Electric and magnetic dipole strength in 112,114,116,118,120,124Sn

Abstract

Background: Inelastic proton scattering at energies of a few hundred MeV and very forward angles including 0 • has been established as a tool for the study of electric and magnetic dipole strength distributions in nuclei. The present work reports a systematic investigation of the chain of stable even-mass tin isotopes.Methods: Inelastic proton scattering experiments were performed at the Research Center for Nuclear Physics, Osaka, with a 295 MeV beam covering laboratory angles 0 • − 6 • and excitation energies 6 − 22 MeV. Cross sections due to E1 and M 1 excitations were extracted with a Multipole Decomposition Analysis (MDA) and then converted to reduced transition probabilities with the "virtual photon method" for E1 and the "unit cross section method" for M 1 excitations, respectively. Including a theory-aided correction for the high excitation energy region not covered experimentally, the electric dipole polarizability was determined from the E1 strength distributions.Results: Total photoabsorption cross sections derived from the E1 and M 1 strength distributions show significant differences compared to those from previous (γ, xn) experiments in the energy region of the IsoVector Giant Dipole Resonance (IVGDR). The widths of the IVGDR deduced from the present data with a Lorentz parameterization show an approximately constant value of about 4.5 MeV in contrast to the large variations between isotopes observed in previous work. The IVGDR centroid energies are in good correspondence to expectations from systematics of their mass dependence. Furthermore, a study of the dependence of the IVGDR energies on bulk matter properties is presented. The E1 strengths below neutron threshold show fair agreement with results from (γ, γ ) experiments on 112,116,120,124 Sn in the energy region between 6 and 7 MeV, where also isoscalar E1 strength was found for 124 Sn. At higher excitation energies large differences are observed pointing to a different nature of the excited states with small ground state branching ratios. The isovector spin-M 1 strengths exhibit a broad distribution between 6 and 12 MeV in all studied nuclei.

Fine Structure of the Isovector Giant Dipole Resonance in $^{142-150}$Nd and $^{152}$Sm

“…Some impact of the spreading due to coupling of the 2p2h states to the 1p1h doorway states is seen in the spherical/transitional nuclei, where such calculations beyond RPA are available. At present, it remains open whether information on the fine structure scales can be utilised to improve the development of global energy density functionals, where the isovector dipole response remains an open problem [13]. This question will be addressed in future work.…”

“…These make use of proton inelastic scattering with energies of a few hundred MeV at scattering angles close to θ Lab = 0 • , which requires special experimental techniques [5,6]. Under these kinematic conditions, the background from other nuclear processes has been found to be small in heavy nuclei and its contribution can easily be subtracted [7][8][9][10][11][12][13].…”