Abstract
Background:
In atomic nuclei, a concentration of electric dipole strength around the particle threshold, commonly denoted as pygmy dipole resonance, may have a significant impact on nuclear structure properties and astrophysical scenarios. A clear identification of these states and the structure of this resonance is still under discussion.
Purpose:
We present an experimental and theoretical study of the isospin character of the pygmy dipole resonance and investigation of a splitting of the electric dipole strength previously observed in experiments on N=82 nuclei.
Method: The pygmy dipole resonance has been studied in the semi-magic Z=50 nucleus 124Sn by means of the (α,α′γ) coincidence method at Eα=136MeV using the Big-Bite Spectrometer at the Kernfysisch Versneller Instituut in Groningen, The Netherlands.
Results:
A splitting of the low-energy part of the electric dipole strength was identified in 124Sn by comparing the differential cross sections measured in (α,α′γ) to results stemming from (γ,γ′) photon-scattering experiments. While an energetically lower-lying group of states is observed in both kinds of experiments, a higher-lying group of states is only excited in the (γ,γ′) reaction. In addition, theoretical calculations using the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle-phonon model have been performed. Both calculations show a qualitative agreement with the experimental data and predict a low-lying isoscalar component that is dominated by neutron-skin oscillations as expected for the pygmy dipole resonance. Furthermore, the states at higher energies show a pronounced isovector component and a different radial dependence of the corresponding transition densities as expected for the tail of the giant dipole resonance.
Conclusions:
An experimental signature of the neutron-skin oscillation of the pygmy dipole resonance has been corroborated. The combination of the presented reactions might make it possible to identify states of this resonance.
In atomic nuclei, a concentration of electric dipole strength around the particle threshold, commonly denoted as pygmy dipole resonance, may have a significant impact on nuclear structure properties and astrophysical scenarios. A clear identification of these states and the structure of this resonance is still under discussion.
Purpose:
We present an experimental and theoretical study of the isospin character of the pygmy dipole resonance and investigation of a splitting of the electric dipole strength previously observed in experiments on N=82 nuclei.
Method: The pygmy dipole resonance has been studied in the semi-magic Z=50 nucleus 124Sn by means of the (α,α′γ) coincidence method at Eα=136MeV using the Big-Bite Spectrometer at the Kernfysisch Versneller Instituut in Groningen, The Netherlands.
Results:
A splitting of the low-energy part of the electric dipole strength was identified in 124Sn by comparing the differential cross sections measured in (α,α′γ) to results stemming from (γ,γ′) photon-scattering experiments. While an energetically lower-lying group of states is observed in both kinds of experiments, a higher-lying group of states is only excited in the (γ,γ′) reaction. In addition, theoretical calculations using the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle-phonon model have been performed. Both calculations show a qualitative agreement with the experimental data and predict a low-lying isoscalar component that is dominated by neutron-skin oscillations as expected for the pygmy dipole resonance. Furthermore, the states at higher energies show a pronounced isovector component and a different radial dependence of the corresponding transition densities as expected for the tail of the giant dipole resonance.
Conclusions:
An experimental signature of the neutron-skin oscillation of the pygmy dipole resonance has been corroborated. The combination of the presented reactions might make it possible to identify states of this resonance.
| Original language | English |
|---|---|
| Article number | 064331 |
| Journal | Physical Review C |
| Volume | 85 |
| Issue number | 6 |
| DOIs | |
| Publication status | Published - 28 Jun 2012 |
| Externally published | Yes |