Low-energy dipole strength inSn112,120

Abstract

The 112,120 Sn(γ, γ ) reactions below the neutron separation energies have been studied at the superconducting Darmstadt electron linear accelerator S-DALINAC for different endpoint energies of the incident bremsstrahlung spectrum. Dipole strength distributions are extracted for 112 Sn up to 9.5 MeV and for 120 Sn up to 9.1 MeV. A concentration of dipole excitations is observed between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays to excited states is estimated in a statistical model. A fluctuation analysis is applied to the photon scattering spectra to extract the amount of the unresolved strength hidden in background due to fragmentation. The strength distributions are discussed within different model approaches such as the quasiparticle-phonon model and the relativistic time blocking approximation allowing for an inclusion of complex configurations beyond the initial particle-hole states. While a satisfactory description of the fragmentation can be achieved for sufficently large model spaces, the predicted centroids and total electric dipole strengths for stable tin isotopes strongly depend on the assumptions about the underlying mean field.

La137,138,139(n,γ) cross sections constrained with statistical decay properties of Kheswa1, Wiedeking2, Brown3 et al. 2017Phys. Rev. C

“…Instead of using discrete spectroscopic tools, a broad range of techniques has been developed to extract statistical properties, below or in the vicinity of S n , such as the nuclear level density (NLD) and γ-ray strength function (γSF) which are measures of the average nuclear response. Some of the commonly used experimental methods include (i) (γ,γ') scattering using mono-energetic beams [1,2] or Bremsstrahlung photon sources [3,4], (ii) (n, γ) measurements with thermal/cold neutron beams [5,6], average resonance capture [7], (iii) two-step cascade methods using thermal neutrons [8] or charged particle reactions [9], and (iv) isoscalar sensitive techniques [10][11][12][13].…”

Dipole response in neutron-rich nuclei within self-consistent approaches using realistic potentials

“…In stable nuclei, dense discrete spectra of dipole states were detected in (γ, γ ) [3][4][5][6][7][8][9] and in (α, α γ) experiments [10][11][12]. Moreover, fairly complete dipole spectra below and above the neutron decay threshold were produced with great accuracy through inelastic proton scattering experiments [13,14].…”

Exotic nuclear matter

“…The theoretical results have been used successfully to analyse the data of different experiments, e.g. [6,[8][9][10][11]13]. Although in many cases the overall features are already reasonably obtained by singlephonon QRPA calculations, multi-phonon effects are indispensable for a proper description of the fragmentation pattern and the total strength of pygmy modes [5].…”

“…In recent years nuclear density functional theory and nuclear many-body methods -ranging from HFB and (Q)RPA theory to the QPM approach of Soloviev et al -have been used to study systematically ground state properties and multipole response functions of exotic nuclei, especially in the region of the N = 82 and the Z = 50 shell closures, see e.g. [1] and detailed studies are found in [2][3][4][5][6][7][8][9][10][11][12][13][14]. None of those systems can be explored without an adequate reaction theory.…”