School of Physics - Theses

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    A spectroscopic and chromatographic study of the photochemical properties of daylight fluorescent paint
    Hinde, Elizabeth ( 2009)
    Daylight fluorescent pigments fade rapidly, accompanied by a chronology of colour change. Fluorescence is a photo-physical phenomenon which involves emission of light from an excited state. Fluorescent dyes thus have a high susceptibility of being promoted to an excited state; a characteristic in the case of organic fluorophores which infers vulnerability toward photo-bleaching. Multiple organic fluorescent dyes are routinely incorporated into a given daylight fluorescent pigment, to either additively fluoresce or interact through energy transfer. The organic fluorescent dyes employed invariably differ in photo-stability, and upon loss of each species of fluorophore an abrupt colour change is observed. The collective result of this fading behaviour is that in a short period of time a daylight fluorescent paint layer will be of a different hue, devoid of luminosity. As consequence it is almost impossible to colour match a faded daylight fluorescent paint layer without the hues diverging asynchronously, or ascertain the original palette of a daylight fluorescent artwork after a protracted period of time. The predicament is exacerbated by the fact that there is no standard method in cultural material conservation, of documenting daylight fluorescent colour in a painting photographically or colorimetrically. The objective of this thesis is to investigate the photochemical behaviour of daylight fluorescent pigments, to ensure best practice in the preservation of artworks that contain daylight fluorescent paint. Fluorimetrie and chromatographic analysis of the DayGlo daylight fluorescent pigment range at the constituent dye level, prior to and during an accelerated light ageing program formed the basis of the experimental. Given the limited selection of fluorescent dyes suitable for daylight fluorescent pigment manufacture, it is anticipated that the results attained for the DayGlo range will be applicable to all daylight fluorescent media encountered in cultural material. Experimental data revealed the manner in which the fluorescent dyes behind each DayGlo daylight fluorescent pigment were formulated, and provided explanation for the 1colour changes observed upon fading. A prognosis of when and why a daylight fluorescent palette experiences hue shift and the implications this has for display is presented. Methodology for imaging daylight fluorescence, identification of the constituent fluorescent dyes in a daylight fluorescent pigment and colour matching a daylight fluorescent paint layer are presented and applied in-situ, to case studies possessing a daylight fluorescent palette.
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    Studies for the adaptation of a field ionization ion source for a proton microprobe
    Colman, Robert Alan ( 1989)
    A major factor limiting the resolution of the Scanning Proton Microprobe is the brightness of the primary beam supplied by the accelerator. The recent development of a field ionization proton source, which is up to five orders of magnitude brighter than the present source, holds the promise of substantially improved resolution in MP. The optics of the Pelletron accelerator were studied to determine the expected resolution improvement to the MP beam from the installation of the new source. The optics of the field ionization source region were studied using the charge simulation method. First order effective source size was calculated for field ionization tips, and calculations carried out to determine the contribution of aberrations to source size. Tip size and applied voltage to maximize source brightness were also investigated. The present electrostatic lens was investigated for use with the field ionization source, and found to be unsuitable unless very high voltages were to be applied. A range of alternative two and three element electrostatic lenses was investigated. Three element lenses were found to be more flexible, and generally had lower aberrations than two element lenses. Various designs of three element lenses were examined, and accelerating and decelerating modes discussed for all lenses. Accelerating lenses, although optically superior, were generally found to require unacceptably high applied voltages in order to achieve focusing. Decelerating lenses were investigated in further detail, and the geometry of promising lenses varied to attempt to reduce aberrations. Calculations suggested the best alternative to the present lens to be a miniaturized variation of the decelerating Riddle lens. A full scale version of this lens was studied on a specially constructed electron optical bench. The two grid method was used to measure cardinal points for the lens, as well as chromatic and spherical aberrations. The values of measured optical properties were found to correspond well with theoretical calculations for the same lens over the voltage range of most importance, suggesting that a reduced scale version of the same lens would be suitable for use with the field ionization source. The optics of the accelerating column were also investigated using the finite element method. Cardinal elements were extracted for a range of source lens operating voltages, permitting the calculation of accelerator object positions for a focus at the analysing magnet object slits. Chromatic and spherical aberrations of the accelerating column were also determined, and their effect on beam brightness for various source and lens configurations discussed. Finally all ion optical elements were combined and the final brightness, and expected MP beam resolution determined for a range of optical combinations Conclusions were drawn on the most appropriate optical configuration of the accelerator. Further work required for the installation of the source was also discussed.
