Dynamische Symmetrien von Atomkernen an Unterschalenabschlüssen

In dieser Arbeit wurden die Einflüsse der Unterschalenabschlüsse bei Neutronenzahlen N=40 und N=56 auf die Kernstruktur der umliegenden Atomkerne untersucht. Der Schwerpunkt war hierbei der Kern 70Zn, der im Rahmen dieser Arbeit durch eine ganze Reihe von Experimenten untersucht wurde. Es wurde zunächst ein Photonenstreu-Experiment an der Universität Stuttgart durchgeführt, um die Lebensdauer des 2+2-Zustandes in 70Zn zu überprüfen. Des Weiteren wurde 70Zn mit monoenergetischen Neutronen an der University of Kentucky untersucht. Diese Messung konnte viele entscheidende Korrekturen im tiefenergetischen Termschema von 70Zn anbringen und zeigt generelle Probleme bei den bekannten experimentellen Daten der Zink-Isotope auf. Weiterhin wurden an der Yale University mit der Methode transienter Magnetfelder magnetische Momente kurzlebiger Zustände in 70Zn vermessen. Als Konsequenz dieser Resultate wurde in dieser Arbeit gezeigt, dass der Kern 70Zn durch einen F-Spin-symmetrischen IBM-2 -Hamiltonian in der dynamischen Symmetrie U(5) beschrieben werden kann. Es wurde eine neue Deutung für das ungewöhnliche energetische Verhalten der Zustände 0+2 und 2+3 gegeben. Für den Zustand 2+3 wurde dabei das Zerfallsverhalten als Signatur der Proton-Neutron gemischt-symmetrischen Anregung 2+1,ms gedeutet. Weiterhin wurden Kandidaten für gemischt-symmetrische Zustände höherer Phononenordnung vorgestellt. Hier konnten Effekte von starken Zustandsmischungen nachgewiesen werden. Das ungewöhnliche Verhalten der 2+1,ms-Zustände in den gerade-gerade Zink-Isotopen wurde als Brechung der F-Spin-Symmetrie beim Übergang in ein Isospin-symmetrisches System gedeutet. Es wurden ebenfalls Experimente mit radioaktiven Ionen-Strahlen der Kerne 88Kr und 92Kr vorgestellt um den aktuellen Stand der Entwicklung dieser Technik aufzuzeigen. Es wurde gezeigt, welche Aufgaben angegangen werden müssen, um in Zukunft die Spektroskopie gemischt-symmetrischer Anregungen mit radioaktiven Ionen-Strahlen möglich zu machen

Comprehensive test of nuclear level density models

For the last two decades, experimental information on nuclear level densities for about 60 different nuclei has been obtained on the basis of the Oslo method. While each of these measurements has been typically compared to one or a few level density models, a global study including all the measurements has been missing. The present study provides a systematic comparison between Oslo data and six global level density models for 42 nuclei for which $s$-wave resonance spacings are also available. We apply a coherent normalization procedure to the Oslo data for each of the six different models, all being treated on the same footing. Our quantitative analysis shows that the constant-temperature model presents the best global description of the Oslo data, closely followed by the mean-field plus combinatorial model and Hartree-Fock plus statistical model. Their accuracies are quite similar, so that it remains difficult to clearly favour one of these models. When considering energies above the threshold where the experimental level scheme is complete, all the six models are shown to lead to rather similar accuracies with respect to Oslo data. The recently proposed shape method can, in principle, improve the situation since it provides an absolute estimate of the excitation-energy dependence of the measured level densities. We show for the specific case of $^{112}$Cd that the shape method could exclude the Hartree-Fock plus statistical model. Such an analysis remains to be performed for the bulk of data for which the shape method can be applied to the Oslo measurements before drawing conclusions on the general quality of a given nuclear level density model.Comment: 11 pages, 7 figures, published in PR

