Dipole response inTe128,130below the neutron threshold

Abstract

Background: Numerous studies of the ground-state decay of the Pygmy Dipole Resonance (PDR) have been carried out in the past. However, data on the decay of the PDR to low-lying excited states is still very scarce due to limitations of the sensitivity to weak branching transitions of experimental setups.Purpose: We present a detailed examination of the low-energy dipole response of 128 Te and 130 Te below their neutron separation thresholds of 8.8 MeV and 8.5 MeV, respectively.Methods: Photonuclear reactions with the subsequent γ-ray spectroscopy of the decay channel with continuous-energy bremsstrahlung at varying endpoint energies and linearly polarized quasi-monochromatic γ-ray beams with energies ranging from 2.7 MeV to 8.9 MeV in steps of roughly 250 keV were used for probing the decay behavior of the low-energy dipole response in 128 Te and 130 Te. In addition, ( γ, γ γ ) reactions were used to study the population of low-lying states of 128 Te.Results: Spin-parity quantum numbers and reduced transition probabilities are determined for individual photo-excited states. The analysis of average decay properties for nuclear levels in narrow excitation-energy bins enable the extraction of photoabsorption cross sections, average branching ratios to the 2 + 1 state, and the distinction between E1 and M 1 transitions to the ground state and to the 2 + 1 state accounting for resolved and unresolved transitions. Conclusions: Above 5 MeV, the experimental data are in reasonable agreement to calculations within the quasi-particle phonon model. The major fraction of the ground-state decay channel is due to E1 transitions, while less than 5 − 10 % stem from M 1 transitions. Furthermore, first direct experimental evidence is provided that the population of the 2 + 1 state of 128 Te via primary γ-ray transitions from excited states in the PDR region from 5 to 9 MeV is dominated by E1 transitions of 1 − states.

Comprehensive Test of the Brink-Axel Hypothesis in the Energy Region of the Pygmy Dipole Resonance

“…Applying statistical model corrections of the branching ratios, the deduced photoabsorption cross sections agree within the considerable model dependence of such a procedure discussed above. We note that the recent realization of ðγ; γ 0 γ 00 Þ experiments in combination with quasimonoenergetic photon beams from laser Compton backscattering promises a competitive extraction of photoabsorption cross sections below threshold in the future [65].…”