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    Application of a scanning proton microprobe as a diagnostic tool and the development of a high brightness ion source
    Allan, Garry Lindsay ( 1989)
    This thesis concerns both the application and future development of a Scanning Proton Microprobe (SPMP). The work involved the use of a microprobe in a biological project which placed heavy demands on beam brightness, and also a program to investigate and address the demand for brighter microprobe beams. The thesis thus falls naturally into two distinct, though related, sections. The SPMP has been applied to the study of Menkes' disease, a copper-dependent genetic disorder. The disease is expressed in fibroblast cells, and the SPMP was used to map elemental distributions within both normal and Menkes' fibroblasts. An elevated level of intracellular copper was observed within Menkes' cells enabling individual cells to be identified as normal or Menkes' depending upon the copper content of the cell. Subcellular structure within fibroblasts was investigated by using the microprobe as a Scanning Transmission Ion Microscope (STIM). It was shown that this technique affords sufficient resolution to image the nuclear membrane and nucleoli. However, at this resolution, insufficient beam current was available to permit elemental distributions to be obtained. The elemental content of subcellular and subnuclear components is of fundamental importance to biochemical processes within the cell and to the expression of Menkes' disease. Hence an increase in the resolution of the SPMP is of major importance provided that the beam current can be maintained at levels acceptable for elemental analysis. Such a significant improvement in microprobe resolution can only be achieved with a brighter primary beam from the accelerator. This requires a brighter ion source. The performance of the existing RF ion source has been studied on a suitable test-bench, and its brightness measured. The possible use of alternative ion sources offering significant gains in. brightness was investigated, and an ion source using the process of field ionization was designed and built. Field ionization sources use a sharply pointed emitter as the site for ion production. This gives these sources an intrinsically high brightness, but in general they have not been designed so as to produce currents suitable for use in an electrostatic accelerator. The present field ionization source was optimized to produce a maximum current whilst being sufficiently rugged and compact to withstand use within the accelerator. The beam brightness achieved with this source offered a significant increase in source brightness with sufficient current to provide stable operation of the accelerator. The successful implementation of this source would produce a major improvement in the spatial resolution available for imaging and elemental analysis with the microprobe.
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    UHE neutrino detection using the lunar Čerenkov technique
    McFadden, Rebecca Angela ( 2009)
    This thesis investigates Ultra High Energy (UHE) neutrino detection using the lunar Čerenkov technique. UHE neutrinos may hold the key to understanding the origin of the most energetic particles observed in nature, the UHE Cosmic Rays (CR). UHECR traveling over distances larger than ∼80 Mpc will lose energy to photo-pion production, causing a suppression of the cosmic ray spectrum at the highest energies. However, significant information is preserved in the spectrum of neutrinos produced as a result of these interactions and UHE neutrino astronomy may therefore be able to provide more insight into the origin of the UHECR. Direct detection of UHE neutrinos is very difficult due to their extremely small interaction cross-sections. Instead, they may be detected indirectly via observation of the Askaryan effect (1) in the lunar regolith. Using the Moon as a large volume neutrino detector, coherent radio Čerenkov emission from neutrino-induced cascades in the lunar regolith can be observed with ground-based telescopes. This thesis explores detection issues associated with using this technique including an investigation into alternative planetary detectors, the phenomenology of the radio Čerenkov emission and the effect of ionospheric dispersion on Čerenkov pulse propagation. The results of this investigation were used to design a detector system for a series of experiments performed at the Australia Telescope Compact Array. The experiments made use of a 600-MHz RF signal available at the ATCA which required the development of custom-designed ionospheric dedispersion and pulse detection hardware. Approximately 36.5 hours of lunar data were taken over three observations. Earlier runs were scheduled to target a broad region surrounding the galactic centre, which was chosen as it harbors the closest super massive black hole and potential accelerator of UHECR. Scheduling of the final ATCA experiment in May 2008 was influenced by results published by the Pierre Auger Observatory in late 2007, which showed a statistical correlation between observations of the highest energy CR and the matter distribution in the local universe as represented by nearby active galactic nuclei (2). These experiments offered an increase in exposure to the regions of Sagittarius A* and Centaurus A. However, analysis of the data revealed that most of the candidate events could be attributed to terrestrial RFI and no candidates events were suspected as potential lunar Čerenkov pulses. A new method to calibrate the dispersive effect of the ionosphere on lunar Čerenkov pulses is also presented. This method exploits radial symmetries of the lunar polarisation distribution to make Faraday rotation measurements in the visibility domain of synthesis array data. The Faraday rotation measurements are combined with geomagnetic field models to estimate the ionospheric Total Electron Content (TEC). An accurate knowledge of the ionospheric TEC can be used to perform pulse dedispersion and recover maximum Čerenkov pulse amplitude before detection. This method of ionospheric calibration is particularly attractive for the lunar Čerenkov technique as it can be used in real time to give values of the ionospheric TEC which are line-of-sight to the Moon.