Preparation and Handling of Methane for Radiocarbon Analysis at Cologneams

CH₄ is the second most important anthropogenic greenhouse gas and originates from different sources. The use of radiocarbon (¹⁴C) analysis of CH₄ opens up the possibility to differentiate geological and agricultural origin. At the CologneAMS facility, the demand for ¹⁴C analysis of CH₄ required the development of a sample handling routine and a vacuum system that converts CH₄ to CO₂ for direct injection of CO₂ into the AMS. We evaluated the processing of CH₄ using several series of gas mixtures of ¹⁴C-free and modern standards as well as biogas with sample sizes ranging from 10 to 50 µg C. The results revealed a CH₄ to CO₂ conversion efficiency of 94–97% and blank values comparable to blank values achieved with our routinely used vacuum system for processing CO₂ samples. The tests with a near modern CH₄:CO₂ biogas mixture gave reproducible results with a near modern ¹⁴C content of 0.967–1.000 F¹⁴C, after applying the background correction.ISSN:0033-822

Independent normalization for gamma-ray strength functions: The shape method

The shape method, a novel approach to obtain the functional form of the γ-ray strength function (γSF), is introduced. In connection with the Oslo method the slope of the nuclear level density (NLD) and γSF can be obtained simultaneously even in the absence of neutron resonance spacing data. The foundation of the shape method lies in the primary γ-ray transitions which preserve information on the functional form of the γSF. The shape method has been applied to 56Fe, 92Zr, and 164Dy, which are representative cases for the variety of situations encountered in typical NLD and γSF studies. The comparisons of results from the shape method to those from the Oslo method demonstrate that the functional form of the γSF is retained regardless of nuclear structure details or Jπ values of the states fed by the primary transitions

Reverse optogenetics of G protein signaling by zebrafish non-visual opsin Opn7b for synchronization of neuronal networks

Opn7b is a non-visual G protein-coupled receptor expressed in zebrafish. Here we find that Opn7b expressed in HEK cells constitutively activates the $G_{i/o}$ pathway and illumination with blue/green light inactivates G protein-coupled inwardly rectifying potassium channels. This suggests that light acts as an inverse agonist for Opn7b and can be used as an optogenetic tool to inhibit neuronal networks in the dark and interrupt constitutive inhibition in the light. Consistent with this prediction, illumination of recombinant expressed Opn7b in cortical pyramidal cells results in increased neuronal activity. In awake mice, light stimulation of Opn7b expressed in pyramidal cells of somatosensory cortex reliably induces generalized epileptiform activity within a short (<10 s) delay after onset of stimulation. Our study demonstrates a reversed mechanism for G protein-coupled receptor control and Opn7b as a tool for controlling neural circuit properties

Lamprey Parapinopsin ("UVLamP"): a Bistable UV‐Sensitive Optogenetic Switch for Ultrafast Control of GPCR Pathways

Optogenetics uses light‐sensitive proteins, so‐called optogenetic tools, for highly precise spatiotemporal control of cellular states and signals. The major limitations of such tools include the overlap of excitation spectra, phototoxicity, and lack of sensitivity. The protein characterized in this study, the Japanese lamprey parapinopsin, which we named UVLamP, is a promising optogenetic tool to overcome these limitations. Using a hybrid strategy combining molecular, cellular, electrophysiological, and computational methods we elucidated a structural model of the dark state and probed the optogenetic potential of UVLamP. Interestingly, it is the first described bistable vertebrate opsin that has a charged amino acid interacting with the Schiff base in the dark state, that has no relevance for its photoreaction. UVLamP is a bistable UV‐sensitive opsin that allows for precise and sustained optogenetic control of G protein‐coupled receptor (GPCR) pathways and can be switched on, but more importantly also off within milliseconds via lowintensity short light pulses. UVLamP exhibits an extremely narrow excitation spectrum in the UV range allowing for sustained activation of the Gi/o pathway with a millisecond UV light pulse. Its sustained pathway activation can be switched off, surprisingly also with a millisecond blue light pulse, minimizing phototoxicity. Thus, UVLamP serves as a minimally invasive, narrow‐bandwidth probe for controlling the G$_{i/o}$ pathway, allowing for combinatorial use with multiple optogenetic tools or sensors. Because UVLamP activated Gi/o signals are generally inhibitory and decrease cellular activity, it has tremendous potential for health‐related applications such as relieving pain, blocking seizures, and delaying neurodegeneration