Dissertations / Theses on the topic 'Ray-based'
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1
Lindeberg, Johan. "X-ray based tree ring analyses /." Umeå : Dept. of Silviculture, Swedish Univ. of Agricultural Sciences, 2004. http://epsilon.slu.se/s299.pdf.
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2
Fernandes, Louis Edward. "Pyroelectric crystal-based X-ray diffractometer." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/40901.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (p. 49-50).
We investigate the use of an Amptek Cool-X X-ray Generator for an instructional tool in the physics of x-rays, as well as a source for x-rays for crystal diffraction experiments. The x-ray source is a solid-state two-phase air-cooled source with a time-varying photon output. Two detectors are used in this experiment, the first being an Amptek X-123 Spectrometer and the second a combination scintillator/Polaroid film setup. We collimate the x-ray beam and determine that the system, although low flux and low resolution, will function as a quick and easy tool for the investigation of x-ray physics.
by Louis Edward Fernandes.
S.B.
3
Cline, David. "Sampling Methods in Ray-Based Global Illumination." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd2056.pdf.
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Ollinger, Christoph German Josef. "A waveguide-based lens-less x-ray microscope." Doctoral thesis, [S.l.] : [s.n.], 2006. http://webdoc.sub.gwdg.de/diss/2006/ollinger.
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Fuhse, Christian. "X-ray waveguides and waveguide-based lensless imaging." Doctoral thesis, [S.l.] : [s.n.], 2006. http://webdoc.sub.gwdg.de/diss/2006/fuhse.
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Durany, Vendrell Jaume. "Geometrical room acoustics: ray based simulation for room acoustics." Doctoral thesis, Universitat Pompeu Fabra, 2016. http://hdl.handle.net/10803/395190.
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L’acústica de sales és la ciència encarregada d’estudiar la propagació del so en
entorns tancats. La informació acústica de qualsevol entorn, coneguda com la
resposta impulsional, s’expressa en termes del camp acústic com una funció de
l’espai i el temps. La formulació analítica de la distribució de les variables del so és,
en general, extremadament complexa d’obtenir i només existeixen solucions
d’escenaris molt simples i irreals. Per tant, l’ús d’ordinadors per solucionar aquest
tipus de problemes ha emergit com una alternativa adequada per calular funcions de
resposta.
En aquesta Tesi ens hem centrat en l’ús de mètodes basats en rajos per calcular
funcions de resposta. Més concretament, presentem el disseny i la implementació
d’un motor de traçat de rajos que calcula funcions de resposta en cualsevol entorn
virtual, obtenint no només la funció de resposta per la presió sinó també pel vector de
velocitats del camp acústic. Amb aquesta informació extra tenim totes les dades
necessàries per modelar la propagació del so i podem de forma natural espacialitzar
un so per qualsevol configuració d’altaveus.
Aquesta recerca contribueix als aspectes principals del càlcul de funcions de resposta
utilitzant mètodes basats en rajos. El motor de traçat de rajos que presentem inclou un
mètode desenvolupat per aplicar la solució analítica de la Funció de Distribució
Acústica de Reflectància Bidireccional (A-BRDF) al Model de Dispersió Basat en
Vectors (VBS), fet que redueix molt notablement el cost computacional.Room acoustics is the science devoted to study sound propagation in enclosures where the sound conduction medium is bounded on all sides by walls, ceiling and floor. The acoustic information of any room, the so-called impulse response, is expressed in terms of the acoustic field as a function of space and time. The analytical formulation of the sound variables distribution is, in general, extremely hard to obtain and there only exist solutions of very simple and unrealistic scenarios. Therefore the use of computers for solving this type of problems has emerged as a proper alternative to calculate impulse responses. In this Thesis we focus on the use of the ray-based methods to compute impulse responses. More precisely, we present the design and implementation of a sound ray tracing engine that computes the impulse response in any given environment not only for the pressure but also for the velocity vector of the acoustic field. With this extra information we have all the necessary data to model the propagation of sound and we can then naturally spatialize the sound to any speakers layout. This research contributes to the main aspects in the computation of impulse responses using a ray-based approach. The presented ray tracing engine includes a method developed to apply the analytical solution for the Acoustic Bidirectional Reflectance Distribution Function (A-BRDF) in the Vector Based Scattering Model (VBS), which reduces dramatically the computational cost.
7
Ciydem, Mehmet. "Ray Based Finite Difference Method For Time Domain Electromagnetics." Phd thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12606633/index.pdf.
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In this study, novel Ray Based finite difference method for Time Domain electromagnetics(RBTD) has been developed. Instead of solving Maxwell&rsquos hyperbolic partial differential equations directly, Geometrical Optics tools (wavefronts, rays) and Taylor series have been utilized. Discontinuities of electromagnetic fields lie on wavefronts and propagate along rays. They are transported in the computational domain by transport equations which are ordinary differential equations. Then time dependent field solutions at a point are constructed by using Taylor series expansion in time whose coefficients are these transported distincontinuties. RBTD utilizes grid structure conforming to wave fronts and rays and treats all electromagnetic problems, regardless of their dimensions, as one dimensional problem along the rays. Hence CFL stability condition is implemented always at one dimensional eqaulity case on the ray. Accuracy of RBTD depends on the accuracy of grid generation and numerical solution of transport equations. Simulations for isotropic medium (homogeneous/inhomogeneous) have been conducted. Basic electromagnetic phenomena such as propagation, reflection and refraction have been implemented. Simulation results prove that RBTD eliminates numerical dispersion inherent to FDTD and is promising to be a novel method for computational electromagnetics.
8
Hirvasniemi, J. (Jukka). "Novel X-ray-based methods for diagnostics of osteoarthritis." Doctoral thesis, Oulun yliopisto, 2015. http://urn.fi/urn:isbn:9789526210384.
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Abstract Osteoarthritis (OA) is the commonest joint disease in the world, and it has a major socioeconomic impact. OA causes progressive degenerative changes in the composition and structure of articular cartilage and subchondral bone. Clinical diagnosis of OA is based on physical examination and qualitative evaluation of changes on plain radiographs. Current clinical imaging methods are subjective or insensitive to early OA changes. Therefore, new methods are needed so as to quantify composition of the cartilage and characteristics of the subchondral bone. The aim of this thesis was to evaluate the potential of clinically applicable X-ray-based methods for the assessment of the cartilage proteoglycan content as well as the structure and density of subchondral bone in a knee joint. Subchondral bone density and structure (local binary patterns, Laplacian, and fractal-based algorithms) analysis methods for two-dimensional (2-D) plain radiographs were validated against three-dimensional (3-D) bone microarchitecture obtained from micro-computed tomography ex vivo and applied to plain radiographs in vivo. Furthermore, a method developed for the evaluation of articular cartilage proteoglycan content from computed tomography (CT) was validated against a delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC), which is widely used as a proteoglycan sensitive method, in subjects referred for an arthroscopy of the knee joint. Subchondral bone density and structure evaluated from 2-D radiographs were significantly related to the bone volume fraction and true 3-D microarchitecture of bone, respectively. In addition, bone density- and structure-related parameters from radiographs were significantly different among subjects with different stages of OA. Cartilage proteoglycan content evaluated from CT was significantly related to dGEMRIC method. Furthermore, dGEMRIC was associated with bone structure from a 2-D radiograph. In conclusion, analysis of bone structure and density is feasible from clinically available 2-D radiographs. A novel CT method sensitive to proteoglycan content should be considered when a 3-D view of cartilage quality is neededTiivistelmä Nivelrikko on maailman yleisin nivelsairaus. Se aiheuttaa merkittävää kärsimystä potilaille, ja sillä on suuri taloudellinen vaikutus yhteiskuntaan. Nivelrikko aiheuttaa palautumattomia muutoksia nivelrustokudoksen ja rustonalaisen luun koostumukseen ja rakenteeseen. Nivelrikon diagnoosi perustuu kliiniseen tutkimukseen ja röntgenkuvien silmämääräiseen arviointiin. Nykyiset nivelrikon kliiniset kuvantamismenetelmät ovat subjektiivisia eivätkä riittävän tarkkoja nivelrikon varhaisten muutosten osoittamiseen, minkä vuoksi rustokudoksen koostumuksen ja rustonalaisen luun muutosten arviointiin tarvitaan uusia menetelmiä. Tämän väitöskirjantyön tarkoituksena oli tutkia uusien röntgensäteilyyn perustuvien menetelmien soveltuvuutta polvinivelen rustokudoksen proteoglykaanipitoisuuden sekä luun tiheyden ja rakenteen arviointiin. Rustonalaisen luun tiheyttä ja rakennetta arvioitiin digitaalisesta röntgenkuvasta tietokonepohjaisilla menetelmillä ja tuloksia verrattiin mikrotietokonetomografiassa nähtävään luun kolmiulotteiseen rakenteeseen. Röntgenkuvasta laskettavia muuttujia verrattiin myös eriasteisesta nivelrikosta kärsivien henkilöiden välillä. Rustokudoksen proteoglykaanipitoisuutta epäsuorasti mittaavaa tietokonetomografiamenetelmää verrattiin vastaavaan magneettikuvausmenetelmään henkilöillä, jotka olivat menossa polven niveltähystykseen. Röntgenkuvasta laskettu rustonalaisen luun tiheys ja rakenne olivat tilastollisesti selkeästi yhteydessä luun tilavuusmäärään ja mikrorakenteeseen, ja ne erosivat eriasteisesta nivelrikosta kärsivien henkilöiden välillä. Proteoglykaanipitoisuutta arvioivien tietokonetomografia- ja magneettikuvausmenetelmien välillä oli tilastollisesti merkitsevä korrelaatio. Ruston proteoglykaanipitoisuutta arvioivan magneettikuvausmenetelmän ja röntgenkuvasta laskettavan luun rakenteen välillä oli myös tilastollinen yhteys. Loppupäätelmänä voidaan todeta, että luun tiheyttä ja rakennetta on mahdollista arvioida kliinisesti saatavilla olevista röntgenkuvista. Tietokonetomografiamenetelmän käyttöä tulee harkita tutkimuksissa silloin, kun rustokudoksen tilasta halutaan kolmiulotteista tietoa
9
Ippolito, Alessandro <1977>. "Oblique ionograms automatic scaling and eikonal based ray tracing." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2015. http://amsdottorato.unibo.it/6888/.
