• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 9
  • 3
  • 1
  • 1
  • Tagged with
  • 13
  • 10
  • 8
  • 5
  • 5
  • 5
  • 3
  • 3
  • 3
  • 3
  • 3
  • 3
  • 2
  • 2
  • 2
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Photodisintegration studies of astrophysically relevant p-nuclei

Nair, Chithra Kumaran 01 October 2013 (has links) (PDF)
The majority of the light elements up to iron (Fe) are formed by successive rounds of thermonuclear fusion burning in the stellar interiors. The nuclei heavier than iron (Z>26) are being synthesized mainly by neutron-capture reactions - the astrophysical r-and s-processes. There are 35 neutron deficient stable isotopes between Se and Hg which are shielded from the rapid neutron capture by stable isobars. These so-called p-nuclei are produced in explosive stellar environments via photodisintegration reactions like (γ,n), (γ,p) and (γ,α) on r- or s-seed nuclei. The reaction rates of the p-nuclei are mostly based on theoretical parameterizations using statistical model calculations. At the bremsstrahlung facility of the superconducting electron accelerator ELBE, photon-induced reactions of the p-nuclei are being studied. In the scope of this thesis work, photodisintegration measurements of the p-nuclei 92Mo and 144Sm have been performed via the photoactivation technique. The residual nuclei resulting from photoactivation were studied via γ-ray spectroscopy. For the decay measurements of short-lived nuclei, a pneumatic delivery system has been used. In the case of 144Sm(°,p) and 144Sm(γ,α) reactions, the activated samarium samples with very low counting statistics were measured at the underground laboratory "Felsenkeller" in Dresden. The experimental activation yields for the 144Sm (γ,n), (γ,p) and (γ, α) and the 92Mo(γ,α) reactions were determined. It is to be emphasized that the (γ,p) and (γ,α) reactions were measured for the first time in a laboratory at astrophysically relevant energies. In all the mentioned experiments, special care was taken to determine the endpoint energy of the bremsstrahlung spectra by using the photodisintegration of deuteron. The 197Au(γ,n)196Au reaction has been established as an activation standard. The photoactivation yields for the 197Au(γ,n) and 144Sm(γ,n) reactions have been compared to the yield calculated using cross sections from previous photoneutron experiments. A comparison of the two data sets leads to a conclusion on the inaccuracies in previous data. The statistical uncertainties involved in the activation experiments are very small except for the case of decay spectra with weak counting statistics. The systematic uncertainties are mostly from the experimental determination of photon flux. A detailed discussion of the overall uncertainty is provided. Hauser-Feshbach statistical model calculations using TALYS and NON-SMOKER codes have been performed for all the concerned reactions. The experimental activation yields, in general, agree within a factor of 2 to the simulated yields using statistical model predictions. The sensitivity of the model codes to the nuclear physics inputs like optical-model potentials, nuclear level densities and γ-ray strength functions has been tested.
2

