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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.
51

The properties of current limited spark chambers

Stubbs, R. J. January 1971 (has links)
An investigation has been made of the characteristics of the current limited spark chamber, in which it was found that certain anomalies existed in its operation. Such discrepancies are not found in the operation of the more conventional types of chamber. They are thought to be due to charge deposited on the dielectric walls producing internal electric fields across the gas which reduce the chamber efficiency. The variation of the clearing field has been investigated in regions far from the discharge, and would appear to be present throughout the chamber. Measurement of the decay constant of the charge agreed well with the value calculated considering the chamber capacity and surface resistivity of the glass, although on application of this constant, to determine equilibrium values of the clearing fields, a second, much longer decay process seemed to be suggested. A theoretical model for handling such small clearing fields has been examined, and applied to the system, thus giving an indication of the electron drift velocity and density of charge deposited on the walls The development of an approximate semiquantitative model of spark breakdown shows how different pulse delays will affect the growth of streamers, and describes qualitatively certain characteristics of the discharges noted during chamber operation. However, the lack of more detailed knowledge of certain discharge parameters does not allow a more detailed comparison.
52

Variable-temperature photoluminescence emission instrumentation and measurements on low yield metals

Armstrong, Helen January 2010 (has links)
Measurements of the photoluminescence emission spectra of 99.999 % purity gold, 99.9999 % purity copper, polycrystalline PbMo6S8 and single crystal YBCO were made for λex = 488 nm as a function of temperature (72 K < T < 300 K), time (t < 12 hours), excitation power (P < 120 mW) and position on the sample using a high sensitivity instrument which was designed, commissioned and calibrated for this study. We present the first measurements of the photoluminescence emission spectra of gold and copper as a function of temperature which show peak photoluminescence emission intensity increasing by approximately a factor of two for gold and a factor of five for copper between 300 K and 79 K. Full width half maximum (FWHM) and peak photoluminescence emission wavelength showed no dependence upon temperature. The spectra compare well to published data and data modelled using theories presented in the literature. Variable temperature measurements on the superconductors PbMo6S8 and YBCO in their normal state show peak photoluminescence intensity increasing by a factor of 1.5 between 300 K and 80 K for PbMo6S8 and a factor of 2 between 300 K and 131 K for YBCO. A decrease in FWHM of 20 - 30 nm is observed with no change in peak photoluminescence wavelength. Measurements for 99.99 % purity single crystal niobium, polycrystalline SnMo6S8 and single crystal DyBCO superconductors are also presented, however, these samples exhibited problems with oxidation, impurities or damage to the sample surface. Two interesting features which remain unexplained from this work include a variation in photoluminescence emission intensity over < 12 hours with a period of ~400 minutes for gold and copper and a continuous decrease in intensity for niobium, SnMo6S8 and YBCO and an increase in photoluminescence emission intensity by a factor of 4 at low temperatures in PbMo6S8, SnMo6S8 and YBCO.
53

Vapour sorption, wavelength tracking and thermo-optic properties of dual slab waveguide interferometers

Cassidy, David Robertson January 2007 (has links)
The dual slab waveguide interferometer is introduced as a device which has many applications in various research areas. Reported is its ability to provide details on the mechanism for the vapour sorption of thin polymer films, the development of the interferometer as a wavelength tracking device for the telecommunications industry and a method to characterise the thermo-optic properties of III-V semiconductor alloys. The vapour sorption mechanisms of thin films of polymers Polyisobutylene (PIB) and Polyvinylpyrrolidone (PVP) on exposure to several solvents are investigated. Coating interferometer chips with a thin layer of polymer and monitoring the interference fringe pattern for changes due to the exposure to a solvent vapour provides information on the mechanism for vapour sorption as one of swelling rather than void-filling. Interferometer sensitivities to vapour concentrations are linear and depend on refractive index differences between polymer and condensed vapour. An interferometer manufactured from III-V semiconductor compounds is developed to produce a device which can operate as a wavelength tracker. Sensitivities of over 6 mrad / pm and 7 mrad / pm for single and dual quaternary systems respectively indicate that a device of length around 5 mm would be capable of detecting picometer input wavelength changes including thermal background noise. The sensitivity to thermal changes provides a simple method for determining the thermo-optic coefficient of two Indium Gallium Arsenide Phosphide (InGaAsP) alloy compositions as (3.15±0.08)x 10(^-4)K(^-1) and (2.60±0.017)x10(^-4)k(6-1) forInGaAsP compounds with bandgap wavelengths around 1.3 μm and 1.15 μm respectively.
54