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A method for automatic scaling of oblique ionograms has been introduced. This method also provides a rejection procedure for ionograms that are considered to lack sufficient information, depicting a very good success rate. Observing the Kp index of each autoscaled ionogram, can be noticed that the behavior of the autoscaling program does not depend on geomagnetic conditions.
The comparison between the values of the MUF provided by the presented software and those obtained by an experienced operator indicate that the procedure developed for detecting the nose of oblique ionogram traces is sufficiently efficient and becomes much more efficient as the quality of the ionograms improves. These results demonstrate the program allows the real-time evaluation of MUF values associated with a particular radio link through an oblique radio sounding. The automatic recognition of a part of the trace allows determine for certain frequencies, the time taken by the radio wave to travel the path between the transmitter and receiver. The reconstruction of the ionogram traces, suggests the possibility of estimating the electron density between the transmitter and the receiver, from an oblique ionogram. The showed results have been obtained with a ray-tracing procedure based on the integration of the eikonal equation and using an analytical ionospheric model with free parameters. This indicates the possibility of applying an adaptive model and a ray-tracing algorithm to estimate the electron density in the ionosphere between the transmitter and the receiver An additional study has been conducted on a high quality ionospheric soundings data set and another algorithm has been designed for the conversion of an oblique ionogram into a vertical one, using Martyn's theorem. This allows a further analysis of oblique soundings, throw the use of the INGV Autoscala program for the automatic scaling of vertical ionograms.
10
Bardi, Niki. "Tissue equivalent X-ray dosimetry based on carbon nanotubes." Thesis, University of Surrey, 2017. http://epubs.surrey.ac.uk/841216/.
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11
Lee, Kyoung-Jin. "Efficient ray tracing algorithms based on wavefront construction and model based interpolation method." Texas A&M University, 2005. http://hdl.handle.net/1969.1/3771.
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Understanding and modeling seismic wave propagation is important in regional and
exploration seismology. Ray tracing is a powerful and popular method for this purpose.
Wavefront construction (WFC) method handles wavefronts instead of individual
rays, thereby controlling proper ray density on the wavefront. By adaptively controlling
rays over a wavefront, it efficiently models wave propagation. Algorithms for a
quasi-P wave wavefront construction method and a new coordinate system used to
generate wavefront construction mesh are proposed and tested for numerical properties
and modeling capabilities. Traveltimes, amplitudes, and other parameters, which
can be used for seismic imaging such as migrations and synthetic seismograms, are
computed from the wavefront construction method. Modeling with wavefront construction
code is applied to anisotropic media as well as isotropic media. Synthetic
seismograms are computed using the wavefront construction method as a new way
of generating synthetics. To incorporate layered velocity models, the model based
interpolation (MBI) ray tracing method, which is designed to take advantage of the
wavefront construction method as well as conventional ray tracing methods, is proposed
and experimental codes are developed for it. Many wavefront construction
codes are limited to smoothed velocity models for handling complicated problems
in layered velocity models and the conventional ray tracing methods suffer from the
inability to control ray density during wave propagation. By interpolating the wavefront
near model boundaries, it is possible to handle the layered velocity model as well
as overcome ray density control problems in conventional methods. The test results
revealed this new method can be an effective modeling tool for accurate and effective
computing.
12
Dammertz, Holger [Verfasser]. "Acceleration methods for ray tracing based global illumination / Holger Dammertz." Ulm : Universität Ulm. Fakultät für Ingenieurwissenschaften und Informatik, 2011. http://d-nb.info/1016659350/34.
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13
Men, Shuang. "X-ray photoelectron spectroscopy of ionic liquid-based catalytic systems." Thesis, University of Nottingham, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.557146.
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The negligible volatility of ionic liquids allows them to be studied by applying ultrahigh vacuum (UHV) techniques, i.e. X-ray photoelectron spectroscopy (XPS). Consequently, this Thesis describes the XPS of ionic liquid-based metal catalytic systems. Correlation between the binding energy derived from XPS and reaction performance is extensively discussed and used to design a catalytic system for the Suzuki cross coupling reaction. To allow comparable information for the solute dissolved in ionic liquids to be obtained, pure ionic liquids and ionic liquid mixtures are analysed. Fitting models of C 1 s spectra for different ionic liquid families are presented. Charge correction methods, specifically the influence of anion basicity on charge correction, are discussed in detail. The cation-anion interaction is investigated for different families of ionic liquids with single and mixed anions. The interaction between catalysts and ionic liquids can also be demonstrated by XPS. The formation of palladium-containing ylidene complexes in ionic liquids is confirmed by XPS and ESI-MS. The influence of anion basicity on the measured binding energy of the palladium centre is explored. The binding energies of Pd 3d for a range of solutions are plotted against several established metrics, i.e. proton affinity, interaction energy and hydrogen bond acceptor ability (P), and are used to reflect the reaction rates for a model Suzuki reaction. After a proper understanding of the correlation between the binding energy of a metal centre and the reaction rate, a palladium-based catalytic system may be optimised for the Suzuki reaction. The reaction rates observed in ionic liquids with differing basicity anions are monitored and correlated back to the binding energy. XPS is also applied to investigate the interaction of catalysts with ligands in ionic liquid-based rhodium systems. The formation of different rhodium ligand-containing complexes is detected by XPS. The influence of anion basicity on the formation of different rhodium-containing complex is discussed. The measured binding energy of the rhodium centre is correlated to the reaction selectivity in the hydroformylation reaction.
14
Gee, Ian Andrew. "X-ray photoelectron spectroscopy studies of oxide based glass systems." Thesis, University of Warwick, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.365230.
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VIEIRA, PEDRO SAMPAIO. "A SPINE 3D MODELING SYSTEM BASED ON X-RAY IMAGES." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2009. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=14919@1.
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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRONos dias atuais, pesquisas envolvendo a computação gráfica e a área médica, têm contribuído muito para a evolução tecnológica de exames e diagnósticos. Uma vertente desses trabalhos está relacionada diretamente à reconstrução 3D de estruturas anatômicas do corpo humano, em específico a coluna vertebral. O sedentarismo e a alta dependência dos computadores vêm aumentando e agravando os problemas posturais das pessoas. Por esse motivo, novas técnicas de reconstrução 3D baseada em exames de tomografia computadorizada (TC), ressonância magnética (RM) e raios-x são desenvolvidas, tornando as avaliações clínicas cada vez mais precisas. Neste trabalho é proposto um sistema de modelagem 3D baseado em radiografias digitais com a finalidade de recriar a coluna vertebral em um ambiente virtual. A recuperação das informações tridimensionais de cada vértebra ajuda a melhorar a avaliação feita atualmente com base apenas em imagens 2D. A técnica utilizada no desenvolvimento do método se baseia na estereoradiografia. E a utilização de radiografias, em relação à TC, reduz consideravelmente o tempo de exposição do paciente à radiação, além de ser mais acessível à população pelo seu menor custo. Os resultados obtidos apresentaram uma boa precisão do sistema. Além do mais, o método proposto atingiu resultados bem próximos aos de pesquisas baseadas em TC e RM, onde os dados de entrada são bem mais legíveis do que as imagens de raiosx.
Research involving computer graphics and laboratory exams has contributed much to the quality of the Medical diagnose. One aspect of these researches is directly related to 3D reconstruction of anatomical structures of the human body, especially the spine. The sedentary lifestyle and the high dependence of computers have increased the postural problems of the population. Therefore, new techniques for 3D reconstruction based on Computed tomography (CT), magnetic resonance imaging (MRI) and x-ray images are required, in order to make the clinical evaluation increasingly accurate. This work proposes a 3D modeling system based on x-ray images that yields a virtual spine model. The recovery of three-dimensional information of each vertebra helps improve the assessment currently made using only 2D images. The technique used here is based on stereo radiographic. The use of x-ray images instead of CT, significantly reduces the exposure time of the patient to radiation, and is more useful to the general population due to its lower cost. The results presented here show good accuracy despite its lower cost. The proposed method has achieved results very close to those based on expensive CT or MRI, where the input image is better than x-ray images.
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Souza, Geovane Grossi Araújo de. "X-Ray fluorescence imaging system based on Thick-GEM detectors." Universidade de São Paulo, 2019. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-21032019-233121/.