Photodisintegration studies of astrophysically relevant p-nuclei

Nair, Chithra Kumaran 01 October 2013 (has links)
The majority of the light elements up to iron (Fe) are formed by successive rounds of thermonuclear fusion burning in the stellar interiors. The nuclei heavier than iron (Z>26) are being synthesized mainly by neutron-capture reactions - the astrophysical r-and s-processes. There are 35 neutron deficient stable isotopes between Se and Hg which are shielded from the rapid neutron capture by stable isobars. These so-called p-nuclei are produced in explosive stellar environments via photodisintegration reactions like (γ,n), (γ,p) and (γ,α) on r- or s-seed nuclei. The reaction rates of the p-nuclei are mostly based on theoretical parameterizations using statistical model calculations. At the bremsstrahlung facility of the superconducting electron accelerator ELBE, photon-induced reactions of the p-nuclei are being studied. In the scope of this thesis work, photodisintegration measurements of the p-nuclei 92Mo and 144Sm have been performed via the photoactivation technique. The residual nuclei resulting from photoactivation were studied via γ-ray spectroscopy. For the decay measurements of short-lived nuclei, a pneumatic delivery system has been used. In the case of 144Sm(°,p) and 144Sm(γ,α) reactions, the activated samarium samples with very low counting statistics were measured at the underground laboratory "Felsenkeller" in Dresden. The experimental activation yields for the 144Sm (γ,n), (γ,p) and (γ, α) and the 92Mo(γ,α) reactions were determined. It is to be emphasized that the (γ,p) and (γ,α) reactions were measured for the first time in a laboratory at astrophysically relevant energies. In all the mentioned experiments, special care was taken to determine the endpoint energy of the bremsstrahlung spectra by using the photodisintegration of deuteron. The 197Au(γ,n)196Au reaction has been established as an activation standard. The photoactivation yields for the 197Au(γ,n) and 144Sm(γ,n) reactions have been compared to the yield calculated using cross sections from previous photoneutron experiments. A comparison of the two data sets leads to a conclusion on the inaccuracies in previous data. The statistical uncertainties involved in the activation experiments are very small except for the case of decay spectra with weak counting statistics. The systematic uncertainties are mostly from the experimental determination of photon flux. A detailed discussion of the overall uncertainty is provided. Hauser-Feshbach statistical model calculations using TALYS and NON-SMOKER codes have been performed for all the concerned reactions. The experimental activation yields, in general, agree within a factor of 2 to the simulated yields using statistical model predictions. The sensitivity of the model codes to the nuclear physics inputs like optical-model potentials, nuclear level densities and γ-ray strength functions has been tested.
3

Kernels of localities

Grazian, Valentina, Henke, Ellen 04 April 2024 (has links)
We state a sufficient condition for a fusion system to be saturated. This is then used to investigate localities with kernels: that is, localities that are (in a particular way) extensions of groups by localities. As an application of these results, we define and study certain products in fusion systems and localities, thus giving a new method to construct saturated subsystems of fusion systems.
4

Komplexität und Stabilität von kernbasierten Rekonstruktionsmethoden / Complexity and Stability of Kernel-based Reconstructions

Müller, Stefan 21 January 2009 (has links)
No description available.
5

Estrogen Receptor-Beta Dependent Activities of Dietary Compounds in a Genetically Modified Rat Raphe Nuclei-Derived Cell Line

