3,991 research outputs found
A search for TeV gamma ray emission from X-ray binary stars
No full textThis work is concerned with the detection of pulsed TeV gamma ray emission from a number of X-ray binary systems by the use of the atmospheric Cerenkov technique. Chapters 1 and 2 give an overview of the development of gamma ray astronomy, with emphasis placed on progress made in the detection of TeV gamma rays by their Cerenkov radiation in the atmosphere. Chapters 3 and 4 describe the University of Durham atmospheric Cerenkov telescopes, which were used to make the observations reported in this work, and the standard data processing and analysis procedures adopted. The main part of the thesis deals with the application of these techniques to observations of five X-ray binaries. After a review of the properties of such objects in Chapter 5, Chapters 6 and 7 deal specifically with the results for two of the systems considered to be among the most likely candidates to give a detectable TeV photon flux; Centaurus X-3 and Vela X-1. A study of all data recorded on Cen X-3 over the course of six years suggests the presence of a weak gamma ray flux pulsed at the X-ray period. Previous reports of stronger emission near the ascending node of the orbit are confirmed here. For Vela X-1, the analysis of a dataset recorded during a single dark moon interval reveals evidence for two short outbursts of pulsed TeV gamma ray emission. Chapter 8 reports the series of observations made of SMC X-1, 4U1626-67 and X0G21-72, and upper limits are placed on the TeV gamma ray emission from each. Finally, the results reported here are compared with the predictions of a number of theoretical models, some of which are found to give good agreement with the limits and detections derived in this work. A discussion of the status of this field and future observational prospects is also given
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View synthesis for depth from motion 3D x-ray imaging.
No full textThe depth from motion or kinetic depth X-ray imaging (KDEX) technique is designed to enhance the luggage screening at airport checkpoints. The technique requires multiple views of the luggage to be obtained from an arrangement of linear X-ray detector arrays. This research investigated a solution to the unique problems defined when considering the possibility of replacing some of the X-ray sensor views with synthetic images. If sufficiently high quality synthetic images can be generated then intermediary X-ray sensors can be removed to minimise the hardware requirements and improve the commercial viability of the KDEX technique. Existing image synthesis algorithms are developed for visible light images. Due to fundamental differences between visible light and X-ray images, those algorithms are not directly applicable to the X-ray scenario. The conditions imposed by the X-ray images have instigated the original research and novel algorithm development and experimentation that form the body of this work. A voting based dual criteria multiple X-ray images synthesis algorithm (V-DMX) is proposed to exploit the potential of two matching criteria and information contained in a sequence of images. The V-DMX algorithm is divided into four stages
Microstructure Control of Fire-resistant, Low-alloy Steel; An in-situ 3D X-ray Diffraction and A Small-angle X-ray Scattering Study
No full textThe research presented in this thesis aims at deepening our understanding of the formation of the microstructure of steel during thermal processing in order to control the microstructure and thereby improve the fire-resistance of low-alloy steel. The strength of steel decreases during a fire mainly due to the following changes in the microstructure: 1) increased dislocation motion at elevated temperatures, facilitating plastic deformation, 2) coarsening of the microstructure by grain growth and coarsening of precipitates, which reduce the pinning effect on dislocations, 3) the phase transformation from ferrite to austenite can result in a coarser-grained single-phase structure, and 4) grain boundary sliding. Therefore it is very important to understand the kinetics for the formation of precipitates, dislocation structures, and grains and sub-grains during the austenite-to-ferrite phase transformation during thermal processing. In-situ characterization of these microstructural features is possible by using large-scale synchrotron radiation facilities. The precipitate size distribution evolution during thermal processing can be studied in-situ by small-angle x-ray scattering (SAXS). The nucleation and growth of grains during solid-state phase transformations can be studied in-situ by means of three-dimensional x-ray diffraction (3DXRD) microscopy. The evolution of the dislocation structure during thermal processing can be studied by combining SAXS and 3DXRD. Chapter 2 introduces the basic concepts of microstructures and mechanisms to strengthen low-alloy steel at elevated temperatures. Firstly, the different types of dislocations and slip mechanisms in steel are reviewed. Secondly, the main characteristics of low and high-angle grain boundaries are presented. Thirdly, the main mechanisms for the loss of strength of steel at high temperature are discussed briefly. Lastly, a review of the literature for improving the fire-resistance of steel is presented. Chapter 3 introduces the theory of small-angle X-ray scattering (SAXS) and 3 Dimensional X-ray diffraction (3DXRD) microscopy using synchrotron radiation for microstructural analysis. In the section related to SAXS, the experimental method and data analysis strategies which are used for measuring the characteristics of precipitates and dislocation walls are described in detail. In the section related to 3DXRD, the experimental method, the pre-processing steps for the data analysis, and the 3DXRD data analysis procedure applied for finding grain characteristics are described. Chapter 4 presents the evolution of the size distribution of the (Fe,Cr)-carbides and dislocation structures in steel during a heat treatment as investigated by in-situ SAXS using synchrotron radiation with a new data analysis strategy. The size distribution of the (Fe,Cr)-carbides during heat treatments is determined from the isotropic component of the SAXS patterns. Bright-field transmission electron microscopy (BF-TEM) and high-resolution