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GEMs (Gas Electron Multiplier) and Thick-GEMs (Thick-Gas Electron Multiplier) are MPGDs (Micropattern Gas Detector) that make part of the new generation of gaseous detectors, allowing high counting rates, low cost when compared to solid state detectors, high radiation hardness and gain when using multiple structures. Besides that, the handling and maintenance of these detectors is relatively simple, being versatile to detect different types of radiation. Therefore, these detectors are an effective alternative to build imaging systems with large sensitive area. This work consists in the study and characterization of a set of gaseous detectors, more specifically the Thick-GEMs produced in the High Energy Physics and Instrumentation Center at IFUSP, which were tested showing promising results in terms of gain, energy resolution and operational stability. However, due to the low signal-to-noise ratio of the Thick-GEMs, the X-ray fluorescence imaging system was mounted using GEMs. During this work the necessary software tools for image processing and reconstruction were developed as a parallel study in computational simulations to better understand the operation of gaseous detectors. X-ray fluorescence techniques are essential in areas such as medicine and the study of historical and cultural heritage since they are non-invasive and non-destructive. Techniques to check the authenticity of masterpieces are required and museums are gradually becoming more interested in the Physics and instrumentation needed to characterize their patrimony.Os GEMs (Gas Electron Multiplier) e Thick-GEMs (Thick-Gas Electron Multiplier) são estruturas do tipo MPGD (Micropattern Gas Detector) que fazem parte da nova geração de detectores de radiação a gás e permitem altas taxas de contagens, baixo custo quando comparados com os detectores de estado sólido, uma elevada resistência à radiação e ganhos elevados, quando utilizadas estruturas múltiplas para multiplicação. Além disso, o manuseio e manutenção desses detectores é relativamente simples, sendo versáteis em relação à montagem podendo detectar diferentes tipos de radiação. Sendo assim, a utilização desses detectores é uma alternativa eficiente para montar um sistema de imagem com grande área sensível. Este trabalho consiste no estudo e caracterização de um conjunto de detectores gasosos, mais especificamente os Thick-GEMs produzidos pelo grupo de Física de altas energias e Instrumentação do IFUSP, que foram testados para serem empregados em um sistema de imagem de fluorescência de raios-X. Os Thick-GEMs testados apresentaram resultados promissores em termos de ganho, resolução em energia e estabilidade operacional. No entanto, devido à baixa relação sinal-ruído, um sistema de imagem de fluorescência de raios-X foi montado utilizando GEMs. Durante o trabalho as ferramentas de software necessárias para processamento e reconstrução de imagens foram desenvolvidas, assim como um estudo paralelo de simulações computacionais para entender melhor o funcionamento de detectores gasosos. Técnicas como o imageamento por fluorescência de raios-X são de suma importância pois são consideradas não invasivas e não destrutivas. Sua utilização tem uma importância imprescindível nas áreas da medicina e na análise de patrimônios histórico e cultural. Atualmente, a verificação e validação de autenticidade de obras é um requisito obrigatório e alguns museus começam a se interessar cada vez mais em áreas da Física e da instrumentação necessária para caracterizar o seu patrimônio.
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Samothrakitis, Stavros. "Neutron & X-ray scattering studies of Fe-based materials." Thesis, University of Birmingham, 2018. http://etheses.bham.ac.uk//id/eprint/8459/.
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Small-angle scattering technique uses the scattering of radiation (e.g. neutrons or X-rays) at small angles to probe large-scale structures withjn matter, up to thousands of Angstroms. It is proven a valuable tool for investigating precipitation in reactor pressure vessel (RPV) steels and Fe-Ga alloys offering a statistical average over a large volume of samples. RPV steels, being of crucial importance for the longevity of a nuclear reactor, have been a long-standing theme for investigations. The main topics of such investigations are the effects of irradiation upon the steels and the consequent implications on their macroscopic properties. In this thesis, small-angle neutron scattering is employed to investigate irradiation induced precipitates in low- and high-Cu RPV steels. After irradiations with protons to low damage levels, precipitates could be clearly observed only in the high-Cu RPV steels. Stable preirradiation formed features are attributed to precipitation of carbides. Fe-Ga binary alloys have attracted much attention due to the still unexplained high magnetostriction they exhibit. To investigate the composition of nanoheterogeneities in a Fe-Ga sample, anomalous small-angle X-ray scattering is employed exploiting the energy dependence of the Fe and Ga atoms near their respective absorption edges. The nanoprecipitates are found to have a Fe3Ga stoichiometry.
18
Chan, Raymond C. "Model-based estimation of arterial diameter from X-ray angiograms." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/41346.
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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1996.Includes bibliographical references (leaves 122-124).
by Raymond C. Chan.
M.S.
19
Cipiccia, Silvia. "Compact gamma-ray sources based on laser-plasma wakefield accelerator." Thesis, University of Strathclyde, 2011. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=23936.
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Laser-plasma wakefield accelerator (LWFA) is a promising novel technology that is introducing miniaturization to the accelerator world: the unprecedented gradient of acceleration shrinks the accelerator down to table-top size. Moreover, the LWFA comes with an embedded light source: electrons, while accelerating, undergo betatron oscillatory motion that results in synchrotron radiation emitted in a narrow cone along the direction of propagation. In this thesis we study theoretically and we prove experimentally a new regime of betatron oscillation that occurs when electrons experience the electromagnetic field of the laser during acceleration and oscillate resonantly at the laser frequency or its sub-harmonics. The signature of the harmonically resonant betatron (HRB) regime is a large oscillation amplitude and consequently prolific emission of high energy photons up to the MeV range. The HRB source has unique properties: very short pulse length (~10 fs), small source size (few microns), high peak brightness of the order of 1023 photons/s mm2 mrad2 0.1% B.W., which is comparable with a third generation light source. These properties make the source particularly appealing for the life sciences and medical and security applications. As a part of a future applications project, we give the scaling of the photon energy as a function of laser intensity and plasma density, which could extend toward tens of MeV. The thesis also investigates another gamma-ray source that utilises beams from the LWFA: bremsstrahlung radiation from high energy electrons interacting with metal targets. We study the electron beam and target parameters to optimize the emission process and the gamma-ray beam properties to match potential application requirements, such as radioisotope generation via photonuclear process. The results of a proof of concept experiment are presented and compared with simulations. Finally, we investigate numerically the possibility of generating a converging gamma ray beam based on the bremsstrahlung process. The results are encouraging, and the potential impact of a compact converging gamma-ray beam source is discussed with particular attention to medical applications in cancer treatment.
20
Sheikh, Muhammad Imran. "Ray-optics based propagation tools for future wireless communication networks." Thesis, University of Birmingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633234.
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The thesis is concerned with the current use of ray-optics based propagation prediction tools for the determination of wideband characteristics of the wireless urban propagation channel. Reliable prediction of propagation channel parameters such as delay spread, coherence bandwidth etc. is important for the deployment of wideband systems. Deterministic propagation models, based on ray-optical techniques make use of site-specific topographic databases, and aimed to yield reliable area coverage and wideband propagation channel parameters. Such models determine wideband wireless channel characteristics by Fourier transforming the radio channel impulse response at a center frequency. This can lead to errors in the derived wideband parameters. A three-dimensional ray-optics based propagation prediction tool is developed. The model uses image theory, Geometrical Optics (GO) and the Uniform Theory of Diffraction (UTD) to determine wideband characteristics of the urban wireless communication channel. Higher order diffractions which can contribute significantly in shadow region are incorporated in addition to first order diffractions. The model is thoroughly validated with single frequency and swept frequency measurements in scaled down controlled environment. Systematic errors are identified in deriving wideband response. Errors (up to 27 dB and 4.1 rad) in the lit regions corresponds to the individual tiny deep nulls which are quite insignificant. In the shadow region average errors gradually increases away from the center frequency and rises up to 1 dB and 0.05 rad. Error sign reversal is observed in the transition regions. Errors corresponding to realistic bandwidths are less than 0.5 dB and 0.03 rad. Errors in frequency correlation function corresponds to shadow regions and frequency separation of 0.25 GHz to 7 GHz. It is concluded that the coherence bandwidth provides more information of the dispersive nature of the channel compared to delay spread.
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Tseng, Hsin-Wu. "Task-Based Image Quality Assessment in X-Ray Computed Tomography." Diss., The University of Arizona, 2015. http://hdl.handle.net/10150/593630.
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In X-Ray CT, there is always a desire to maintain the image quality while reducing the radiation dose. Recently several dose reduction approaches in both software and hardware have been developed to achieve the goal of making radiation as low as possible. Thus, the assessment of image quality becomes an important factor for routine quality control of medical X-Ray devices. In this work, task-based image quality measurements using model observers were used to evaluate the performance of X-Ray CT systems. To evaluate the dose reduction ability, detection tasks as well as combined detection and estimation tasks were considered. In detection tasks and combined detection and estimation tasks, the channelized Hotelling observer (CHO) and channelized scanning linear observer (CSLO) (with Dense Difference of Gauss channels) were employed respectively. They were used to evaluate the dose reduction capability of the iterative reconstruction algorithm developed by GE compared to the traditional reconstruction algorithm, filtered backprojection (FBP). Additionally, CHO and CSLO were also used for optimization of CT protocols. Our methods were also applied to Cardiac CT systems for temporal resolution evaluations. Two reconstruction algorithms, FBP and the motion correction algorithm, Snapshot Freeze (SSF), operated at two heart-beating rates with two reconstruction windows were quantitatively evaluated using task-based measurements. Finally, due to the huge demand of data acquisitions in the conventional channelized model observers, a proposed High-Dose-Signal-LOOL CHO/CSLO (HL-CHO/CSLO) that could efficiently reduce the data requirement has also been investigated in the pure detection, and combined detection and estimation task. In all studies, the practicality and the use of real data is emphasized. The results of all these studies demonstrate the usefulness of the task-based measurements of image quality in X-Ray CT imaging.
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Santos, Ana. "Advanced X-ray photoelectron spectroscopy of N-based ionic liquids." Thesis, University of Nottingham, 2018. http://eprints.nottingham.ac.uk/49383/.
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X-ray photoelectron spectroscopy (XPS) has been used to probe the influence of cation structure on fundamental interactions within Ionic Liquid (IL) systems. A series of guanidinium-based ILs have been investigated and cation-anion interactions have been studied to determine the effect of a highly charge delocalised cationic core on the electronic environment of the anion. These and sulfur-based cations were developed, and 13C-NMR comparisons addressed, along with the influence of sulfur functionalisation upon the electronic environments of the cationic component. These materials were then used to probe the composition and structural orientation of ions of the IL surface with resource to angle-resolved XPS (ARXPS) and particularly, energy-resolved XPS (ERXPS). Appropriate methodologies for both techniques were developed and fully verified, and the position of the anion ([NTf2]-) relative to cation (imidazolium, guanidinium and thioimidazolium) was found to support previous findings in the literature using other spectroscopic techniques. Additionally, ERXPS allowed for relative depth profiles not only of the anionic component, but also of the sulfur in the cationic component for the thioimidazolium bis(trifluoromethanesulfonyl)imide IL series.
