High-sensitivity investigation of low-lying dipole strengths in 120Sn


High-sensitivity investigation of low-lying dipole strengths in 120Sn

Müscher, M.; Wilhelmy, J.; Savran, D.; Schwengner, R.; Massarczyk, R.; Grieger, M.; Isaak, J.; Junghans, A. R.; Kögler, T.; Ludwig, F.; Symochko, D.; Takacs, M. P.; Tamkas, M.; Wagner, A.; Zilges, A.

Background: The term Pygmy Dipole Resonance (PDR) denotes 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 (g,g') reactions on 112,116,120,124Sn 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 Sn. Furthermore, previous results from elastic photon scattering experiments on 120Sn are in disagreement with corresponding (p,p') Coulomb excitation data. To examine this discrepancy an additional high-sensitivity bremsstrahlung experiment on 120Sn was performed.
Method: The Nuclear Resonance Fluorescence (NRF) method is used which bases on real photon scattering. The bremsstrahlung experiment presented in this work was performed with a maximum energy of E = 9.5 MeV at the gELBE facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Besides a state-to-state analysis, the quasi-continuum was investigated as well.
Results: Above Ex = 4 MeV 236 dipole transitions were clearly identified and 168 of those were observed for the first time. Assuming that all analyzed transitions have electric dipole character the summed electric dipole strength equals B(E1) = 374(35) e2 fm2 (0.54(5) % of the TRK sum rule) for transitions from 4 MeV to Sn = 9.1 MeV. This is an enhancement of a factor 2.3 compared to the previously published 120Sn(g,g') results.
Especially, the observation of many weaker transitions in the state-to-state analysis lead to this increase. The photo-absorption cross sections deduced from the quasi-continuum analysis are about two times higher than the results of the (p,p') experiment.
Conclusion: The newly extracted summed B(E1) value of the state-to-state analysis is larger than those of 112,116Sn and smaller than that of 124 Sn. The difference between the present (g,g') data and the results of the inelastic proton scattering experiment above 6.3 MeV is still striking. The deviation may be explained by unobserved decay branchings and unresolved strength. Up to now, there is no explanation for the discrepancy between the extracted photo-absorption cross sections of the analysis of the quasi-continuum and the (p,p')
measurement. Additional experiments may shed light on this deviation.

Keywords: Photon scattering; Photoabsorption cross section; Electromagnetic transition strengths

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Publ.-Id: 30867