Microchannel plates in astronomy

Price, Gareth James January 2001 (has links)
This thesis describes both round-pore microchannel plates (MCPs) used in energetic pho¬ton and particle detectors and their square-pore offspring, micropore optics (MPOs), used to focus x-rays. A Monte Carlo electron raytracing software package is described that is used to predict the energy and angular distribution of electrons (EDOE and ADOE) in a microchannel electron multiplier's output charge cloud, including saturated operation. The model is shown to agree with experimental evidence. The addition of a micromachined electrostatic lens to the end of a microchannel is modelled and found to have no beneficial effects upon the EDOE and ADOE of the channel. The current state of the art planar and slumped 'lobster eye' square-packed MPOs are evaluated. The best focus (5' FWHM) from a large format (61mm x 56mm), small chan¬nel (10μm side length) planar MPO is reported, together with the observation of high energy (~50keV →65keV) x-ray focusing from large (500:1) aspect ratio channels. The alignment of many small lobster eye MPOs to create a large optic for the Lobster-ISS instrument is discussed and the alignment jig constructed for this purpose is used to measure the bias angles of a Lobster specification MPO. The bias angle is found to be 4 ± 1.5'. The concept of the microchannel conic approximation to the Wolter type I and II x- ray lenses is reviewed. A radially-packed twin MPO Wolter approximation is then tested, which while of poor quality, demonstrates true Wolter II imaging with a peak gain greater than unity. Currently proposed (UK) astronomical instruments that employ MPOs are then discussed in the light of the results from the current generation of MPOs.
55

Energy selecting electron microscopy

Philip, John George January 1973 (has links)
No description available.
56

Applications of the weak-beam technique of electron microscopy

Jenkins, M. L. January 1973 (has links)
No description available.
57

Detection problems with the displays of radioisotope imaging devices

Sharp, Peter F. January 1973 (has links)
The final interpretation of the image produced by a radioisotope imaging device depends upon a human observer. The object of this investigation is to see what factors influence the observer's response, The interpretation of a display record is defined as the detection of a perturbation in an otherwise normal pattern, distribution, of radioisotope. For the purpose of this investigation the simplest normal pattern is chosen, that of a uniform distribution of radioisotope. The perturbation, or target, consists of a circular area of radioisotope, 2 cm. in diameter, superimposed on the normal pattern, or background. The observer's ability to detect the target is measured by his visual response i.e. the percentage of the number of times he manages to detect the target. Three types of displays are investigated: a gamma-camera oscilloscope display, a multi-element display and a rate-meter controlled display with colour-coded statistical bonds. The test-pattern is looped on each display and the resulting display records are shown to a group of observers. The observer's results are plotted in the form of visual response curves, each of which shows visual response as a function of target Intensity for a given value of background intensity. These curves are measured for a series of values of the background intensity. The visual response curves are found to be ogival in shape. This is due both to the day-to-day variation in the responses given by an observer and to the fact that the visual appearance of a target is not completely defined by its Intensity measured in units of counting-density or counting-rate. The target intensity that produces a visual response of 50% is taken as representative of the detectability of the target at a particular background intensity. The ratio of this target intensity to the background intensity is called the 50% contrast. With the oscilloscope display the 30% contrast was found to decrease with increasing background intensity. The two other displays differ from the oscilloscope display in that they are bend-limited. There are only a limited number of different 'intensity' unite with which to display the information. The multi-element display has 8 different grey shades end the counting-rate display 6 colours. Therefore a range, or band, of values of the counting-density or counting-rate is assigned to each of the display 'intensity units. In the multi-element display the position of the limits defining each band is fixed. This results in e saw-tooth shaped variation of the 50% contrast with background intensity. However on the rate-meter controlled display the position of the bands can be altered. With the bends always positioned symmetrically about the mean background intensity the 50% contrast decreases monotonically with increasing background intensity. The performances of the displays are intercompared. The performance of a display is expressed by the surface defined by the axes showing visual response, the ratio of the target-to-background intensity, and the background intensity. A good display is one which has low values of the target-to-background ratio, for a given visual response, end which shows a rapid rise in visual response with increasing target intensity. The multi-element and counting-rate displays are both shown to be superior to the oscilloscope display end comparable to each other in performance. A theory is also proposed to explain the manner in which such perturbations might be detected by the visual system. It is suggested that the observer identifies as the perturbation any one area whose intensity is much greater than that of any other area of comparable size found elsewhere in the background. This contrasts with the statistical theories which compare the target with the average of the signals from the background. The limitations of this way of measuring display performance are indicated and, in conclusion, further areas of work are suggested.
58