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Smith, Jonathan Chase. "Examining soil based construction materials through X-ray computed tomography." Thesis, Durham University, 2015. http://etheses.dur.ac.uk/11113/.
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X-ray computed tomography (XRCT) enables the non-destructive analysis of samples internal structures down to a sub-micron resolution and has been used to examine the macrostructure of unstabilized soil based construction materials (SBCMs) alongside experiments on the materials unconfined compressive strength. SBCMs are manufactured mixtures of clay, sand and gravel which should be considered as highly unsaturated compacted soil where suction is the key source of strength. The use of XRCT in geotechnical literature is comprehensively reviewed before three laboratory investigations are described. Firstly crack propagation in SBCMs following unconfined compression is investigated and key lessons about XRCT scanning highlighted. Secondly the impact of altering sample size to match optimum XRCT scanning conditions is explored through experiments on void size distribution and unconfined compressive strength. Finally the effects of adding expansive clay to SBCM mixes on macrostructure are investigated and insights on how the unconfined compressive strength develops as SBCM dries are given. Conclusions from this thesis have applicability to both the SBCM industry, as the insights into the fundamental behaviour of SBCM can be used to inform building practice, and geotechnical researchers where the extensive use and development of XRCT can be applied to investigate the internal structure of a wide range of geotechnical materials.
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Agnarsson, Joakim. "Simulation of a radar in Flames : a ray based radar model." Thesis, Uppsala universitet, Institutionen för informationsteknologi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-201093.
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In this report a radar model is modeled and implemented in a simulation framework called Flames. The focus is to investigate how and if electromagnetic waves emitted from the radar antenna can be simulated by different methods and how these methods can be implemented to work in real-time simulations. The radar model developed in this report is based on ray tracing and ray propagation techniques developed by the author. The model considers varying refractive indices that describes both standard atmospheres and atmospheres where ducting occur. Preparatory studies are also made to model ground reflection by ray tracing techniques. Other simulation techniques, such as the Finite-Difference Time-Domain method and the Split-Step Parabolic Equation method, are furthermore considered for radar simulation applications. The results show that ray tracing in conjunction with geometrical optics is a valid method for simulating electromagnetic waves in simple atmosphere models.
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Zanazzi, Enrico. "Characterization of X-ray detectors based on organic semiconducting single crystals." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2015. http://amslaurea.unibo.it/8750/.
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Negli anni recenti, lo sviluppo dell’elettronica organica ha condotto all’impiego di materiali organici alla base di numerosi dispositivi elettronici, quali i diodi ad emissione di luce, i transistor ad effetto di campo, le celle solari e i rivelatori di radiazione. Riguardo quest’ultimi, gli studi riportati in letteratura si riferiscono per la maggiore a dispositivi basati su materiali organici a film sottile, che tuttavia presentano problemi relativi ad instabilità e degradazione. Come verrà illustrato, l’impiego di singoli cristalli organici come materiali alla base di questi dispositivi permette il superamento delle principali limitazioni che caratterizzano i rivelatori basati su film sottili. In questa attività sperimentale, dispositivi basati su cristalli organici semiconduttori verranno caratterizzati in base alle principali figure di merito dei rivelatori. Tra i campioni testati, alcuni dispositivi basati su singoli cristalli di 6,13-bis (triisopropylsilylethynyl)-pentacene (TIPS-Pentacene) e 5,6,11,12-tetraphenyltetracene (Rubrene) hanno mostrato interessanti proprietà e sono stati quindi maggiormente studiati.
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Prakoonwit, Simant. "The reconstruction and manipulation of object-based 3D X-ray images." Thesis, Imperial College London, 1996. http://hdl.handle.net/10044/1/7330.
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Poulsen, Henrik. "Potential of GPU Based Hybrid Ray Tracing For Real-Time Games." Thesis, Blekinge Tekniska Högskola, Avdelningen för för interaktion och systemdesign, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:bth-3488.
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The development of Graphics Hardware Technology is blazing fast, with new and more improved models, that out spec the previous generations with leaps and bounds, before one has the time to digest the potential of the previous generations computing power. With the progression of this technology the computer games industry has always been quick to adapt this new power and all the features that emerge as the graphic card industry learn what the customers need from their products. The current generations of games use extraordinary visual effects to heighten the immersion into the games, all of which is thanks to the constant progress of the graphics hardware, which would have been an impossibility just a couple of years ago. Ray tracing has been used for years in the movie industry for creation of stunning special effects and whole movies completely made in 3D. This technique for giving realistic imagery has always been for usage exclusively for non-interactive entertainment, since this way of rendering an image is extremely expensive when it comes to computations. To generate one single image with Ray Tracing you might need several hundred millions of calculations, which so far haven’t been proven to work in real-time situations, such as for games. However, due to the continuous increase of processing power in Graphical Processing Units, GPUs, the limits of what can, and cannot, be done in real-time is constantly shifting further and further into the realm of possibility. So this thesis focuses upon finding out just how close we are to getting ray tracing into the realm of real-time games. Two tests were performed to find out the potential a current (2009) high-end computer system has when it comes to handling a raster - ray tracing hybrid implementation. The first test is to see how well a modern GPU handles rendering of a very simple scene with phong shading and ray traced shadows without any optimizations. And the second test is with the same scenario, but this time done with a basic optimization; this last test is to illustrate the impact that possible optimizations have on ray tracers. These tests were later compared to Intel’s results with ray tracing Enemy Territory: Quake Wars.
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Vine, David John. "New quantitative methods in analyser-based phase contrast X-ray imaging." Monash University. Faculty of Science. School of Physics, 2008. http://arrow.monash.edu.au/hdl/1959.1/57830.
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New quantitative methods are developed for analyser-based phase contrast imaging (ABI) with hard X-rays. In the first instance we show that quantitative ABI may be implemented using an extended incoherent source. Next, we outline how complex Green’s functions may be reconstructed from phase contrast images and we apply this method to reconstruct the thick perfect crystal Green’s function associated with an ABI imaging system. The use of quantitative ABI with incoherent X-ray sources is not widespread and the first set of results pertains to the feasibility of quantitative ABI imaging and phase retrieval using a rotating anode X-ray source. The necessary conditions for observation of ABI phase contrast are deduced from elementary coherence considerations and numerical simulations. We then focus on the problem of extracting quantitative information from ABI images recorded using an extended incoherent X-ray source. The results of an experiment performed at Friedrich-Schiller University, Germany using a rotating anode X-ray source demonstrate the validity of our approach. It is shown that quantitative information may be extracted from such images under quite general and practicable conditions. We then develop a new use for phase contrast imaging systems that allows the Green’s function associated with a linear shift-invariant imaging system to be deduced from two phase contrast images of a known weak object. This new approach is applied to X-ray crystallography where the development of efficient methods of inferring the phase of rocking curves is an important open problem. We show how the complex Green’s function describing Bragg reflection of a coherent scalar X-ray wavefield from a crystal may be recovered from a single image over a wide range of reciprocal space simultaneously. The solution we derive is fast, non-iterative and deterministic. When applied to crystalline structures for which the kinematic scattering approximation is valid, such as thin crystalline films, our technique is shown to solve the famous one-dimensional phase retrieval problem which allows us to directly invert the Green’s function to retrieve the depth-dependent interplanar spacing. Finally we implement our Green’s function retrieval method on experimental data collected at the SPring-8 synchrotron in Hyogo, Japan. In the experiment we recorded analyser-based phase contrast images of a known weak object using a thick perfect silicon analyser crystal. It is then demonstrated that these measurements can be inverted to recover the complex Green’s function associated with the analyser crystal Bragg peak. The reconstructed Green’s function is found to be in good agreement with the prediction of dynamical diffraction theory.
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Yang, Yong Suk Tony. "Time resolved X-ray studies of crystallization in Zr-based glasses." Thesis, McGill University, 1990. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=74548.
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We have developed new techniques, which enable us to study the kinetics of phase transitions in situ. By resistive self heating of samples, heating rates of over 10$ sp4$ K/s have been achieved. Our methods allows us to acquire x-ray diffraction patterns as quickly as every 3 ms.We have used these new techniques of time resolved x-ray diffraction experiments to study the crystallization kinetics of amorphous NiZr$ sb2,$ CoZr$ sb2,$ FeZr$ sb2$ and Ni$ sb{36.5}$Zr$ sb{63.5}.$ The experiments were performed at beamline X20C National Synchrotron Light Source.
We have mostly focussed on the crystallization behaviour at high temperatures and therefore faster kinetics. We have found many new results and unexpected metastable phases in these systems. Most of these phases would not be seen using conventional techniques showing the importance of in situ time resolved structural measurements to study the kinetics of phase transitions.
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Nussey, James Peter. "Terrestrial and space-based applications of microchannel plate X-ray optics." Thesis, University of Leicester, 2005. http://hdl.handle.net/2381/8992.