Amer, Dena Ahmed Mohamed 21 July 2011 (has links) (PDF)
Estrogens greatly affect the activity and connectivity of serotonergic neural cell populations, which extend from clusters of nuclei in the brainstem, termed the raphe nuclei, where estrogen receptor β is the most abundantly expressed estrogen receptor subtype. Estrogenic effects on the raphe nuclei are primarily important for influencing various neuropsychological behaviors, including depression, mood swings and anxiety behaviors. Because of this connection, phases of intense hormone fluctuations for instance during menopause are often associated with several mood disturbances that often reduce the quality of life of menopausal women. Accordingly, long-term use of hormone replacement therapy appeared to be the method of choice for many menopausal women to help alleviate vasomotor symptoms, which may include neuropsychological changes such as depression. However, given the limitations and number of serious health risks attributed to hormone replacement therapy, natural compounds such as phytoestrogens are receiving widespread awareness due to their occurrence in medicinal plant extracts and a wide variety of food items including dietary supplements with respective health claims. Flavonoids, particularly the isoflavones and the naringenin-type flavanones, belong to a group of polyphenolic plant-derived secondary metabolites known to possess estrogen-like bioactivities. Nevertheless, little is known about their transactivational activity and their potential to regulate endogenous gene expression of estrogen responsive genes in the raphe nuclei due to the lack of suitable cellular models expressing sufficient amounts of functional estrogen receptor β. Hence, a raphe nuclei-derived cell line that expresses a functional estrogen receptor β was sought as a model to investigate effects of flavonoids in vitro. In this regard, RN46A-B14 cells derived from embryonic day 13 rat medullary raphe nuclei were primarily used in this study as the main cellular model. Nonetheless, expression of endogenous estrogen receptor β in these cells was not sufficient to pursue downstream investigations of estrogen-dependent activities. To overcome this deficit, a rat raphe nuclei-derived in vitro model that overexpresses a functional estrogen receptor β was initially established (herein termed RNDA cells) by stably transducing its parent cell line, RN46A-B14 cells, with a suitable lentiviral expression vector encoding a human estrogen receptor β gene. The stable expression and the functional characterization of the transgenic receptor was confirmed by Western blot analysis and luciferase reporter gene assays, respectively. The same reporter gene assay was used to scrutinize the transactivational activity of the flavonoids in RNDA cells. Key results revealed that Genistein, Daidzein, Equol, Naringenin and 8-Prenylnaringenin demonstrated high transactivational activity in a concentration-dependent manner by stimulating luciferase expression from an estrogen responsive element-regulated reporter gene construct transiently transfected in RNDA cells. Low transactivational activity was observed in RNDA cells in response to increasing concentrations of 7-(O-prenyl)naringenin-4'-acetate. However, no transactivational activity was noticed in response to 6-(1,1-Dimethylallyl)naringenin in the studied cell model. All effects elicited by the flavonoids were antagonized by the pure estrogen receptor antagonist, Fulvestrant, indicating that all substances act by binding to and activating the transgenic ERβ. Additional effects were observed in RNDA cells in response to a co-treatment of 1 µM of either Genistein or Daidzein, but not Equol, with 10 nM 17β-Estradiol. Slight antagonistic effects were observed in the same studied cell line when either 8-Prenylnaringenin or 7-(O-prenyl)naringenin-4'-acetate, but not Naringenin or 6-(1,1-Dimethylallyl)naringenin, were co-added with 17β-Estradiol. Results from the reporter gene assays were validated on the basis of regulation of mRNA expression of estrogen responsive genes following the global assessment of 17β-Estradiol-induced gene expression in this cell line using a DNA microarray technique. Out of 212 estrogen-regulated genes with at least two-fold change of expression, six were selected according to specific features of estrogenic regulation of expression. The expression of the six selected 17β-Estradiol-regulated genes was validated using quantitative real-time PCR analysis. The regulation of mRNA expression of the selected genes in response to the tested flavonoids was then investigated in RNDA cells. Additionally, because RNDA cells encode a temperature-sensitive mutant of the Simian Virus 40 large T-antigen, their neuronal differentiation is constitutive upon shifting them from conditions promoting proliferation (permissive temperature) to differentiation (non permissive temperature). Hence, the regulation of mRNA expression of the selected genes in response to the tested flavonoids was additionally investigated as RNDA cells differentiate. In RNDA cells grown under proliferative conditions, 17β-Estradiol up-regulated mRNA expression of camello-like 5, sex determining region Y-box 18 and keratin type I cytoskeletal 19. Similar effects were observed in response to 8-Prenylnaringenin, Genistein, Daidzein and Equol. In addition, 17β-Estradiol down-regulated mRNA expression of neurofilament medium polypeptide and zinc finger DHHC-type containing 2. Similar effects were observed in response to 8-Prenylnaringenin, Naringenin, Genistein, Daidzein and Equol. Yet, no effect was observed on the regulation of mRNA expression of solute carrier family 6 member 4 in response to 17β-Estradiol or the flavonoids in RNDA cells grown under proliferative conditions. When RNDA cells were shifted to conditions promoting differentiation, changes in cell morphology, in mRNA expression levels and in responsiveness towards 17β-Estradiol or the flavonoids were observed. These expression studies additionally highlighted some of the genes as indicator genes for RNDA cellular differentiation. The newly established RNDA cell line should prove useful to elucidate basic physiological properties of estrogen receptor β in the raphe nuclei. The present study should serve as the basis to help shed light on molecular and cellular mechanisms following the action of phytoestrogens, endocrine disruptors or other exogenous estrogen receptor ligands in neural cell populations, particularly the raphe nuclei, for further applications within the brain.
6