transmission electron microscopy (HR-TEM) reveal the nearly spherical morphology of the precipitates. Additional measurements have been carried out on a single crystal of ferrite containing (Fe,Cr)-carbides by combining 3DXRD and SAXS during rotation of the specimen at room temperature in order to understand the origin of the streaks in the SAXS-pattern. From simulations based on the theory of SAXS from dislocations we derive that the measured streaks, in the 2D SAXS-patterns including the spottiness, correspond to a dislocation structure of symmetric low-angle tilt boundaries, which in turn corresponds to the crystallographic orientation gradient in the single crystal of ferrite as measured by 3DXRD. Chapter 5 presents the effect of niobium and the grain boundary density on the fire-resistance of Fe-C-Mn steel. Two steels are used: Fe-C-Mn steel and Fe-C-Mn-Nb steel with an atomic ratio Nb/C=1.3. Two different sets of heat treatments are carried out before the fire-test. The first set of heat-treatments consists of heating the steel to 1100°C to bring all niobium in solid solution and austenitizing before continuous cooling at three different cooling rates during the austenite-to-ferrite phase transformation. The second set of heat treatments consists of heating the steel to bring all niobium in solid solution and austenitizing, rapid cooling to room temperature, reheating to 600?C, and annealing for different times before rapid cooling to room temperature. The fire-tests are carried out on specimens by applying a force equal to 60% of the room temperature yield strength at elevated temperature using a Gleeble thermo-mechanical simulator. In this study two techniques are used for analysing the microstructure: 1) Electron Back Scattered Diffraction (EBSD) and, 2) Optical microscopy. In this chapter we show that the low-angle and high-angle grain-boundary densities increase with increasing cooling rate during the austenite to ferrite phase transformation for the niobium containing steel, whereas total grain boundary density decreases with increasing the annealing time at 600°C. We show that the addition of 0.10 wt.% niobium to Fe-C-Mn steel increases the failure temperature of steel by 92°C during a fire-resistance test. Moreover, we demonstrate that the failure temperature increases linearly up to an additional 45°C with increasing grain boundary density from 0.06 to 0.64 ?m-1 for the niobium-containing steel. In Chapter 6 the effect of niobium in solid solution and NbC-precipitates on the austenite-to-ferrite phase transformation kinetics is investigated. In order to separate the effects of NbC-precipitates and Nb in solid solution on the phase transformation kinetics, we use three high-purity Fe-C-Mn steels with different niobium concentrations and one without niobium. Three-dimensional x-ray diffraction (3DXRD) microscopy is used at a third-generation synchrotron radiation facility to study in-situ and simultaneously the nucleation and growth of individual ferrite grains in the bulk of steel during the austenite/ferrite phase transformation. The measured nucleation rate is compared to the classical nucleation theory (CNT) to determine the nucleation parameters. The effects of NbC-precipitates and the concentration of Nb in solid solution on the incubation time, frequency factor, and the activation energy for ferrite nucleation are quantified. The experimentally measured nucleation start temperature is 40-115°C lower than predicted by Thermo-calc for the four alloys under ortho-equilibrium conditions, depending on the concentration of niobium. The frequency factor for nucleation is found to decrease exponentially with increasing concentration of Nb in solid-solution. The ?-parameter, which contains information about the shape of the nucleus and the interfacial energies that are involved in the nucleation process, increases with increasing concentration of Nb in solid solution. The dependence of ?-parameter on the concentration of Nb in solid solution is best described by a pill-box type of geometry of the nucleus and a square root dependence of the ?/?-grain boundary energy with the Nb concentration in solid solution. The ratio of the density of ferrite grains to the density of austenite grains decreases by more than 25% due to the formation of NbC-precipitates, which can be interpreted as reduction of nucleation probability of potential nucleation sites for ferrite, i.e. grain corners, by NbC-precipitates. The ratio of the density of ferrite grains to the density of austenite grains does not depend significantly on the amount of niobium in solid solution. The delay in the start of the transformation as a function of the concentration of niobium in solid solution is found to depend on three factors: 1) the segregation of Nb to the ?/?-grain boundaries, which increases the activation energy for nucleation, 2) reduction of nucleation probability of potential nucleation sites by NbC-precipitates, and 3) diffusion of Nb-atoms from the ?/?-grain boundaries back into the matrix during the nucleation. The drag-effect of niobium in solid-solution on the growth of individual ferrite grains is quantified for different Nb-concentrations. In this work we follow the ‘dissipation’-approach initially developed by Hillert and Sundman et al. and further developed by Odqvist et al. By measuring the velocity of the interface and estimating the chemical driving force, the pressure due to curvature, and the pinning pressure, we determined the dissipation of Gibbs energy caused by the diffusion of the solute atoms being dragged along with the migrating interface, ?Gdiff (J/mol), as a function of the Nb-concentration in solid solution. The drag effect increases with increasing concentration of Nb. Chapter 7 presents an in-situ study in which the partial 3D microstructure of the austenite and ferrite phases before and after the transformation are characterized and in which the nucleation rate of ferrite is measured in an Fe-C-Mn alloy. Separating the experimentally observed ferrite nucleation rate in the bulk and in the surface region of the sample shows that the maximum nucleation rate is reached at lower temperature in the surface region than in the bulk. The activation energy for the nucleation of ferrite in the surface region is higher than in the bulk. An explanation for this could be the application of a thin nickel coating