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This thesis describes the continued development of square pore microchannel plate (MCP) X-ray optics with a view to their application on terrestrial and space-based instrumentation. The development of a lab-based Imaging X-ray Fluorescence Spectrometer (IXRFS) is described, incorporating an MCP optic to relay fluorescent flux, excited from uniformly illuminated target samples onto a Charge Coupled Device (CCD). Applications in mineralogy and scientific instrument calibration are investigated, with elemental maps spatially correlated to observed sample features in each case. The soft X-ray focusing properties of one planar and two slumped MCP optics are evaluated with a view to their application on the future Lobster-ISS all-sky monitor. Best FWHM foci of 9.4' x 7.5' (planar) and 9.1' x 9.1' (slumped) are reported. Observed temporal variation in the angular resolution of a slumped MCP optic is attributed to the mechanical deformation of the optic during focusing. Multifibre misalignment is determined to be the major source of error in the focusing properties of the planar MCP. The metallisation of the reflecting surfaces of MCP optics improves their high energy and wide angle response; metal films of iridium, ruthenium and nickel, deposited onto MCP optics using the Atomic Layer Deposition (ALD) and Electroless Nickel (EN) methods are characterised using the techniques of Auger Electron Spectroscopy (AES) and X-ray Reflectivity (XRR). Ruthenium and nickel-coated MCPs exhibit a shortfall in low-grazing angle reflectivity, not observed in the case of iridium, which is modelled with an RMS surface roughness of 26 Å. The Lobster-ISS all-sky monitor is modelled to a range of specifications using a Monte-Carlo raytrace code to optimise performance. Iridium-coated MCP optics are found to provide the best modelled sensitivity for a variety of astrophysical objects. In this configuration, Lobster-ISS is modelled to have a 1 day limiting sensitivity of 0.32 mCrab.
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Kelly, Kathryn L. "Phase contrast imaging with a laboratory-based microfocus X-ray source." Thesis, Durham University, 2007. http://etheses.dur.ac.uk/2453/.
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This project looks at the design of an in-line X-ray phase contrast imaging system, based around the Bede Microsource®, on a suitable scale for imaging small pathological samples. The system is applied to two test objects to assess feasibility of its use in practice. The system comprises: microfocus X-ray source, copper and rhodium targets, phase object and detector. By altering sample position, phase contrast is seen to vary proportionally with propagation distance. By increasing source size, phase contrast is seen to degrade with increasing source dimensions. It is furthermore determined that transverse coherence in the direction perpendicular to a sample boundary dominates the result. Using different operating voltages with an image manipulation approach, the importance of monochromaticity is investigated. It is determined that less polychromatic X-rays provide better phase contrast. However, applying a filter method of monochromation reduces phase contrast due to beam hardening, so this method is unsuitable for phase imaging with this system. By increasing sample thickness, phase contrast is seen to plateau at a certain thickness, dependant on X-ray energy, revealing a practical limitation of the system. Introduction of water and glass to the system is seen to be detrimental to phase contrast, due to beam hardening. However phase contrast is not destroyed by the presence of water. When imaging a mammographic test object comprising low-contrast details and breast tissue morphology, the system detects all low-contrast details extremely well. Furthermore, using a phase object together with breast tissue morphology, with the harder rhodium X-rays, the system detects an anomaly of -500J.!m through the noise of simulated breast tissue. Finally, we see the difference in two scorpion sting images: an absorption image taken at the beginning of the project and a phase image taken after the system has been improved.
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Williams, Jamie Oscar David. "From terahertz to X-ray : developing new graphene-based photodetector technologies." Thesis, University of Leicester, 2018. http://hdl.handle.net/2381/42405.
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The latest technological developments have resulted in a push for faster, cheaper and simpler photodetector technologies across a wide range of temperatures, wavelengths and sensitivities for use in industrial and research applications. Graphene, a 2D allotrope of carbon, is seen as an interesting route for future photodetectors. Recent research into graphene has focussed on fundamental physics, fabrication processes and future commercial applications. Fundamental research has demonstrated many interesting properties, including the potential for high carrier mobility, high conductivity, approximately constant photon absorption and extreme tensile strength. These properties led to promising developments for graphene-based photodetectors, such as the demonstration of ultrafast photodetection on a femtosecond timescale for pulsed lasers. This document discusses novel graphene-based photodetector technologies from concept to theory, design, fabrication and experimental demonstration. Three detectors, from terahertz to X-ray, were fabricated in essentially the same graphene field effect transistor (GFET) structure, with photons coupling to different components of the detector to provide a measurable photosignal. A simulated cryogenic, colour sensitive, bilayer graphene single photon counting photodetector exploited the tuneable band gap of bilayer graphene to trade-off resolution against temperature to enable higher temperature operation, requiring less costly and complex cryogenics, with photons coupling directly to the bilayer graphene. The passive terahertz detector utilised photons from a broadband terahertz source coupling to antennae to generate a photoresponse via the Dyakonov-Shur effect with an NEP=0.85±0.15μWHz-0.5, with further work ongoing to demonstrate narrowband terahertz detection. The X-ray GFET was developed to investigate the energy sensitivity to X-ray photons coupling to the absorber based on work in the literature, where charge carrier modulation generates a field that changed the conductivity of the graphene channel. Using pulsed optical lasers to probe the behaviour and sensitivity of the detector gave ΔE~480keV (for E=30MeV) with a photoresponse dependent on the gate voltage. No X-ray sensitivity was observed for Fe-55 sources, but it was observed for an X-ray generator; this inconsistency possibly suggests a different mechanism, such as bolometry, to that proposed previously in the literature.
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Romell, Jenny. "Comparison of Grating- and Speckle-Based X-Ray Phase-Contrast Imaging." Thesis, KTH, Biomedicinsk fysik och röntgenfysik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-189056.
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Forde, Declan J. "FTIR and X-ray investigation of triphenylene based discotic liquid crystals." Thesis, Sheffield Hallam University, 2000. http://shura.shu.ac.uk/19655/.
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Novel disc like molecules based on hexa-n-alkoxy benzoates of triphenylene were synthesised at Hull university. The compounds exhibited thermotropic liquid crystalline behaviour. The compounds differed chemically based upon the number and position of methyl additions to the ester benzoate linkage. Unsymmetrical compounds based on hexa-n-alkoxy triphenylenes were also examined. A number of techniques were employed to observe and measure the physical properties of these compounds. Polarising optical microscopy was used to observe and record the phase behaviour. Typical schlieren nematic textures were often observed in the liquid crystalline phase. The transition temperatures of the phase transitions were recorded to within +/-0.1 °C. Methyl additions to the ester benzoate linkage plays a major role in determining transition temperatures and also the ranges of liquid crystal phase. X-ray diffraction investigations allowed the molecular planar spacings to be measured, use of a heating stage enabled measurements to be taken in the liquid crystalline phase. All the samples produced a diffuse broad diffraction ring in the liquid crystalline phase, indicating that the samples are not highly ordered and that they are likely to have adopted a hexagonal packing arrangement. Planar spacings measured were in the range 22 - 30A, only one sample, DB26, showed a diffraction ring corresponding to a planar spacing of 4.1A, indicating that molecular columns or partial columns were able to form from molecules stacking one on top of another. Thus methyl groups on the ester benzoate linkage disrupt the formation of columns, in turn reducing transition temperatures. A number of methods of successfully aligning the discotic materials using surface treatments are presented. Rubbed PVA and HTAB layers aligned the samples hometropically, while SiO deposited layers aligned the samples homogeneously. (The SiO deposition used an evaporation angle of 45°, a deposition angle of 5° is commonly used to obtain homotropic alignment of calamitic materials.) Methods that produced homogeonous alignment of calamitic materials produced hometropically aligned discotic materials and vice-versa. FTIR allows conformational information about a molecule to be determined. The CH[2] wagging region was investigated to determine conformational information relating to the alkyl arms. Combination of FTIR and aligned samples allowed the alignment process to be investigated, it was determined that the alignment of the molecules occurs in a series of stages, the triphenylene cores align first on cooling, followed by the ester benzoate linkages and finally the alkyl arms. Computer modelling simulations allowed various molecular conformations to be observed, combination with X-ray diffraction data allowed molecular structures to be generated. The software allowed various molecular dimensions to be easily measured and the effect and extent of interdigitation of molecular arms to be observed. The molecular dynamics calculations were only able to calculate energy minimisations for crystalline structures, but the crystalline results offered valuable insights into the liquid crystalline structures and behaviour.
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Song, Jungki. "Metrology and mechanics for manufacturing space-based x-ray grating spectrometers." Thesis, Massachusetts Institute of Technology, 2021. https://hdl.handle.net/1721.1/130846.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, February, 2021Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 223-231).
Small errors in critical dimensions (CDs) or deformation of optical components can lead to severe performance degradation in high-resolution imaging and spectroscopy tools. Consistent innovations towards more precise optical elements and assembly procedures have led to high-resolution optical systems in many fields - including telescope, microscopy, lithography, and display technologies. A space x-ray telescope needs more stringent requirements as it observes distant space objects using a limited number of x-ray photons in a harsh space environment. The optical instruments for x-ray telescopes need to be high-resolution, efficient in collecting x-ray photons, and lightweight. Optical elements in x-ray telescopes have large-apertures (typically around 1-2 m²) which are realized by populating them with > 1000 high-quality optical sub-elements (i.e., mirrors or gratings).
In this thesis, we limit our attention to an xray grating spectrometer, one of the essential elements in x-ray telescopes. It is placed downstream of the focusing optics and prior to the x-ray detector to disperse nonmonochromatic x rays from distant sources for space-based x-ray spectroscopy. A critical-angle transmission (CAT) grating, a lightweight, freestanding, high-aspect ratio x-ray grating with 200-nm period and 4 [mu]m depth, is a building block for grating spectrometers. More than 1000 high-quality CAT gratings need to be manufactured and precisely aligned within tolerance to build future CAT grating spectrometers. This thesis attacks this manufacturing challenge through 1) inventing metrologies for characterizing CDs, 2) developing alignment processes, and 3) performing design and analysis of CAT grating structural supports. First, a metrology to characterize period variation of CAT gratings was developed.
