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[en] The so-called pygmy dipole resonance (PDR) appears as a concentration of E1 strength below the well-known isovector giant dipole resonance and was observed in medium-heavy and heavy neutron-rich nuclei. Growing interest in the PDR is driven by its implications for fundamental questions concerning, e.g., the equation of state of nuclear matter under isospin asymmetric conditions as present in neutron stars, the origin and abundance of the elements in the universe, and isospin symmetry breaking. To approach a comprehensive understanding of the structure of the PDR throughout the nuclear landscape, different experimental techniques are applied using complementary probes on stable and unstable nuclei. An overview of recent results and future perspectives is presented.
[en] To gain additional information on nuclear structure from particle-induced reactions, the new silicon-detector array SONIC with 8 ΔE-E-telescopes was installed inside the existing γ array HORUS consisting of 14 HPGe detectors. The main purpose of the combined setup will be the study of inelastic scattering experiments using p, d and α beams delivered by the Cologne 10 MV Tandem accelerator. Since the excitation of the target nucleus is uniquely determined by the energy loss of the projectile, gates on the excitation of specific levels or on a certain final level can be set. Due to the angular granularity of SONIC and HORUS, angular correlations between the ejectile and the emitted γ ray can be measured, which gives access to the spin of the level. Using the particle identification capabilities, light ejectiles (p to α) can be easily distinguished, allowing the study of weaker reaction channels, e.g. inelastic scattering, even if the cross section is dominated by others, e.g. transfer reactions. As a first physics test case, the reaction 92Mo(p,p'γ) was measured. Preliminary results of this experiment are shown, concentrating on states and decay properties of the Pygmy Dipole Resonance.
[en] The Pygmy Dipole Resonance (PDR) has been studied extensively in the past decade. However, one important property, the γ-decay branching ratio to excited states, is still not known systematically. To access this observable, the particle-γ coincidence method is used at the dedicated SONIC rate at HORUS setup. It consists of up to twelve silicon detectors for particle identification and ejectile energy determination and the 14 HPGe detector array HORUS for high-resolution γ-ray spectroscopy. Due to the good energy resolution of the silicon detectors, a narrow gate on a specific excitation energy can be set, which allows for a sensitive and straightforward state to state determination of branching ratios. Branching ratios for 1- states in 92Mo and 94Mo from (p,p'γ) experiments and the decay pattern of 1- states in 120Sn from a (d,pγ) experiment will be shown, as well as possible theoretical interpretations. Together with γ-decay studies from the γ3 setup at HIγS, these experiments allow for a systematic study of the PDR decay pattern to better understand the underlying structure of low-lying E1 strength.
[en] The combined setup SONIC and HORUS consists of the γ-ray spectrometer HORUS with 14 HPGe detectors and the recently commissioned particle spectrometer SONIC with up to 8 ΔE-E silicon detectors. This setup is used to measure the ejectile of a nuclear reaction (p, d, t, or α) in coincidence with the deexciting γ rays emitted by the recoil nucleus. By requiring a certain ejectile energy (e.g. the excitation of a level), a very clean γ spectrum is obtained, in which only physically related events remain. Measuring the angular correlations between the coincident ejectiles and γ-rays allows spin assignments to excited nuclear levels by comparison to theoretical particle-γ angular correlations. An overview of the experimental setup is given, and preliminary p-γ angular correlations measured in a recent 92Mo(p,p'γ) experiment are shown.
[en] The experimental evidence for the presence of octupole vibrational states in 168Yb is presented. In order to populate the excited states in this nucleus, the 166Er(α,2nγ)168Yb fusion evaporation reaction was used with a beam energy of 24 MeV. Using the coincidence method, the level scheme was corrected and extended up to 3 MeV, both for the positive and negative parity states. In a second step, the lifetimes of five excited states were measured by using the fast timing method with the Bucharest HPGe and LaBr3:Ce detector array using the triple-γ coincidence method. Reduced E1 and E2 transition probabilities were extracted from the measured lifetimes and compared, when possible, with similar observables in neighboring isotopes, showing a smooth behavior with increasing mass. The positive and negative-parity states revealed by this experiment are compared with the Interacting Boson Model and are found to be in good agreement.
[en] In the last decade, the Pygmy Dipole Resonance (PDR) has attracted a lot of interest both in experimental and theoretical nuclear physics. However, some key observables are still not easily accessible. One of these is the decay branching of the PDR to excited states, which is a sensitive measure of the wave functions. To gain access to this observable, the new setup SONIC rate at HORUS consisting of silicon and HPGe detectors at the Institute for Nuclear Physics in Cologne was used to investigate the two molybdenum isotopes "9"2Mo and "9"4Mo in inelastic proton scattering experiments. In these experiments, both the energy of the γ-ray and of the particle were measured in coincidence with high energy resolution and in the offline analysis, specific excitation and de-excitation patterns were studied. The results of the "9"2Mo(p,p'γ) experiment are presented, giving insight into the state-to-state decay behaviour of the PDR. Additionally, preliminary results of the "9"4Mo(p,p'γ) experiment will be shown, focussing on the same observable for this non-magic nucleus.
[en] The excitation of states belonging to the Pygmy Dipole Resonance (PDR) by a single-neutron transfer-reaction was investigated in "1"2"0Sn, using a "1"1"9Sn(d,pγ)-experiment and the combined setup SONIC rate at HORUS at the 10 MV Tandem accelerator in Cologne. The setup consisting of 14 HPGe and 6 ΔE-E silicon telescope detectors for the coincident detection of γ-rays and charged particles, respectively, enables an offline selection of excitation and deexcitation channels. First results show that dipole states in the PDR region, i.e. 5 MeV to 9 MeV, could be excited by the reaction. These dipole states were identified as PDR states by comparison with data from a Nuclear Resonance Fluorescence experiment. The contribution will present the experiment and principles of the data analysis needed to select transitions from or to J"π=1"- states. Using results from the experiment, features of the PDR are discussed, including possible particle-hole configurations.
[en] The excitation of states belonging to the Pygmy Dipole Resonance (PDR) in 120Sn was observed in a 119Sn(d,pγ) experiment, using the SONIC rate at HORUS setup at the 10 MV Tandem accelerator in Cologne. The setup, consisting of 6 ΔE-E silicon and 14 HPGe detectors, allows a selection of reaction, excitation, and deexcitation channels in an offline analysis, due to complete kinematics. Several excited states were identified as Jπ = 1(-) states by comparison with results from a nuclear resonance fluorescence experiment, establishing the (d,p) reaction as an additional tool to study the PDR. Preliminary analysis shows that a γ-decay branching to the first 2+ state is observed and branching ratios can be determined. The contribution presents the experiment and principles of the data analysis needed to select transitions from Jπ = 1- states. Furthermore, the latest status of the analysis is presented, including branching ratios and branching-corrected B(E1) values, which will allow a more stringent comparison with recent inelastic proton scattering data.