Low-energy dipole strength in 112,120Sn

TL;DR

This study investigates low-energy dipole excitations in 112Sn and 120Sn using photon scattering, revealing concentration regions, estimating missing strength, and comparing results with advanced nuclear models.

Contribution

It provides detailed dipole strength distributions in tin isotopes and evaluates the effectiveness of different theoretical models in describing these excitations.

Findings

Dipole strength concentrated between 5 and 8 MeV.

Estimated missing strength due to unobserved decays.

Model predictions depend on assumptions about the mean field.

Abstract

The 112,120Sn(gamma,gamma') 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 112Sn up to 9.5 MeV and for 120Sn 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.