Metrology repeatability of 0.004 nm rms was achieved, successfully characterizing period variations of 0.018 nm rms (1 sigma) over large-area CAT gratings patterned with traditional interference lithography. The demonstrated metrology uncertainty and period variations fulfill the requirements for future x-ray telescope missions. Second, alignment metrology and protocols were developed, demonstrating an ability to align multiple CAT gratings to satisfy alignment requirements ( <6 arcmin or 0.1 deg). The developed alignment protocol is reliable and scalable for flight-level alignment, for which a large volume (>1000) of CAT gratings need to be aligned in a fast and accurate manner. Third, a metrology to characterize grating bar tilt variations was developed using small-angle x-ray scattering and a laser setup. The developed metrology demonstrated repeatability of <0.01 deg (1 [sigma]) and accuracy of ~0.08 deg (4.8 arcmin).
It successfully characterized bar tilt angle variations from CAT gratings and results agree well with synchrotron measurements. It enabled us to close the loop in process optimization for CAT grating fabrication, and contributed towards suppressing bar tilt (or blaze) error within tolerance ( <6 arcmin or 0.1 deg). Fourth, analytical and finite element studies were performed to design CAT grating structural supports that minimize x-ray blockage at a given stiffness. In-plane and out-of-plane stiffness of several 2D-lattice topologies were examined. A triangular lattice shows 23-580% on stiffness improvement ( depending on mode of stiffness) over the current hexagonal lattice design for the same open area fraction. Adopting the new 2D lattice design is expected to increase open area fraction by ~5% without compromising stiffness.
by Jungki Song.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineering
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Luz, Pedro Hugo Ferreira Natal da. "Development of neutron and X-ray imaging detectors based on MHSP." Doctoral thesis, Universidade de Aveiro, 2009. http://hdl.handle.net/10773/2682.
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Doutoramento em FísicaA Micro-Hole & Strip Plate (MHSP) é uma microestrutura desenvolvida recentemente em instituições universitárias portuguesas. Neste trabalho, o seu desempenho como detector para imagiologia é explorado, tendo como objectivo a imagiologia não só de neutrões, mas também de raios X. A aplicação do método da divisão resistiva de carga é aplicada a uma MHSP especialmente desenhada para sensibilidade em posição. Várias abordagens em termos da electrónica de aquisição de sinal são testadas, sem perder de vista a relação desempenho-preço. Resoluções espaciais abaixo de 1 mm foram obtidas com a MHSP a operar em xénon e tetrafluorometano, com um sistema de detecção a um preço modesto quando comparado com as alternativas, e apropriado para inúmeras aplicações em imagiologia de neutrões e de raios X.
The Micro-Hole & Strip Plate (MHSP) is a Micropattern Gaseous Detector developed recently in Portuguese institutions. In this work, its performance as a imaging detector is characterised, aiming neutron imaging, but also with results in X-ray imaging. The application of the method of resistive charge division is used in a MHSP specially designed for position sensitivity. Several different electronic approaches for signal collection are tested, always taking into account the performance-price ratio. Position resolutions below 1 mm were achieved with the MHSP operating in xenon and in tetrafluoromethane, in a very cost effective detector setup, suitable for many applications in neutron and X-ray imaging.
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Grätz, Matthias. "Characterisation and application of a laser-based hard X-ray source." Lund : Lund Institute of Technology, 1998. http://catalog.hathitrust.org/api/volumes/oclc/68945036.html.
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Zhou, Tunhe. "Laboratory X-Ray Phase-Contrast Imaging : Methods and Comparisons." Doctoral thesis, KTH, Biomedicinsk fysik och röntgenfysik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-192593.
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X-ray phase-contrast imaging has seen rapid development in recent decades due to its superior performance in imaging low-absorption objects, compared to traditional attenuation x-ray imaging. Having higher demand on coherence, x-ray phase-contrast imaging is performed mostly at synchrotrons. With the development of different imaging techniques, and the development of laboratory sources and x-ray optics, x-ray phase-contrast imaging can now be implemented on laboratory systems, which is promising and practical for broader range of applications. The subject of this thesis is the implementation, development and comparison of different laboratory phase-contrast methods using a liquid-metal-jet source. The three x-ray phase-contrast imaging methods included in this thesis are the propagation-, grating-, and speckle-based techniques. The grating-based method has been implemented on a laboratory system with a liquid-metal-jet source, which yields several times higher brightness than a standard solid-anode microfocus source. This allows shorter exposure time or a higher signal-to-noise ratio. The performance of the grating-based method has been experimentally and numerically compared with the propagation-based method, and the dose required to observe an object as a function of the object’s diameter has been investigated with simulations. The result indicates a lower dose requirement for the propagation-based method in this system but a potential advantage for the grating-based method to detect relatively large samples using a monochromatic beam. The speckle-based method, both the speckle-tracking and speckle-scanning techniques, has been implemented on a laboratory system for the first time, showing its adaptability to radiation of low temporal coherence. Tomography has been performed and shows the potential applications of this method on quantitative analysis on both absorption and phase information of materials. As a basis for further optimization and comparisons to other methods, the noise properties of the differential phase contrast of the speckle-based method have been studied and an analytical expression for the noise variance introduced, showing a similarity to the grating-based method.Faskontrastavbildning med röntgenstrålning är en teknik som har utvecklats kraftigt de senaste årtiondena, eftersom den fungerar bättre än traditionella, absorptionsbaserade röntgenundersökningar för objekt med låg absorption. Den har dock höga krav på koherens, vilket gjort att den huvudsakligen används vid stora synkrotron-anläggningar. Tack vare utveckligen av nya avbildningstekniker, laboratoriekällor och röntgenoptik kan numera faskontrast användas även med laboratoriesystem, vilket är lovande då tekniken kan användas vid ett större antal olika tillämpningsområden Denna avhandling syftar till att tillämpa, utveckla och jämföra olika faskontrastmetoder i laboratoriemiljö, med en metallstråleröntgenkälla. De tre faskontrastmetoderna som behandlas i denna avhandling är propogation, gitter och speckelbaserad faskontrast. Den gitterbaserade metoden har implementerats i ett laboratoriesystem med en metallstrålekälla som ger flera gånger högre radians än en vanlig, fast mikrofokuskälla. Den högre radians en möjliggör kortare exponeringstider eller högre signal-brusförhållande. Den gitterbaserade tekniken har jämförts experimentellt och numeriskt med den propageringsbaserade metoden. Den strålningsdos som krävs för observera ett objekt, som funktion av dess diameter, har jämförts för de båda teknikerna, den här gången via simuleringar. Resultaten visar på en lägre strålningsdos för den propagationsbaserade tekniken i detta fall, men även att det finns en potentiell fördel för den gitterbaserade tekniken för något större objekt med monokromatisk röntgenstrålning. Speckelbaserade tekniker, nämare bestämt den som bygger på att spåra speckel och den som bygger på att scanna diffusorn, har för första gången implementerats i laboratoriemiljö. Därmed har visats att de fungerar även för strålning med låg tidskoherens. Tekniken har även använts för tomografi och visar möjliga tillämpningar inom kvantitativ analys av material. För att förenkla framtida optimeringar och jämförelser av tekniken med andra metoder, har brusegenskaperna för den speckelbaserade metoden studerats och visat sig likna den gitterbaserade metoden.
QC 20160921
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Carroli, Marco. "Novel materials for direct X-ray detectors based on semiconducting organic polymers." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2015. http://amslaurea.unibo.it/9345/.
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Conventional inorganic materials for x-ray radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, me- chanical sti ffness, lack of tissue-equivalence and toxicity. Semiconducting organic polymers represent an alternative and have been employed as di- rect photoconversion material in organic diodes. In contrast to inorganic detector materials, polymers allow low-cost and large area fabrication by sol- vent based methods. In addition their processing is compliant with fexible low-temperature substrates. Flexible and large-area detectors are needed for dosimetry in medical radiotherapy and security applications. The objective of my thesis is to achieve optimized organic polymer diodes for fexible, di- rect x-ray detectors. To this end polymer diodes based on two different semi- conducting polymers, polyvinylcarbazole (PVK) and poly(9,9-dioctyluorene) (PFO) have been fabricated. The diodes show state-of-the-art rectifying be- haviour and hole transport mobilities comparable to reference materials. In order to improve the X-ray stopping power, high-Z nanoparticle Bi2O3 or WO3 where added to realize a polymer-nanoparticle composite with opti- mized properities. X-ray detector characterization resulted in sensitivties of up to 14 uC/Gy/cm2 for PVK when diodes were operated in reverse. Addition of nanoparticles could further improve the performance and a maximum sensitivy of 19 uC/Gy/cm2 was obtained for the PFO diodes. Compared to the pure PFO diode this corresponds to a five-fold increase and thus highlights the potentiality of nanoparticles for polymer detector design. In- terestingly the pure polymer diodes showed an order of magnitude increase in sensitivity when operated in forward regime. The increase was attributed to a different detection mechanism based on the modulation of the diodes conductivity.
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Flórez, Díaz Jorge Eliécer. "Improvements in the ray tracing of implicit surfaces based on interval arithmetic." Doctoral thesis, Universitat de Girona, 2008. http://hdl.handle.net/10803/7885.