at the surface of specimen, which might result in interfaces between austenite and nickel grains that have lower energy than grain boundaries between austenite grains in the bulk. Future work could focus on deepening our understanding of the effect of low-angle grain boundaries on the fire-resistance of steel containing precipitates. Hitherto, it was assumed that low-angle grain boundaries do not contribute substantially to the strength of steel. However, this research demonstrates that low-angle grain boundaries do contribute substantially to the strength of steel at room temperature and they improve the fire-resistance. How does the interaction between a dislocation and a low-angle grain boundary lead to strengthening of the steel? How do the low-angle grain boundaries form during the austenite-to-ferrite phase transformation in niobium containing steel? What are the prerequisites for the formation of low-angle boundaries? Would the low-angle boundaries also form in steel containing different alloying elements than niobium? Which methods can be used to introduce an even higher density of grain boundaries? From a more application oriented point of view, future research could focus on testing the fire-resistant steel that is presented in chapter 5 under more realistic fire-conditions. Fire-tests could be performed in tensile-mode rather than compression-mode. Moreover, the creep-resistance of this alloy could be investigated to explore its potential applications in gas turbines for power generation and steel-pipes for the transport of hot-gasses from sea to land. An investigation into the low-temperature properties of the steel would also be very interesting in order to test the suitability of the steel for arctic conditions.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
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Enhanced colour encoding of materials discrimination information for multiple view dual-energy x-ray imaging
No full textThis thesis reports an investigation into dual-energy X-ray discrimination techniques. These techniques are designed to provide colour-coded materials discrimination information in a sequence of perspective images exhibiting sequential parallax. The methods developed are combined with a novel 3D imaging technique employing depth from motion or kinetic depth effect (KDE). This technique when applied to X-ray images is termed KDEX imaging and was developed previously by the university team for luggage screening applications at security checkpoints. A primary motivation for this research is that the dual-energy X-ray techniques, which are routinely incorporated into ‘standard’ 2D luggage scanners, provide relatively crude materials discrimination information. In this work it was critical that robust materials discrimination and colour encoding process was implemented as the sequential parallax exhibited by the KDEX imagery may introduce colour changes, due to the different X-ray beam paths associated with each perspective image. Any introduction of ‘colour noise’ into the resultant image sequences could affect the perception of depth and hinder the ongoing assessment of the potential utility of the dual-energy KDEX technique. Two dual-energy discrimination methods have been developed, termed K-II and W-E respectively. Employing the total amount of attenuation measured at each energy level and the weight fraction of layered structures, a combination of the K-II and the W-E techniques enables the computation and extraction of a target objects’ effective atomic number (Zeff) and its surface density (ρS) in the presence of masking layers
Very high energy cosmic gamma rays from radio and x-ray pulsars
No full textThis thesis is concerned with the detection of very high energy cosmic gamma-rays from isolated pulsars and X-ray binary sources using the atmospheric Cerenkov technique. A general introduction to gamma ray detection techniques is followed by adscription of the properties of atmospheric Cerenkov radiation and a discussion of the principles of the atmospheric Cerenkov technique. The Mark I and Mark II gamma-ray telescopes operated in Dugway, Utah by the University of Durham between 1981 and 1984 are briefly described. There follows a discussion of the results from observations at many different wavelengths of Cygnus X-3. This object was observed by the Durham group between 1981 and 1983 in Dugway Utah and also in Durham during autumn 1985. The detection in the Dugway data of the 4.8 hr X-ray period and the possible detection of a19.2 day intensity variation are considered. The discovery of a 12.59 ms pulsar in data taken on Cygnus X-3 in 1983 is described. Evidence is presented which suggests this periodicity is also present at a weaker level in earlier data and also in the data taken in Durham in 1985.Results from observations of PSR1937+21 , PSR1953+29and six radio pulsars , are presented. The design and construction of the Mark III telescope, now operating in Narrabri , N.S.W. , is described in detail. Preliminary results from observations with the Mark III telescope of three objects, LMC X-4, the Vela pulsar and CentaurusX-3, are presented, with particular reference to periodicities inherent in the sources. An observation of the supernova in the Large Magellanic Cloud is discussed. A brief discussion of the mechanisms by which V.H.E. gamma-rays may be produced in isolated pulsars and X-ray binary pulsars is given, followed by a description of the future prospects for the Mark III and Mark IV telescopes
Characterization of Aptamer-Protein Complexes by X-ray Crystallography and Alternative Approaches
No full textAptamers are oligonucleotide ligands, either RNA or ssDNA, selected for high-affinity binding to molecular targets, such as small organic molecules, proteins or whole microorganisms. While reports of new aptamers are numerous, characterization of their specific interaction is often restricted to the affinity of binding (KD). Over the years, crystal structures of aptamer-protein complexes have only scarcely become available. Here we describe some relevant technical issues about the process of crystallizing aptamer-protein complexes and highlight some biochemical details on the molecular basis of selected aptamer-protein interactions. In addition, alternative experimental and computational approaches are discussed to study aptamer-protein interactions.
