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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

A study of artificial satellite resonance orbits due to lunisolar perturbations

Hughes, S. January 1978 (has links)
A study of artificial satellite resonance orbits due to lunisolar perturbations is given. Particular emphasis being placed on the following aspects - 1.The classification of resonance orbits according to their commensurability condition. 2. The form of the commensurability condition when expressed in terms of the orbital elements of a satellite. 3. The predominant resonant terms for each commensurability condition. 4. Criteria which determine the existence or non-existence of a particular commensurability condition.
12

Stellar Xray polarimetry from the Ariel-V satellite

Gowen, Robert A. January 1977 (has links)
X-ray polarisation observations from the Ariel-V satellite are described. Notably, an upper limit to the X-ray polarisation of Sco X-1 was produced, more sensitive than previous results had managed to achieve, and for which constraints could be placed upon models for the X-ray emission mechanism. Analysis of the data provided strong empirical confirmation of the desirability for future polarimeters to incorporate some previously proposed design features. These include utilisation of focusing methods to allow a smaller detector, and hence a reduced background level; increased area of collection, so that much weaker X-ray sources may be usefully observed; and also sufficiently accurate pointing, narrow f.o.v. and a capability for simultaneous measurement of more than one component of linear polarisation, all to suppress possible spurious modulations. It would be expected that the advantages obtained by following the advocated design features would result in a polarimeter sufficiently sensitive to be able to measure X-ray polarization from weak X-ray sources, and to achieve accuracies that have not yet been remotely approached. Characteristics of a Bragg crystal instrument capable of a sufficient degree of broad band X-ray polarimetry to test for the existence of a fast rotating black hole in the Cyg X-1 system, are also described. Finally, the need for X-ray polarisation measurements throughout the field of current X-ray astronomy is illuminated.
13

Quasars within epoch of reionisation

Barrio Madias, F. Eugenio January 2009 (has links)
No description available.
14

Synergism between single event effects and total ionising dose

Dyer, Alexander C. R. January 2015 (has links)
The space industry is continuing to use commercial off the shelf (COTS) devices in satellites where the ionising radiation environment poses a threat. For device qualification, their single event effects (SEE) and total ionising dose (TID) performances are normally assessed separately. However, it has been shown that there can be a synergistic relationship in static random-access memory (SRAM) between TID and SEEs, where by the single event upset (SEU) cross section increases with dose, with some devices showing a significant increase for doses less than those seen by low earth orbit (LEO) satellites. The mechanism behind this effect in older SRAM technologies is believed to be due to threshold voltage shift imbalances of the nMOS transistors of the cross coupled inverter within the cell. This is due to variations in the build up of trapped charge in the nMOS transistors when they are ON or OFF. This degrades the noise threshold of the cell making it more susceptible to upsets when holding the opposite state to which it was irradiated in. For more modern devices the gate oxide is too thin to hold enough trapped charge to cause a significant change in threshold voltage. The mechanism for these modern SRAMs is based on the potential between the gate of the ON nMOS transistor and its substrate, in this situation fringing fields are at their strongest ushering the charge created by ionising radiation towards the boundary of the field oxide and the gate channel. It is at this boundary that a significant amount of trapped charge can create a parasitic leakage current between the transistor's source and drain. This parasitic leakage current then reduces the voltage seen at its drain and hence degrades the cell's noise threshold. The main goal of this work is to determine if these mechanisms behind synergy still have a significant affect on the SEU cross section of modern six transistor (6T) SRAMs built on the 180 and 130 nm fabrication processes. Other non-volatile memory devices have also been tested to see if their memory cell or complex CMOS peripheral circuitry suffer any synergistic effects such as an increase in single event functional interrupt (SEFI) or single event latchup (SEL) with increasing dose. To do this test boards containing the devices were irradiated with Co-60 gamma-rays to 5, 10, 15, 25 krads. These boards, as well as the control group test boards, were then taken to be tested with 23.5, 60.9, 151 and 230 MeV protons to determine the SEE response of the various parts. To help assess these devices a highly adaptable test system was developed consisting of high level control software and a control board. The high level software offers an over-view of key data such as the device under test's (DUT) current consumption, SEFI and SEL notifications and a preview of the incoming results. The control board is based around Texas Instrument's microcontroller, the TMS570, and is capable of testing both serial and parallel devices while offering latchup protection via a selectable current limit. From the testing run carried out in this work it was found that the modern SRAM's tested did not exhibit any significant signs of synergy. However there are concerns over the accuracy of some of the data due to the SRAM's SEL response dominating behaviour. These results would benefit from further testing at lower proton energies and flux to ensure any synergy effect was not obscured by the SRAM's SEL response or being close to saturation at 23.5 MeV. The 110 and 65 nm NOR flash memories tested did not show any SEUs in their main memory sectors, while the 110 nm SONOS flash functionally failed at less than 25 krad. The serial ferroelectric random-access memory (FeRAM) suffered a few SEFI events at both 10 and 15 krad resulting in the device being non-responsive, while the device suffered a transient error where by two groups of four addresses were reported to contain errors. Lastly a new method for determining if a device is susceptible to synergy has been suggested, in addition to recommendations for improving the test methodology used in this work.
15

