Dipole response in Te128,130 below 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 $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$ below their neutron separation thresholds of 8.8 and 8.5 MeV, respectively.Methods: Photonuclear reactions with the subsequent $\ensuremath{\gamma}$-ray spectroscopy of the decay channel with continuous-energy bremsstrahlung at varying endpoint energies and linearly polarized quasimonochromatic $\ensuremath{\gamma}$-ray beams with energies ranging from 2.7 to 8.9 MeV in steps of roughly 250 keV were used for probing the decay behavior of the low-energy dipole response in $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$. In addition, $(\stackrel{P\vec}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}}{\ensuremath{\gamma}}^{\ensuremath{'}\ensuremath{'}})$ reactions were used to study the population of low-lying states of $^{128}\mathrm{Te}$.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 $M1$ 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 quasiparticle phonon model. The major fraction of the ground-state decay channel is due to $E1$ transitions, while less than $5--10$% stem from $M1$ transitions. Furthermore, first direct experimental evidence is provided that the population of the ${2}_{1}^{+}$ state of $^{128}\mathrm{Te}$ via primary $\ensuremath{\gamma}$-ray transitions from excited states in the PDR region from 5 to 9 MeV is dominated by $E1$ transitions of ${1}^{\ensuremath{-}}$ states.