混合分配下之估計模型鑑別力比較 / Comparison of Estimating Discriminatory Power under Mixed Model

廖雅薇 Unknown Date (has links)
銀行在評分模型建置完成後需進行驗證工作,以瞭解評分模型是否能有效評出客戶的風險層級,穩健地估計區別鑑別力指標為驗證工作中的重點。在先前的文獻中假設正常授信戶與違約戶分數分配為常態分配。但在實際資料中,分配未必定為常態。因此本文接著探討在正常授信戶與違約授信戶之分配為混合分配,即兩分數分配為偏斜常態分配下,何種方法可以對於估計AUC具有較高的穩定性。本文比較五種估計AUC的方法,分別為常態核,經驗分配,曼惠尼近似,最大摡似法和EM演算法。模擬結果呈現(1)投信戶組合分配為兩常態分配下,最大摡似法在大部分違約率下都可以得到較窄的信賴區間。(2)組合分配為一常態與一偏斜常態及兩偏斜常態分配下,EM演算法在大部分情況有較窄的信賴區間,其中在兩偏斜常態分配下,表現更佳。(3)曼惠尼近似建構的信賴區間寬度最大,代表曼惠尼近似是較保守的估計方法。 / Banks face discrimination after constructing the rating systems to figure out whether the systems can discriminate defaulting and non-defaulting borrowers. Literature assumed the two score distribuion are normal distributed. However, the real data may not be normal distribuions. We assum the two score distribuions are skewed normal distribuions to discuss which method has more robustness to estimate the AUC value.Under skewed distribution, we propose EM algorithm to estimate the population parametric. If used properly, information about the population properties may be used to get better accuracy of estimation the AUC value.Numerical results show the EM algorithm method , comparing with other methods, has robustness in detect the rating systems have discirmatory power.
7

Estrogen Receptor-Beta Dependent Activities of Dietary Compounds in a Genetically Modified Rat Raphe Nuclei-Derived Cell Line