Instrumentation for measurement of radius and topography of curved surfaces

Siddall, Graham J. January 1975 (has links)
Effective control of the radius and surface geometry of curved surfaces is of paramount importance in precision engineering, particularly within the bearing industry. Stylus tracer instruments can be designed or adapted to measure such surfaces but are limited in their ability to measure radii of curvature. This thesis is concerned with the design and performance of stylus tracer instrumentation capable of both dimensional and surface measurement. After a review of existing techniques, a simple device is described for establishing the suppressed radius of curvature of a spindle type stylus instrument. An accuracy of determination of 2 m is possible over a wide range of radii, using conventional gauge blocks for setting purposes. An account is given of the design, construction and performance of a comprehensive stylus instrument which features measurement of radii within 2 m over a range of 0.25 to 50 mm. This instrument utilises a low-cost dry bearing spindle with negligible 'stick-slip' and with radial and axial errors of rotation of 0.1 m. Surface texture measurement is possible with a resolution of 5 m and a short term repeatability of 10 nm. Surfaces with are lengths of 2 to 360 can be measured utilising electronic control of the position and velocity of the stylus traverse. The problem of setting up an arcuate component on a stylus instrument is examined and a centring aid is devised in the form of a simple iterative algorithm. The effectiveness of this technique, together with the advantages of an integrated instrument approach, are demonstrated by coupling the instrument on-line to a digital computer. An additional radius measuring device, designed specifically for use with a commercial roundness measuring instrument is described. A unique feature of this system is its capability of measuring the size of the least squares reference circle without the need for precise centring of the component.
59

Problems associated with the CdS photovoltaic cell

Buckley, Robert William January 1973 (has links)
No description available.
60

Quantitative methods in electron microscopy

Love, G. January 1978 (has links)
Existing absorption correction programmes used in quantitative electron probe microanalysis are assessed and their limitations discussed. It is shown that the Philibert absorption correction models (Philibert 1963) may be significantly improved by the adoption of new expressions for the a and h terms. A new absorption correction proposed by Bishop (1974) is also investigated and its range of applicability is found to be greater than either of the two Philibert models. Accuracy of the Bishop approach has been improved by introducing a new expression for the mean depth of X-ray emission which is derived from a detailed study of electron-solid interactions using a Monte Carlo simulation of electron trajectories. Data from the Monte Carlo programme are also used in the development of a new atomic number correction. This new correction procedure overcomes some of the limitations in the Duncumb and Reed approach (Duncumb and Reed 1968) and provides an important advance in the field since it permits the computation of correction factors for specimens inclined to the incident electron beam. Applications of quantitative electron probe microanalysis to research in both materials science and biology are described, the new data providing information which has advanced our understanding in diverse fields of study such as growth processes of avian eggshells and oxidation mechanisms in polymeric materials. Complementary research using other electron optical techniques such as scanning and transmission microscopy is also described. These instruments have been employed to study the microstructure of materials and, more particularly, to provide quantitative data on defect populations in thin foils. In addition new techniques of foil thickness measurement are assessed and the prospects for improvement in this area are discussed.

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