hide abstract]
ABSTRACT: Systematic studies of dipole and other multipole excitations in stable and exotic nuclei are discussed theoretically. Exploring the relation of the strengths of low-energy dipole and quadrupole pygmy resonances to the thickness of the neutron (proton) skin a close connection between static and dynamic properties of the nucleus is observed. The fine structure of low-energy dipole strength in 138Ba nucleus is revealed from E1 and spin-flip M1 strengths distributions.
Full-text · Article · Apr 2010 · Modern Physics Letters A
• Source
##### Article: Instantaneous Shape Sampling - a model for the $\gamma$-absorption cross section of transitional nuclei
[Hide abstract]
ABSTRACT: The influence of the quadrupole shape fluctuations on the dipole vibrations in transitional nuclei is investigated in the framework of the Instantaneous Shape Sampling Model, which combines the Interacting Boson Model for the slow collective quadrupole motion with the Random Phase Approximation for the rapid dipole vibrations. Coupling to the complex background configurations is taken into account by folding the results with a Lorentzian with an energy dependent width. The low-energy energy portion of the $\gamma$- absorption cross section, which is important for photo-nuclear processes, is studied for the isotopic series of Kr, Xe, Ba, and Sm. The experimental cross sections are well reproduced. The low-energy cross section is determined by the Landau fragmentation of the dipole strength and its redistribution caused by the shape fluctuations. Collisional damping only wipes out fluctuations of the absorption cross section, generating the smooth energy dependence observed in experiment. In the case of semi-magic nuclei, shallow pygmy resonances are found in agreement with experiment.
Full-text · Article · Sep 2010 · Physical Review C
• Source
##### Article: Isospin Character of the Pygmy Dipole Resonance in Sn 124
[Hide abstract]
ABSTRACT: The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (α, α'γ) coincidence method at Eα=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (α, α'γ) as well as in (γ, γ') reactions and a group of states at higher energies which is only excited in (γ, γ') reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.
Full-text · Article · Nov 2010 · Physical Review Letters