Form and motion of cometary tails

Jones, D. R. L. January 1976 (has links)
This work deals with the geometrical problems inherent in the observation of cometary tails, and with attempts at explanation of their forms. Following the introduction, a review of the geometrical aspects of tail observation is presented in Chapter 2. New formulae are developed for projection from photographic to orbital plane, and for foreshortening etc. Tail orientation is fully treated and expressions are derived for orientation error in this projection. Equations for an elliptic orbit are developed in addition to those for the usual assumption of a parabolic orbit. Perspective error resulting from bisection of apparent tail images is investigated, and appropriate formulae derived. A set of observations of Comet Bennett 1969i is analysed and the results compared with those of other workers. The influence of the solar wind on Type I tails is reviewed in Chapter 3 with special reference to dynamical aberration: some results for Comet Bennett are presented. Oscillation of Type I tails is then discussed. A comparison made between the cases of Comet Burnham 1960 II and Comet Halley 1835 III lends some support to the view that the oscillations are due to external influences on the tail. Formulae are developed for the progressive change in Type I tail orientation to be expected as the comet pursues its orbit. Mechanical theories of Type II tails are reviewed in Chapter 4, special attention being given to tail orientation. The accurate calculation of syndyne and synchrone curves is treated, and the formulae are extended to allow of any value of 1-4. Tail analysis by syndynes and synchrones is considered; results are presented for the time variation in the initial orientation of these curves. Implications for mixed tails and possible coupling between tails are discussed. Finally, Chapter 5 briefly summarises the conclusions and gives suggestions for further work.
16

A surface plasmon resonance-based instrument for the detection of extraterrestrial life

Thompson, Daniel Peter January 2008 (has links)
The subject of this thesis is work relating to the design and testing of the Specific Molecular Identification of Life Experiments, SMILE – a family of Life Marker Chip-type devices currently being developed to search for signs of life on other planets, notably Mars. The SMILE devices will be capable of conducting bioassay measurements on Martian soil samples using molecular receptors to detect target organic molecules (biomarkers). One version of SMILE intends to use the phenomenon of Surface Plasmon Resonance (SPR), combined with fluorescence detection, to probe the sample for biomarkers indicative of the presence of life on the planet, past or present. The main work described in this study falls into two areas; the first is that relating to the design, construction and initial testing of a breadboard demonstrator of combined SPR-Fluorescence detection to understand the practical problems associated with such a system. A theory for the SPR response was developed and fitted to experimental data gathered from the demonstrator with good agreement. The detection of bound analyte via SPR imaging and simultaneous fluorescence imaging was successfully demonstrated. The second part of the thesis describes initial radiation damage tests on fluorescent labelling molecules of the type intended for use in the Life Marker Chip bioassay. The fluorophores were exposed to proton, alpha and gamma radiation at fluences and doses equivalent to those expected for representative mission scenarios. All tested samples survived expected mission fluences, but some effects were detected at higher fluences/doses. Samples were also tested in both desiccated and solute forms, with desiccation found to be preferable for increased radiation tolerance. Lessons learnt from the work are explained, and future work required to progress to a flight version of the SMILE instrument is described.
17