Gamma ray astronomy in the low energy range
No full textA low energy gamma ray telescope, and the results of its observations are described. The telescope consisted of four sodium iodide crystals, each of 120 cm2 area, occulted by lead discs. Charged particles are rejected using scintillation, anti-coincidence shields.The telescope was flown from Palestine, Texas in 1974. Gamma rays were observed from the Crab Nebula, the pulsar NP0 532, and the atmospheric background. No evidence for a gamma ray burst was found, giving an upper limit of less than 5.8 x 103 bursts per year of intensity greater than 1.6 x 10-6 ergs cm-2.Various types of existing, and planned, gamma ray detectors, and successful gamma ray observations are reviewed. Detailed calculations of the detection efficiency of a Double Compton telescope are presented and these results led to the development of the MISO low energy gamma ray telescope.Preliminary calculations of the detection efficiency of an Anticollimated Double Compton telescope are presented, which suggest that further, more detailed investigation of this type of detector would prove fruitful
The 22-Year Hale Cycle in Cosmic Ray Flux – Evidence for Direct Heliospheric Modulation
No full textAbstract
The ability to predict times of greater galactic cosmic ray (GCR) fluxes is important for reducing the hazards caused by these particles to satellite communications, aviation, or astronauts. The 11-year solar-cycle variation in cosmic rays is highly correlated with the strength of the heliospheric magnetic field. Differences in GCR flux during alternate solar cycles yield a 22-year cycle, known as the Hale Cycle, which is thought to be due to different particle drift patterns when the northern solar pole has predominantly positive (denoted as qA>0 cycle) or negative (qA0 cycles than for qA0 and more sharply peaked for qA0 solar cycles, when the difference in GCR flux is most apparent. This suggests that particle drifts may not be the sole mechanism responsible for the Hale Cycle in GCR flux at Earth. However, we also demonstrate that these polarity-dependent heliospheric differences are evident during the space-age but are much less clear in earlier data: using geomagnetic reconstructions, we show that for the period of 1905 – 1965, alternate polarities do not give as significant a difference during the declining phase of the solar cycle. Thus we suggest that the 22-year cycle in cosmic-ray flux is at least partly the result of direct modulation by the heliospheric magnetic field and that this effect may be primarily limited to the grand solar maximum of the space-age
The contribution of symptoms of post-traumatic stress disorder (PTSD), health anxiety and intolerance of uncertainty to distress in Ménière's disease
No full textThis study assessed whether symptoms of post-traumatic stress disorder (PTSD), health anxiety, and intolerance of uncertainty were associated with distress in members of the Ménière's Society (n = 800), and compared the extent of anxiety, depression, intolerance of uncertainty and health anxiety with a healthy control group (n = 484). PTSD symptoms were associated with anxiety, depression, and handicap. Health anxiety was associated with anxiety and depression. Intolerance of uncertainty was directly associated with anxiety; its association with depression and handicap was mediated by PTSD symptoms. The Ménière's group reported more anxiety, depression, and health anxiety than the control group, but were not more intolerant of uncertainty. More than half of the Ménière's group reported experiencing partial or full PTSD symptoms. As PTSD, health anxiety and intolerance of uncertainty are modifiable with psychological treatment, we advise that clinicians should screen patients with Ménière's disease who are particularly distressed
From Canon Road, 900 ft. above river, S.E. to Upper Falls (109 ft. high), Yellowstone Park, U.S.A.
No full textFrom Canon Road, 900 ft. above river, S.E. to Upper Falls (109 ft. high), Yellowstone Park, U.S.A
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