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11.
TA Anisotropy Summary
K. Kawata, Jon Paul Lundquist, 2019, published scientific conference contribution

Abstract: The Telescope Array (TA) is the largest ultra-high-energy cosmic-ray (UHECR) detector in the northern hemisphere. It consists of an array of 507 surface detectors (SD) covering a total 700 km^2 and three fluorescence detector stations overlooking the SD array. In this proceedings, we summarize recent results on the search for directional anisotropy of UHECRs using the latest dataset collected by the TA SD array. We obtained hints of the anisotropy of the UHECRs in the northern sky from the various analyses.
Keywords: cosmic radiation, UHE detector, fluorescence detector, surface, Telescope Array Experiment, anisotropy, experimental results
Published in RUNG: 28.04.2020; Views: 2849; Downloads: 78
.pdf Full text (1,88 MB)

12.
Search for Anisotropy in the Ultra High Energy Cosmic Ray Spectrum using the Telescope Array Surface Detector
R.U. Abbasi, Jon Paul Lundquist, 2017, other component parts

Abstract: The Telescope Array (TA) experiment is located in the western desert of Utah, USA, and observes ultra high energy cosmic rays (UHECRs) in the Northern hemisphere. At the highest energies, E>10~EeV, the shape of cosmic ray energy spectrum may carry an imprint of the source density distribution along the line of sight different in different directions of the sky. In this study, we search for such directional variations in the shape of the energy spectrum using events observed with the Telescope Array's surface detector. We divide the TA field of view into two nearly equal-exposure regions: the "on-source" region which we define as ±30∘ of the supergalactic plane containing mostly nearby structures, and the complementary "off-source" region where the sources are further away on average. We compare the UHECR spectra in these regions by fitting them to the broken power law and comparing the resulting parameters. We find that the off-source spectrum has an earlier break at highest energies. The chance probability to obtain such or larger difference in statistically equivalent distributions is estimated as 6.2±1.1×10−4 (3.2σ) by a Monte-Carlo simulation. The observed difference in spectra is in a reasonable quantitative agreement with a simplified model that assumes that the UHECR sources trace the galaxy distribution from the 2MRS catalogue, primary particles are protons and the magnetic deflections can be neglected.
Keywords: cosmic radiation: UHE, detector: surface, cosmic radiation: spectrum, cosmic radiation: energy spectrum, deflection: magnetic, numerical calculations: Monte Carlo, anisotropy
Published in RUNG: 27.04.2020; Views: 2744; Downloads: 98
.pdf Full text (169,05 KB)

13.
Shaping FEL radiation : from multipulse/multicolor emission to generation of twisted light
Primož Rebernik Ribič, 2016, published scientific conference contribution abstract (invited lecture)

Keywords: radiation, synchrotron, free-electron laser
Published in RUNG: 16.01.2020; Views: 2757; Downloads: 11
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Coherent THz Emission Enhanced by Coherent Synchrotron Radiation Wakefield
Primož Rebernik Ribič, Giovanni De Ninno, 2018, original scientific article

Abstract: We demonstrate that emission of coherent transition radiation by a ∼1 GeV energy-electron beam passing through an Al foil is enhanced in intensity and extended in frequency spectral range, by the energy correlation established along the beam by coherent synchrotron radiation wakefield, in the presence of a proper electron optics in the beam delivery system. Analytical and numerical models, based on experimental electron beam parameters collected at the FERMI free electron laser (FEL), predict transition radiation with two intensity peaks at ∼0.3 THz and ∼1.5 THz, and extending up to 8.5 THz with intensity above 20 dB w.r.t. the main peak. Up to 80-µJ pulse energy integrated over the full bandwidth is expected at the source, and in agreement with experimental pulse energy measurements. By virtue of its implementation in an FEL beam dump line, this work promises dissemination of user-oriented multi-THz beamlines parasitic and self-synchronized to EUV and x-ray FELs.
Keywords: terahertz radiation, free-electron lasers, coherent transition radiation
Published in RUNG: 12.01.2020; Views: 2855; Downloads: 0
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17.
Au and Ag on the Bi2Se3(0001) Surface: Experimental Electronic and Physical Properties
Sandra Gardonio, Mattia Fanetti, Katja Ferfolja, Matjaž Valant, published scientific conference contribution abstract