Amer, Dena Ahmed Mohamed 10 June 2011 (has links)
Estrogens greatly affect the activity and connectivity of serotonergic neural cell populations, which extend from clusters of nuclei in the brainstem, termed the raphe nuclei, where estrogen receptor β is the most abundantly expressed estrogen receptor subtype. Estrogenic effects on the raphe nuclei are primarily important for influencing various neuropsychological behaviors, including depression, mood swings and anxiety behaviors. Because of this connection, phases of intense hormone fluctuations for instance during menopause are often associated with several mood disturbances that often reduce the quality of life of menopausal women. Accordingly, long-term use of hormone replacement therapy appeared to be the method of choice for many menopausal women to help alleviate vasomotor symptoms, which may include neuropsychological changes such as depression. However, given the limitations and number of serious health risks attributed to hormone replacement therapy, natural compounds such as phytoestrogens are receiving widespread awareness due to their occurrence in medicinal plant extracts and a wide variety of food items including dietary supplements with respective health claims. Flavonoids, particularly the isoflavones and the naringenin-type flavanones, belong to a group of polyphenolic plant-derived secondary metabolites known to possess estrogen-like bioactivities. Nevertheless, little is known about their transactivational activity and their potential to regulate endogenous gene expression of estrogen responsive genes in the raphe nuclei due to the lack of suitable cellular models expressing sufficient amounts of functional estrogen receptor β. Hence, a raphe nuclei-derived cell line that expresses a functional estrogen receptor β was sought as a model to investigate effects of flavonoids in vitro. In this regard, RN46A-B14 cells derived from embryonic day 13 rat medullary raphe nuclei were primarily used in this study as the main cellular model. Nonetheless, expression of endogenous estrogen receptor β in these cells was not sufficient to pursue downstream investigations of estrogen-dependent activities. To overcome this deficit, a rat raphe nuclei-derived in vitro model that overexpresses a functional estrogen receptor β was initially established (herein termed RNDA cells) by stably transducing its parent cell line, RN46A-B14 cells, with a suitable lentiviral expression vector encoding a human estrogen receptor β gene. The stable expression and the functional characterization of the transgenic receptor was confirmed by Western blot analysis and luciferase reporter gene assays, respectively. The same reporter gene assay was used to scrutinize the transactivational activity of the flavonoids in RNDA cells. Key results revealed that Genistein, Daidzein, Equol, Naringenin and 8-Prenylnaringenin demonstrated high transactivational activity in a concentration-dependent manner by stimulating luciferase expression from an estrogen responsive element-regulated reporter gene construct transiently transfected in RNDA cells. Low transactivational activity was observed in RNDA cells in response to increasing concentrations of 7-(O-prenyl)naringenin-4'-acetate. However, no transactivational activity was noticed in response to 6-(1,1-Dimethylallyl)naringenin in the studied cell model. All effects elicited by the flavonoids were antagonized by the pure estrogen receptor antagonist, Fulvestrant, indicating that all substances act by binding to and activating the transgenic ERβ. Additional effects were observed in RNDA cells in response to a co-treatment of 1 µM of either Genistein or Daidzein, but not Equol, with 10 nM 17β-Estradiol. Slight antagonistic effects were observed in the same studied cell line when either 8-Prenylnaringenin or 7-(O-prenyl)naringenin-4'-acetate, but not Naringenin or 6-(1,1-Dimethylallyl)naringenin, were co-added with 17β-Estradiol. Results from the reporter gene assays were validated on the basis of regulation of mRNA expression of estrogen responsive genes following the global assessment of 17β-Estradiol-induced gene expression in this cell line using a DNA microarray technique. Out of 212 estrogen-regulated genes with at least two-fold change of expression, six were selected according to specific features of estrogenic regulation of expression. The expression of the six selected 17β-Estradiol-regulated genes was validated using quantitative real-time PCR analysis. The regulation of mRNA expression of the selected genes in response to the tested flavonoids was then investigated in RNDA cells. Additionally, because RNDA cells encode a temperature-sensitive mutant of the Simian Virus 40 large T-antigen, their neuronal differentiation is constitutive upon shifting them from conditions promoting proliferation (permissive temperature) to differentiation (non permissive temperature). Hence, the regulation of mRNA expression of the selected genes in response to the tested flavonoids was additionally investigated as RNDA cells differentiate. In RNDA cells grown under proliferative conditions, 17β-Estradiol up-regulated mRNA expression of camello-like 5, sex determining region Y-box 18 and keratin type I cytoskeletal 19. Similar effects were observed in response to 8-Prenylnaringenin, Genistein, Daidzein and Equol. In addition, 17β-Estradiol down-regulated mRNA expression of neurofilament medium polypeptide and zinc finger DHHC-type containing 2. Similar effects were observed in response to 8-Prenylnaringenin, Naringenin, Genistein, Daidzein and Equol. Yet, no effect was observed on the regulation of mRNA expression of solute carrier family 6 member 4 in response to 17β-Estradiol or the flavonoids in RNDA cells grown under proliferative conditions. When RNDA cells were shifted to conditions promoting differentiation, changes in cell morphology, in mRNA expression levels and in responsiveness towards 17β-Estradiol or the flavonoids were observed. These expression studies additionally highlighted some of the genes as indicator genes for RNDA cellular differentiation. The newly established RNDA cell line should prove useful to elucidate basic physiological properties of estrogen receptor β in the raphe nuclei. The present study should serve as the basis to help shed light on molecular and cellular mechanisms following the action of phytoestrogens, endocrine disruptors or other exogenous estrogen receptor ligands in neural cell populations, particularly the raphe nuclei, for further applications within the brain.
8

