Investigation of decay properties of the Pygmy Dipole Resonance and photon strength functions on excited states in (gamma, gamma' gamma'') reactions
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Abstract
In the scope of this work, the decay properties of 128Te and 130Te are studied via photon-induced reactions exploiting the γ3-setup at the High Intensity γ-ray Source (HIγS) at Duke University. The unique combination of γ-γ coincidence measurements and linearly-polarized and quasi-monochromatic photon beams allows for the investigation of the decay behavior of the Pygmy Dipole Resonance (PDR) in (γ, γ' γ'') reactions. Properties of individual excited states as well as average decay quantities are extracted, such as B(E1) ↑ strength distributions and average branching ratios. The novel experimental technique enables the determination of the photon strength function build on the ground state and, in particular, for the first time on the first excited state 2+ state of 128Te in a model-independent way below the neutron emission threshold. The comparison of the experimental data to calculations within the quasi-particle phonon model (QPM) and the statistical model allows for the interpretation of the measured decay behavior from a microscopic point of view and from the point of an average approach, respectively. The analysis hints to a group of excited states around 6 MeV with a different underlying structure
compared to the rest of the investigated energy region. Furthermore, an indication for the violation of the Brink-Axel hypothesis below an excitation energy of 8 MeV is found for both tellurium isotopes.