Debris in planetary systems

Jackson, Alan Patrick January 2014 (has links)
No description available.
18

Populations in the Kepler field

Farmer, Robert January 2014 (has links)
Using a population synthesis model I have created a synthetic catalogue of stars in the Kepler field of view. This model has then been subjected to the same biases and selection effects inherent in the selection of stars for the Kepler transit survey mission. This produced a synthetic Kepler Input Catalogue (KIC) which was subjected to the Kepler Stellar Classification Program (SCP) method for determining stellar parameters. I achieve a satisfactory match between the synthetic KIC and the real KIC in the logg vs log Teff diagram. I find a median difference of ~Teff = +500 K and ~ ~logg = -0.2 dex for main sequence stars, although there is a large variation across parameter space. I find no significant difference between ~Teff and ~logg for single stars and the primary star in a binary system. I also re-created the Kepler target selection method and found that the binary fraction is unchanged by the target selection. The fraction of main sequence stars in the sample increases from 75% to 80%, and the giant star fraction decreases from 25% to 20%. I have then used the synthetic KIC to build a of synthetic sample of eclipsing binaries (EBs) in the Kepler field. Comparing the synthetic catalogue to the Kepler EB catalogue I find that the Kepler EB pipeline introduces significant biases into the derived temperature ratio and fractional radii. I then tested the effect of different initial mass ratio distributions (IMRDs) and initial binary fraction distributions (IBFDs). At this time, all distributions fail to match the data, such that their parameters can not be constrained. Modelling the population of asteroseismic binaries, where both stars have a detectable asteroseismic signal, have shown a way to constrain the IMRD for equal mass systems. This method is independent of the binary period and orbital orientation. The number of detectable asteroseismic binaries increases from 87 for the IMR parameter s = -0.5 to 256 for s = 1.0. The number of detectable asteroseismic EBs increases from 34.0 ± 6.0 (s = -0.5) to 59.0 ± 6.0 (s = 1.0). This number shows disagreement with the number of actual systems detected (2 for P orb < 40 days), which can not be explained by incompleteness alone.
19

Simulation, calibration & exploitation of the DRIFT-II directional dark matter detector

Ghag, Chamkaur January 2006 (has links)
Cold Dark Matter is believed to constitute almost a quarter of the Universe, most likely in the form of Weakly Interacting Massive Particles (WIMPs), predicted by supersymmetry. This thesis describes the simulation methods, analysis techniques, and results for the calibration of the DRIFT-IIA directional dark matter detector. This is the first module in an array of gas time projection chambers capable of searching for WIMPs with directional sensitivity, localised in the Boulby mine. Analysis of calibration data taken over a period of five months is compared to detailed Monte Carlo simulations using GEANT4, and a number of efficiencies calculated. After simulation and analysis of data taken during neutron source exposures, in a variety of configurations and separated over a number of months, the DRIFT-IIA detector is found to have an efficiency of 94±2(stat)±5(sys)% for the detection of neutron-induced nuclear recoils. This efficiency is reduced to 44±1(stat)±5(sys)% when measures are taken to remove background events from all data, and a remaining population of events from radon progeny recoils is discussed. The gamma ray rejection factor is determined to be better than 10<sup>-5</sup> following similar analysis of data taken during gamma ray source exposures. Neutron and gamma ray flux from rock surrounding the detector is calculated and interaction rates as a result of this radioactivity in an un-shielded DRIFT-IIA found to be 1.3 nuclear recoils per day and under 2.1 electron recoils per day, above detector threshold, with the nuclear recoil rate dropping to under 1 event per year for a shielded detector. Future operation, development and expansion of the DRIFT-II array will lead to increased potential for directional dark matter detection.
20

Tests of schemes to infer stratospheric temperature from satellite measurements

Jackson, David R. January 1990 (has links)
In this thesis we test a retrieval/analysis scheme for inferring stratospheric temperature from satellite observations of radiance. The scheme is similar to that used by the UK Meteorological Office. The retrievals are made by using a multiple linear regression model which regresses radiances against Planck function, whilst the analyses are made using a linear time/space interpolation method. In addition, we compare analyses made using time/space interpolation with analyses made using another analysis scheme which sequentially estimates Fourier coefficients at fixed latitudes using a version of the Kalman Filter. Because of the lack of 'ground truth' observations in the stratosphere, the schemes are tested in simulation experiments. Preliminary tests of the time/space interpolation and sequential estimation analysis schemes are made using idealised radiance fields which resemble observations made by a satellite radiometer in the northern hemisphere winter stratosphere. The regression retrieval scheme and the two analysis schemes arc also tested in a more sophisticated experiment in which the 'true' atmosphere is represented by an atmosphere simulated by a numerical model. Simulated observations are calculated by computing the radiance that would be observed from the 'true' atmosphere by a satellite instrument. The radiances are then retrieved and analysed and the resultant analyses compared with the corresponding 'true' fields. Tests are made using output from a day when a sudden wanning was present. The retrieval scheme is seen to perform less well within the area of the sudden wanning than outside it. However, this may be expected as the vertical structure within the sudden warming is generally too small to be resolved by a satellite instrument. The analysis scheme analyses the stratospheric field well, even in the area of a sudden warming. These results, and results from preliminary tests made using idealised radiance fields, suggest that the analysis is generally of better quality when the distance radius used to select observations for the scheme is small. Results of tests of the sequential estimation scheme reveal that this method also produces satisfactory analyses of idealised radiance and model fields. Constraints of time prevented more rigorous testing of the scheme, but suggestions for further research are given.

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