Abstract: Binary bismuth chalcogenides, Bi2Se3 and Bi2Te3, have been extensively studied as reference topological insulators (TIs). These materials are bulk insulators with topological surface states (TSS) crossing the Fermi level. In contrast to conventional surface states of metals, the TSS are extremely robust against local modifications at the surface, such as adsorbed adatoms, localized defects or changes in the surface termination. This aspect makes the TIs attractive for applications in spintronics, plasmonics, quantum computing and catalysis. A theoretical model of charge transport by the TI surface states predicts that the TSS survive, provided that bonding at the metal/TI interface is weak. Ab-initio calculations have been done to understand the electronic properties of Au, Ni, Pt, Pd and graphene layers in a contact with Bi2Se3. These calculations showed that for Au and graphene the spin-momentum locking of TSS is maintained at the interface. In another theoretical study, Ag and Au thin layers on Bi2Se3 have been predicted to show a large Rashba splitting and a high spin polarization of the Ag quantum wells, providing a great potential for development of the spintronic devices. Finally, the calculations have foreseen that the presence of the robust TSS affects the adsorption properties of metals (Au bi-layer and clusters of Au, Ag, Cu, Pt, and Pd) supported on TI, in some cases resulting in the enhancement of the catalytic processes. Despite the fundamental importance of the metal/TI interfaces and a number of theoretical studies predicting exotic interfacial phenomena, the experimental knowledge about the metals on the TI surfaces is surprisingly limited, especially concerning combined study of morphology, growth mode, electronic and chemical properties. In order to exploit the predicted physical properties of such systems, it is especially important to extend the study above the diluted coverage regime and to understand what is the growth morphology of the metal on the TI surface, to what extent the metal overlayer interacts with the TI substrate, how the TSS change with the presence of the metal overlayer and what is the reactivity of the system at the different stages of the overlayer growth. Within this frame, we present a comprehensive surface sensitive study, of Au and Ag on Bi2Se3 by means of ARPES, XPS, SEM, LEED and XRD. The obtained results allow us to discuss the relation between electronic and physical properties at two of the most important model metal/TI interfaces
Keywords: topological insulator, electronic properties, synchrotron radiation
Published in RUNG: 27.06.2019; Views: 3237; Downloads: 0
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OXYGEN-EXCESS RELATED DEFECTS IN SiO2-BASED MATERIALS: COUPLING THEORY AND EXPERIMENTS
Blaž Winkler, 2019, doctoral dissertation

Abstract: This work is primarily focused on application of standard first-principle computational approaches to model oxygen excess related point defects in amorphous silica. Atomic models with their respective electronic and optical properties are explored together with some conversion mechanisms between defect models. The first chapter overviews extensive literature about the already known properties of oxygen related defects. Second chapter briefly introduces main methods that have been used in this research, in particular Density Functional Theory (DFT) as energy and force engine with short description of minimal energy path (MEP) algorithm used for modeling chemical/migration reactions, GW approximation for charged electronic excitations (band structure) and Bethe-Salpeter Equation (BSE) for neutral excitations (optical absorption and excitonic structure including electron hole interaction). The third chapter is devoted to the presentation of results. Thanks to the calculation of optical properties of peroxy bridge (POL), a correlation has been found between structural disorder, specifically dihedral angle dispersion, and low coupling with light, which has been identified as main reason why no clear absorption bands have been assigned to the POL. Structure and stability of some other defects, like interstitial ozone molecule (ozonyl) and dioxasilirane (silicon analogy of dioxirane), have been studied. These defects are usually not considered as most important species, however their calculated formation energies are lower compared to some known defects, which indicates they might be present in silica. From a detailed study on possible reaction mechanisms, it has been found that ozonyl might be one of the most important intermediate steps for oxygen exchange reactions. Results also show that dioxasilirane can be spontaneously created during the interaction of oxygen with lone pair defects. By exploring different reactions between oxygen and pre-existing oxygen deficiency centers (ODCs), calculations predict two kinds of passivation behaviors: single-barrier reversible mechanisms with the formation of dioxasilirane-like groups, for which the network keeps the memory of the precursory lone pair defects, and single or multiple-barrier mechanisms, for which the network loses its memory, either because of the high reverse barrier or because of a reconstruction. Final part of this research has been devoted to experimental characterization of the response and tolerance of optical fibers loaded with oxygen under irradiation. These include experiments on commercial fiber along with canonical samples (Optical fibers developed with the intention of studying correlations between different fabrication parameters, dopant/impurity concentration and doping concentrations). Studied fibers also include rare-earth doped fibers.
Keywords: Silica, DFT, GW-approximation, Bethe-Salpeter equation, NEB, defect, oxygen, oxygen excess centers, oxygen deficiency centers, optical absorption, optical fibers, radiation induced attenuation.
Published in RUNG: 07.05.2019; Views: 4242; Downloads: 198
.pdf Full text (13,18 MB)

20.
Radiation Effects on Optical Fibers and Fiber-Based Sensors
Sylvain Girard, A. Boukenter, Youcef Ouerdane, Nicolas Richard, Philippe Paillet, Layla Martin-Samos, Luigi Giacomazzi, Claude Marcandella, 2015, independent scientific component part or a chapter in a monograph

Keywords: Optical Fibers, radiation-induced point defects, dosimetry, ab-initio
Published in RUNG: 24.10.2018; Views: 3606; Downloads: 0
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