Damage of bearings caused by electrical discharge currents at large drives derived from latest field research results

Tröger, Sven, Kröger, Matthias 28 February 2020 (has links)
Bearing currents are not all the same. Under certain circumstances and special use cases classic bearing insulations are not sufficient anymore to prevent bearing currents due to the operation of frequency converters. Additional corrective measures have to be implemented to reduce the source of bearing currents the common mode current. The usage of nanocrystalline tape wound cores shows high effectiveness. As part of a big field study with more than 50 large drive trains in the primary industry, the damaging mechanics of bearing currents are examined under real conditions. Of exceptional high interest is the influence of disturbances which can hardly be simulated in the laboratory. Additional to the shielded motor cable parallel installed functional potential equalization cables applied multiple times have almost no effect in regard to reducing the bearing current. With an optimal installed functional potential equalization system more than 95 percent of the common mode current can flow back through the motor cable shield to the converter. The disturbance impact in the field can influence the voltage over the bearing that breakthroughs are favored but also reduced.
9

Theoretical investigations of nuclear quantum effects in weakly bonded metal-molecular interfaces

Fidanyan, Karen 23 March 2023 (has links)
In dieser Dissertation diskutiere ich theoretische Methoden zur Simulation von Grenzflächen zwischen Metallen und Molekülen auf atomarer Maßstabsebene, die für die Speicherung und Erzeugung "sauberer" Energie von Bedeutung sind, und wende sie an. Wir verwenden die Dichtefunktionaltheorie für das elektronische Subsystem und verschiedene Methoden wie die (quasi-)harmonische Näherung und die Pfadintegral-Molekulardynamik, um die Quanteneigenschaften des nuklearen Subsystems zu berücksichtigen. Wir berechnen den Isotopeneffekt auf die Arbeitsfunktion von Cyclohexan, das an der Rh(111)-Oberfläche adsorbiert wird, ein Effekt, der sich aus der Elektron-Phonon-Kopplung nur dann ergibt, wenn die nuklearen Freiheitsgrade quantenmechanisch behandelt werden. Deuteriertes Cyclohexan C6D12 hat einen größeren Adsorptionsabstand als gewöhnliches Cyclohexan. Pfadintegral-Molekulardynamiksimulationen zeigen auch eine temperaturabhängige Renormierung der elektronischen Zustandsdichte in diesem System. Schließlich befassen wir uns mit Oberflächenreaktionen auf einer geladenen metallischen Oberfläche. Wir stellen unsere Implementierung der Nudged-Elastic-Band-Methode (NEB) im i-PI-Paket vor und diskutieren ihre Leistungsfähigkeit. Anschließend setzen wir die Methode ein, um die Energiebarriere der Wasserspaltungsreaktion auf einer Pd(111)-Oberfläche zu berechnen, die einem elektrischen Feld unterschiedlicher Intensität ausgesetzt ist. Wir zeigen, dass die niedrigste Dissoziationsbarriere auftritt, wenn das Feld eine Stärke erreicht, die eine geometrische Frustration des auf der Oberfläche adsorbierten Wassermoleküls hervorruft, und dass die Nullpunktenergiebeiträge zur Barriere dieser Reaktion über den weiten Bereich der auf das System angelegten elektrischen Feldstärken nahezu konstant bleiben. Wir erklären dies durch eine gegenseitige Aufhebung der Rot- und Blauverschiebungen einzelner Schwingungsmoden zwischen Reaktant und Übergangszustand. / In this thesis, I discuss and apply theoretical methods for simulating interfaces between metals and molecules of relevance to "clean" energy storage and production on an atomistic scale. We use density-functional theory for the electronic subsystem and various methods such as (quasi-)harmonic approximation and path integral molecular dynamics to account for quantum properties of the nuclear subsystem, determining which methods are sufficient to grasp the essential phenomena while remaining computationally affordable. We calculate isotope effect on the work function of cyclohexane adsorbed on Rh(111) surface, an effect that emerges from electron-phonon coupling only when the nuclear degrees of freedom are treated quantum-mechanically. Deuterated cyclohexane C6D12 has larger adsorption distance than ordinary cyclohexane. Path integral molecular dynamics simulations also show a temperature-dependent renormalization of the electronic density of states in this system, induced by both thermal and quantum fluctuations of nuclei. Finally, we address surface reactions on a charged metallic surface. We present our implementation of the nudged elastic band (NEB) method in i-PI package and discuss its performance. We then employ the method to calculate the energy barrier of water splitting reaction on a Pd(111) surface subjected to electric fields of different strengths. We show that the lowest dissociation barrier takes place when the field reaches a strength that induces a geometric frustration of the water molecule adsorbed on the surface, and that the zero-point energy contributions to the barrier of this reaction remain nearly constant across the wide range of electric field strengths applied to the system. We explain this by a mutual cancellation of the red and blue shifts of individual vibrational modes between reactant and transition states.
10