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Las superfícies implícitas son útiles en muchas áreasde los gráficos por ordenador. Una de sus principales ventajas es que pueden ser fácilmente usadas como primitivas para modelado. Aun asi, no son muy usadas porque su visualización toma bastante tiempo. Cuando se necesita una visualización precisa, la mejor opción es usar trazado de rayos. Sin embargo, pequeñas partes de las superficies desaparecen durante la visualización. Esto ocurre por la truncación que se presenta en la representación en punto flotante de los ordenadores; algunos bits se puerden durante las operaciones matemáticas en los algoritmos de intersección. En este tesis se presentan algoritmos para solucionar esos problemas. La investigación se basa en el uso del Análisis Intervalar Modal el cual incluye herramientas para resolver problemas con incertidumbe cuantificada. En esta tesis se proporcionan los fundamentos matemáticos necesarios para el desarrollo de estos algoritmos.Implicit surfaces are useful in many areas related to computer graphics. One of their main advantages over other representations is that they can be easily used as primitives for modeling. However, they are not widely used for this purpose because the models created with implicit surfaces take a long time to be rendered. When a precise visualization of an implicit surface is required, the best option is to use ray tracing. However, thin features can be missed in models that have thin parts. These problems are caused by the truncation performed in the floating-point representation in the computers: come bits ae lost in the mathematical operations during the intersection test. In this work, algorithms to deal with those problems are presented. The research is based on Modal Interval Analysis that includes tools for solving quantified uncertainty. Modal Interval Analysis gives the mathematical foundations used in the development of these algorithms.
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Berglund, Magnus. "A compact soft X-ray microscope based on a laser-plasma source." Doctoral thesis, KTH, Physics, 1999. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-2845.
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Hamann, Borrero Jorge Enrique. "X-ray studies of magnetism and electronic order in Fe-based materials." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-64870.
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The structure and magnetism of selected compounds of the pnictides iron based superconductors with chemical formula LnO{1-x}FeAsFx (Ln = La,Sm and Ce), commonly known as 1111, and of rare earth iron borates RFe3(BO3)4 (R = Tb, Gd, Nd and Y), were studied by means of hard x-ray diffraction.
For the 1111 pnictides compounds, Rietveld refinement of powder x-ray diffraction measurements at room temperature reveals, that the ionic substitution of O by F has no effect on the structure of the FeAs layers of tetrahedra, whereas the major changes takes place in the LnO layer. These changes are reflected as a shrinkage of the crystal lattice, specially in the c direction. Additionally, a study of the temperature dependent structure of the Sm and Ce-1111 compounds was performed and an estimation of the the structural transition temperature was obtained. The results of the structural measurements, combined with electrical resistivity and µSR, were used to construct the Sm and Ce-1111 phase diagrams. These phase diagrams are characterized by two regions, consisting of a spin density wave (SDW) state and a superconducting state, which are sharply separated upon doping. Considering the different Ln ion, upon F doping the transition temperatures are more efficiently suppressed in Ce-1111 as compared to Sm-1111. More intriguingly, for the Ce case, a coexistence region between static magnetism and superconductivity without an orthorhombic distortion has been observed. Further analysis of the width of the Bragg peaks reveals strong lattice fluctuations towards phase transitions, which are reflected in magnetic and transport properties. Moreover, a strong damping of the lattice fluctuations is observed at Tc for superconducting Sm-1111 samples, giving experimental evidence of competing orders towards phase transitions in the iron pnictides.
Regarding the iron borates, non-resonant x-ray scattering studies have shown several new diffraction features, from the appearance of additional reflections that violate the reflection conditions for the low temperature crystal structure, to the emerging of commensurate superlattice peaks that appear below TN. A detailed analysis of the structure factors and q dependencies of the earlier reflections, demonstrate their magnetic nature. Additional resonant x-ray magnetic scattering experiments on NdFe3(BO3)4 were performed at the Nd L2,3 and Fe K edges. The results show that the magnetization behavior is different for the Nd and for the Fe sublattices. Moreover, we find that the magnetization of the Nd sublattice is induced by the Fe magnetization. The temperature dependent measurements also show a commensurate to incommensurate transition where the magnetic structure changes from a commensurate collinear structure, where both Nd and Fe moments align in the hexagonal basal plane, to an incommensurate spin helix structure that propagates along c. When a magnetic field is applied, the spin helix is destroyed and a collinear structure is formed where the moments align in a direction perpendicular to the applied magnetic field. Moreover, the critical field at which the spin helix is destroyed is the same field at which the magnetic induced electric polarization is maximum, thus, showing that the spin helix is not at the origin of the electric polarization.
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Chabior, Michael. "Contributions to the characterization of grating-based x-ray phase-contrast imaging." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-81705.
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In this work, a characterization and optimization of the grating-based x-ray imaging technique is presented. The investigations are introduced by analytical considerations, are underpinned with numerical simulations and validated using exemplary experiments. A detailed examination of the image formation in a grating interferometer is given, highlighting the dependence of the measured signal on the profile of the gratings. Subsequently, it is shown analytically and in experiments that grating-based imaging can be performed using three basic grating arrangements, which differ in their requirements on grating fabrication and experimental implementation. By a characterization of the measurement signal for each arrangement, a dependence of the signal strength on the sample position within the interferometer is identified. The consecutive evaluation of the impact of this position dependence on radiographic and tomographic data leads to the derivation of optimized reconstruction algorithms and to a correction of resulting image artifacts. Additionally, it is shown that the simultaneous measurement of attenuation and phase images allows the determination of the atomic number of the sample, opening new possibilities for material discrimination. Apart from these investigations on the contrast formation, various imperfections of the technique are investigated: The properties of the image noise are examined in a detailed statistical analysis, yielding a fundamental understanding of the signal-to-noise behavior of the three available contrast channels. Additionally, beam-hardening artifacts at polychromatic x-ray sources are investigated and their correction by a linearization approach is resented.
By a subsequent analysis of the influence of various different grating imperfections on the image quality, tolerance limits for grating fabrication are specified. Furthermore, analytical considerations show that gratings with a duty cycle of 1/3 are advantageous with respect to the signal-to-noise ratio in comparison to common gratings with a duty cycle of 1/2. In conclusion, the results, concepts and methods developed in this work broaden the understanding of grating-based x-ray imaging and constitute a step forward towards the practical implementations of the technique in imaging applications.
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Plogmaker, Stefan. "Techniques and Application of Electron Spectroscopy Based on Novel X-ray Sources." Doctoral thesis, Uppsala universitet, Yt- och gränsskiktsvetenskap, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-168799.
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The curiosity of researchers to find novel characteristics and properties of matter constantly pushes for the development of instrumentation based on X-radiation. I present in this thesis techniques for electron spectroscopy based on developments of X-ray sources both in time structure and energy. One part describes a laser driven High-Harmonic Generation source and the application of an off-plane grating monochromator with additional beamlines and spectrometers. In initial experiments, the source is capable of producing harmonics between the 13th and 23rd of the fundamental laser 800 nm wavelength. The intensity in the 19th harmonic, after monochromatization, was measured to be above 1.2·1010 photons/second with a repetition rate of 5 kHz. The development of a chopper system synchronized to the bunch clock of an electron storage ring is also presented. The system can be used to adjust the repetition rate of a synchrotron radiation beam to values between 10 and 120 kHz, or for the modulation of continuous sources. The application of the system to both time of flight spectroscopy and laser pump X-ray probe spectroscopy is shown. It was possible to measure triple ionization of Kr and in applied studies the valence band of a laser excited dye-sensitized solar cell interface. The combination of the latter technique with transient absorption measurements is proposed. The organic molecule maleic anhydride (MA) and its binding configuration to the three anatase TiO2 crystals (101), (100), (001) has been investigated by means of Xray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine structure Spectroscopy (NEXAFS). The results provide information on the binding configuration to the 101 crystal. High Kinetic Energy Photoelectron Spectroscopy was used to investigate multilayers of complexes of iron, ruthenium and osmium. The benefit of hard X-rays for ex-situ prepared samples is demonstrated together with the application of resonant valence band measurements to these molecules.
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Wang, Te-Kang. "A powder X-ray diffraction study of two aluminum-lithium based alloys." Thesis, Monterey, California. Naval Postgraduate School, 1991. http://hdl.handle.net/10945/43736.
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Approved for public release; distribution is unlimited.The microstructures of solution treated, quenched and aged A1-2.5 wt. %Li and A1-2.0 wt.%Li-2.76 wt.%Mg-1.03 wt.%Cu alloys were studied by powder X- ray diffraction. The as-quenched alloys showed extensive X-ray line broadening due to particle size effects and the intensity measurements indicated a significant amount ordering in the as-quenched state. These results were interpreted using a 'spinodal ordering' model which suggests that A1-Li-based alloys order during quenching and then spinodally decompose into regions of order and disorder so that the final microstructure comprises small ordered regions (size 40 nm) in a disordered matrix Studies on the aged A1-2.5 wt.%Li alloy indicated that after initial short-time aging, particle growth follows Ostwald ripening kinetics in agreement with previous work. Studies on the aged quaternary alloy indicated that T1 and S phases grow in this system so that precipitates are not a dominant strengthening mechanism in this alloy.
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Xu, Feng, Yik-Kiong Hue, and Satya P. Ponnaluri. "Channel Modeling Based on Bidirectional Analytic Ray Tracing and Radiative Transfer (RT²)." International Foundation for Telemetering, 2012. http://hdl.handle.net/10150/581636.
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ITC/USA 2012 Conference Proceedings / The Forty-Eighth Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2012 / Town and Country Resort & Convention Center, San Diego, CaliforniaThe extremely large electrical-size and complexity of terrain scene poses great challenge in channel modeling of aeronautic telemetry. It becomes even more difficult if severe multipath and fading present due to scattering and attenuation of ground, terrain objects and precipitation [Rice, 2004]. This is critical in more sophisticated test scenarios involving low flying unmanned air vehicles and helicopters tested over water at high sea states, in hilly terrain, or even over urban environment. Conventional ray tracing and simple Fresnel reflection are not sufficient to characterize such complex channels. Hence, the novel bidirectional analytic ray tracing and radiative transfer (RT²) is proposed for advanced telemetry channel modeling.
