High-sensitivity investigation of low-lying dipole strengths in Sn120

Abstract

Background: The term Pygmy Dipole Resonance (PDR) denotes an accumulation of electric dipole excitations below and around the neutron separation threshold. It may be important, e.g., for the nucleosynthesis of heavy nuclei or the symmetry energy in the Equation of State (EoS). For a deeper understanding of the PDR, systematic studies are essential.Purpose: The tin isotopic chain is a well-suited candidate to investigate the systematics of the PDR, and the ($\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}}$) reactions on $^{112,116,120,124}\mathrm{Sn}$ have already been measured in experiments using bremsstrahlung. It was claimed that the extracted electric dipole transition strengths of these isotopes increase with increasing neutron-to-proton ratio with the exception of $^{120}\mathrm{Sn}$. Furthermore, previous results from elastic photon scattering experiments on $^{120}\mathrm{Sn}$ are in disagreement with corresponding $(p,{p}^{\ensuremath{'}})$ Coulomb excitation data. To examine this discrepancy an additional high-sensitivity bremsstrahlung experiment on $^{120}\mathrm{Sn}$ was performed.Method: The Nuclear Resonance Fluorescence (NRF) method is used, which is based on the scattering of real photons. The bremsstrahlung experiment presented in this work was performed with a maximum energy of ${E}_{\ensuremath{\gamma},\mathrm{max}}=9.5\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ at the $\ensuremath{\gamma}\mathrm{ELBE}$ facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Besides a state-to-state analysis, the quasicontinuum was investigated as well.Results: Above ${E}_{x}=4$ MeV 228 dipole transitions were clearly identified; 163 were observed for the first time. Assuming that all identified dipole transitions have electric dipole character the summed electric dipole strength equals $\ensuremath{\sum}B(E1)\ensuremath{\uparrow}=369(49)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{4pt}{0ex}}{e}^{2}{\text{fm}}^{2}$ [which amounts to 0.58(8)% of the Thomas-Reiche-Kuhn sum rule] for transitions from 4 MeV to ${S}_{n}=9.1$ MeV. This is an enhancement of a factor 2.3 compared to the previously published $^{120}\mathrm{Sn}(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ results. This increase can be explained by the contribution of many weak, previously not included transitions in the state-to-state analysis. The photoabsorption cross sections deduced from the quasicontinuum analysis exceed those of the $(p,{p}^{\ensuremath{'}})$ experiment in average by about $50%$ between 5.9 and 8.7 MeV.Conclusion: The newly extracted summed $B(E1)$ value of the state-to-state analysis is larger than those of $^{112,116}\mathrm{Sn}$ and smaller than that of $^{124}\mathrm{Sn}$. The difference between the electric dipole transition strengths deduced from isolated peaks of the present ($\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}}$) data and those from the inelastic proton scattering experiment above 6.3 MeV is still striking. The analysis of the photoabsorption cross section including the quasicontinuum of levels overcomes this problem and the results are in the order of magnitude of the $(p,{p}^{\ensuremath{'}})$ data and continue the $(\ensuremath{\gamma},n)$ cross sections at the neutron separation threshold.