Search for axion-like particles through their effects on the transparency of the universe with the fermi large area telescope

Gallardo Romero, Galo 26 June 2020 (has links)
Axionartige Teilchen sind pseudoscalare Teilchen welche in Theorien jenseits des Standardmodells vorhergesagt werden. Falls ein axionartiges Teilchen innerhalb eines kosmischen magnetischen Felds gebildet wird, wird dieses nicht durch das Hintergrundlicht absorbiert. Daher kann es kosmische Distanzen überbrücken bevor es wieder in ein Photon zurück oszilliert. Dieser Effekt erhöht die Reichweite der Gammastrahlung im Universum. Im Rahmen dieser Dissertation werden Daten des Fermi Large Area Telescopes, aufgenommen über eine Zeitraum von sechs Jahren, systematisch analysiert. Hierbei wird nach axionartigen Teilchen mit Hilfe von Transparenzeffekten des Universums gesucht. In diesem Zusammenhang werden verschiedene Modelle des extragalaktischen Hintergrundlichts mit und ohne Berücksichtigung axionartiger Teilchen verglichen. Hierfür werden Likelihood-Funktionen für das höchst energetische Photon verschiedener entfernter Quelle kombiniert. Diese sind aktive galaktische Kerne mit einer Rotverschiebung z ≥0.1 des Second Catalog of Hard Fermi-LAT Sources. Unter den Annahmen einer intergalaktischen magnetischen Feldstärke von B = 1 nG und einer Kohärenzlänge von s = 1 Mpc wurde keine Veränderungen der Transparenz durch axionähnliche Teilchen nachgewiesen. Für eine Masse eines axionartigen Teilchens mit m≅ 3.0 neV wird eine Photonen-Axion Kopplungskonstante über 10(^11) GeV(^−1) ausgeschlossen. / Axion-like particles, pseudo-scalar particles that arise in theories beyond the Standard Model, mix with photons in the presence of magnetic fields. If an axion-like particle is produced within a cosmic magnetic field, it evades extragalactic background light absorption and thus it can survive cosmological distances until oscillating back into a photon. This leads to an increased transparency of the Universe to gamma rays. In the scope of this thesis, we search for transparency effects compatible with the existence of axion-like particles with six years of data from the Fermi Large Area Telescope. We derive and combine the likelihoods of the highest-energy photon events from a sample of hard distant sources, in order to compare models that include axion-like particles and models with only extragalactic background light. The sources are active galactic nuclei from the Second Catalog of Hard Fermi sources at redshift z≥0.1. For values of the intergalactic magnetic field strength B = 1 nG and coherence length s = 1 Mpc, we find no evidence for a modified transparency induced by axion-like particles and therefore we set upper limits. We exclude photon-axion coupling constants above 10(^11) GeV(^−1) for axion masses m≅ 3.0 neV.

Page generated in 0.05 seconds