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Junejo, Faraz. "X-ray based machine vision system for distal locking of intramedullary nails." Thesis, Loughborough University, 2009. https://dspace.lboro.ac.uk/2134/35949.
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In surgical procedures for femoral shaft fracture treatment, current techniques for locking the distal end of intramedullary nails, using two screws, rely heavily on the use of two-dimensional X-ray images to guide three-dimensional bone drilling processes. Therefore, a large number of X-ray images are required, as the surgeon uses his/her skills and experience to locate the distal hole axes on the intramedullary nail. The long-term effects of X-ray radiation and their relation to different types of cancer still remain uncertain. Therefore, there is a need to develop a surgical technique that can limit the use of X-rays during the distal locking procedure. A Robotic-Assisted Orthopaedic Surgery System has been developed at Loughborough University named Loughborough Orthopaedic Assistant System (LOAS) to assist orthopaedic surgeons during distal-locking of intramedullary nails. It uses a calibration frame and a C-arm X-ray unit. The system simplifies the current approach as it uses only two near-orthogonal X-ray images to determine the drilling trajectory of the distal-locking holes, thereby considerably reducing irradiation to both the surgeon and patient. The LOAS differs from existing computer-assisted orthopaedic surgery systems, as it eliminates the need for optical tracking equipment which tends to clutter the operating theatre environment and requires care in maintaining the line of sight. Additionally use of optical tracking equipment makes such systems an expensive method for surgical guidance in distal-locking of intramedullary nails. This study is specifically concerned with the improvements of the existing system.
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Alamoudi, Dalal. "Dose enhancement near metal interfaces in synthetic diamond based X-ray dosimeters." Thesis, University of Surrey, 2016. http://epubs.surrey.ac.uk/812968/.
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Diamond is an attractive material for medical dosimetry due to its radiation hardness, fast response, chemical resilience, small sensitive volume, high spatial resolution, near-tissue equivalence, and energy and dose rate independence. These properties make diamond a promising material for medical dosimetry compared to other semiconductor detector materials and wider radiation detection applications. This study is focused on one of the important factors to consider in the radiation detector; the influence of dose enhancement on the photocurrent performance at metallic interfaces in synthetic diamond radiation dosimeters with carbon based electrodes as a function of bias voltages. Monte Carlo (MC) simulations with BEAMnrc code were carried out to simulate the dose enhancement factor (DEF) and compared against the equivalent photocurrent ratio from experimental investigation. MC simulations show that the sensitive region for the absorbed dose distribution covers a few micrometers distances from the interface. Experimentally, two single crystal (SC) and one polycrystalline (PC) samples with carbon based electrodes were used. The samples were each mounted inside a tissue equivalent encapsulation design in order to minimize fluence perturbations. Copper, Gold and Lead have been investigated experimentally as generators of photoelectrons using 50 kVp and 100 kVp X-rays relevant for medical dosimetry. The results show enhancement in the detectors' photocurrent performance when different metals are butted up to the diamond detector. The variation in the photocurrent ratio measurements depends on the type of diamond samples, their electrode fabrication and the applied bias voltages indicating that the dose enhancement from diamond-metal interface modifies the electronic performance of the detector.
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Inwood, David Warwick. "X-ray and electrochemical studies of bimetallic Pt-based oxygen reduction electrocatalysts." Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/417989/.
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Dealloying carbon supported Pt alloy nanoparticles has been shown to particles with a Pt rich outer shell surrounded by an alloy rich core that are highly active electrocatalysts for the oxygen reduction reaction, which is of interest for use in fuel cell cathodes. The structure of these materials as well as how the size, elemental distribution and composition changes during fuel cell operation is important. The catalysts were subjected to an accelerated stability test under similar conditions to those experienced in fuel cell cathodes. At various points throughout the test, the ECSA was recorded and samples taken for ex situ analysis. A variety of x-ray based spectroscopic techniques including XPS, XRD and XAS were used to investigate how the catalyst structure has been affected by the test. TEM will also be used. The 5 nm Pt/C and equivalent alloy catalysts were shown to be stable under these conditions with no significant change in structure or surface area. This shows that the protocol used here does not fully represent the conditions experienced in the fuel cell where degradation is observed. In comparison, the ECSA of 2 nm Pt/C sample was greatly decreased. Further testing for either a longer duration or using higher acid concentration is required to differentiate between the stability of the 5 nm nanoparticle samples. Additionally, as measurements of the electrocatalytic activity made using the RDE technique differ significantly to the performance obtained in an MEA an alternative method was proposed. The GDE combines the advantages of the RDE system in terms of speed of testing and the quantity of catalyst required, with a more accurate representation of the conditions experienced in a fuel cell i.e ability to access the high current density regime. This method was shown to compare favourably with other electrode configurations from the literature such as floating electrodes in terms of ease of use and similarity to results from testing in PEMFC MEAs. Several issues remain with the system, notably quantifying the amount of the catalyst actually utilised, although this does also allow the study of fuel cell related phenomena such as flooding of catalyst layers.
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Eloh, Komlavi Sényo. "FFT-based modelling of X-Ray Diffraction peaks : application to dislocation loops." Electronic Thesis or Diss., Université de Lorraine, 2020. http://www.theses.fr/2020LORR0097.
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Dans ce travail, nous proposons et testons une méthode numérique originale de simulation de diagrammes de diffraction des Rayons X par des monocristaux. Cette méthode repose sur l'utilisation d'algorithmes numériques de type Transformée de Fourier rapide (fast Fourier transform (FFT) en anglais) pour le calcul des champs mécaniques provenant de chargement extérieurs et/ou des défauts linéaires comme des boucles de dislocations. Ces défauts sont modélisés par des champs de déformation libres de contraintes (eigenstrains) dans une microstructure périodique soumise à des chargements thermomécaniques. Dans la première partie, nous présentons une approche améliorée du calcul des champs mécaniques par les algorithmes de types FFT qui permet d'obtenir les champs mécaniques locaux de manière précise et sans oscillation numérique aux discontinuités. Cette amélioration est due à l'utilisation d'un opérateur de Green discret consistent et périodique. Celui-ci est obtenu en résolvant l'équation de Lippmann-Schwinger dans l'espace de Fourier et en faisant une discrétisation spatiale appropriée. L'opérateur de Green modifié de degré quatre permet de calculer pour tous les voxels les valeurs du champ de déformations et de contraintes. Nous proposons aussi un opérateur de Green périodique d'ordre trois qui permet de calculer le champ de déplacement. Le champ de déplacement calculé est ensuite corrigé par une méthode de sous voxélisation qui supprime les artefacts apparaissant dans le cas des boucles de dislocations inclinées vis-à-vis de la grille de référence. Des exemples numériques sur des cas de références montrent l'effet des opérateurs de Green pour le calcul des champs mécaniques locaux sans oscillation et l'efficacité de la méthode de sous voxélisation. Le champ de déplacements final obtenu est la donnée d'entrée de la simulation de diagrammes de diffraction aux rayons X. La méthode de simulation des diagrammes de diffraction des Rayons X de monocristaux CFC est ensuite présentée. Le matériau diffractant est modélisé par un volume représentatif contenant des boucles de dislocations qui glissent dans des plans de types (111). Nous calculons l'amplitude puis l'intensité du faisceau diffracté au voisinage d'un nœud du réseau réciproque. La distribution 3D de l'intensité diffractée est ensuite traitée pour obtenir des diagrammes 2D et 1D qui seront analysés. Les premières simulations démontrent tout d'abord la suppression des artefacts sur les diagrammes de diffraction qui sont dus aux oscillations des champs mécaniques non corrigés. Les pics de diffraction sont analysés par différentes méthodes statistiques (transformée de Fourier de l'intensité, méthode des moments statistiques d'ordres supérieurs) qui permettent d'évaluer les paramètres de la distribution des dislocations (densité, facteur de polarisation, etc.) et de les comparer avec les valeurs théoriquesIn this work, we propose and test an original numerical method of simulation of X-ray diffraction peaks by single crystals. This method is based on the use of Fast Fourier Transform (FFT) algorithms for the calculation of mechanical fields resulting from external loading and / or linear defects such as dislocation loops. These defects are modeled by stress-free strain fields (eigenstrains) in a periodic microstructure subjected to thermomechanical loadings. In the first part, we present an improved approach by FFT-type algorithms which allows to accurately obtain the local mechanical fields without numerical oscillation at material's discontinuities. This improvement is due to the use of a discrete and periodic Green operator. This is obtained by solving the Lippmann-Schwinger equation in the Fourier space and using an appropriate spatial discretization. The fourth order modified Green operator allows to calculate the values of the strain and stress fields at all voxels. We also propose a third order periodic green operator to compute the displacement field. The computed displacement field is then corrected by a sub-voxelization method which removes the artifacts appearing in the case of dislocation loops inclined with respect to the reference grid. Numerical examples on reference cases show the effect of the Green operators for the calculation of local mechanical fields without oscillation and the efficiency of the sub-voxellization method. The final displacement field obtained is the input data of the simulation of X-ray diffraction patterns. The method of simulation of X-ray diffraction peaks of FCC (Face-Centered Cubic) single crystals is then presented. The diffracting material is modelled by a representative volume containing dislocation loops in (111) slip planes. We calculate the amplitude then the intensity of the diffracted beam near a node of the reciprocal lattice. This 3D distribution of the diffracted intensity is processed to obtain 1D diagrams that will be analyzed. The simulations demonstrate foremost the elimination of the artifacts on the diffraction diagrams which are due to the oscillations of the uncorrected mechanical fields. The diffraction peaks are analyzed by different statistical methods (Fourier transform of intensity, method of moments, etc.) which allow to evaluate the distribution parameters of dislocations (density, polarization factor, etc.) and compare them with their theoretical values