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    Atomic resolution microscopy using electron energy-loss spectroscopy
    Witte, C. ( 2008)
    This thesis explores the theory of electron energy-loss spectroscopy (EELS) in atomic resolution electron microscopy. The first unequivocal evidence of the effective nonlocal potential in momentum-transfer-resolved EELS is presented. For suitable geometries, the nonlocal potential can be well approximated by a local potential. In scanning transmission electron microscopy (STEM) the validity of this is mainly influenced by the detector size and, contrary to conventional wisdom, a thin annular detector does not allow direct image interpretation. It is found that the best way to ensure the potential is well approximated by a local potential is to use a detector with a large collection angle. To simplify computation and interpretation it is desirable to make the single-channelling approximation. In this approximation only the elastic scattering of the probe before the ionisation event is modelled. It is shown how this approximation breaks down for the small detectors used in momentum-transfer-resolved EELS and this is confirmed with experimental results. Double-channelling calculations, where the channelling of the probe both before and after the ionisation event are modelled, can also be simulated. An alternative approximation for small detectors that includes double channelling and is more applicable for momentum-transfer-resolved EELS is also presented. Beyond chemical information, the fine structure of an absorption edge gives bonding and electronic information. Incorporating fine structure into channelling theory allows the exploration of the effects of channelling on fine structure. The weighting of the two different spectra in graphite, as a function of incident probe tilt in momentum-transfer-resolved EELS, is calculated using double-channelling simulations. This is combined with experimental data and multivariate statistical analysis to extract the two physical spectra, greatly simplifying the analysis of a large data set. The effect of the nonlocal potential and channelling on site-specific electronic structure analysis by channelling EELS is examined. It is found that using a large on-axis detector can make the interaction effectively local, leading to a greater change in the spectra as a function of sample tilt. Alternatively offsetting the detector can achieve similar results but at the cost of greater statistical noise. Channelling calculations were combined with the program FEFF and the full energy differential cross section was calculated from first principles for the aluminium K edge as a function of sample tilt in nickel aluminate spinel. Qualitative agreement with experiment was found but quantitative agreement will require further investigation. The theory of fine structure in STEM was examined, using strontium titanate to see how the high spatial resolution of STEM can be used in conjunction with energy-loss near-edge spectroscopy measurements. The possibility of imaging unoccupied electron molecular orbitals using STEM was also examined.
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    Studies in phase and inversion problems for dynamical electron diffraction
    Faulkner, Helen Mary Louise ( 2003)
    This thesis examines problems in electron diffraction and related areas of theoretical optics. It begins with a study of the phase of a quantum mechanical wave function and the behaviour of phase vortices and vortex cores. Several rules for vortex core evolution are given and simulated vortex trajectories are studied. These simulations show that in electron microscopy at atomic resolution and in other similar situations, vortices occur in the wave functions very frequently. This means any image processing methods which deal with the wave function phase must permit vortices to occur. In this context a number of methods of phase retrieval are compared and evaluated. The criteria of evaluation are the accuracy of the phase retrieval, its ability to cope with vortices, its numerical stability and its required computational resources. The best method is found to be an iterative algorithm similar in approach to the Gerchberg-Saxton method, but based on a through focal series of images. Using this phase retrieval method as an essential tool, the thesis continues with a study of inverse problems in electron optics. The first problem considered is that of using a set of images taken to characterise the coherent aberrations present in a general imaging system. This problem occurs in many areas of optics and is studied here with a focus on transmission electron microscopy. A method of using software to simultaneously determine aberrations and subsequently remove them is presented and tested in simulation. This method is found to have a high level of accuracy in aberration determination. The second inverse problem studied in this thesis is the inversion problem in dynamical electron diffraction. This problem is solved for a periodic object, giving an accurate and unique solution for the projected potential in the multiple scattering case. An extension of this solution to objects which are non-periodic in the direction of the incident wave is investigated. Finally a model computation solving the general inversion problem for dynamical diffraction in an aberrated transmission electron microscope is performed, illustrating this and previous material and summing up the advances presented in this work.
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    Neutrino oscillations and the early universe
    Bell, Nicole Fiona ( 2000)
    We construct a model which provides maximal mixing between a pseudo-Dirac Vµ/VT pair, based on a local U(1)Lµ-LT symmetry. Its strengths, weaknesses and phenomenological consequences are examined. A new intermediate range force is predicted, mediated by the light gauge boson of U(1)Lµ-LT. Through the mixing of µ, T and e, this force couples to electrons and thus may be searched for in precision “gravity” experiments.The generation of relic neutrino asymmetries in the early universe via the mechanism of partially coherent active-sterile neutrino oscillations is considered. We study how an approximate evolution equation for the growth of the asymmetry can be extracted from the exact Quantum Kinetic Equations which describe the evolution of the neutrino ensemble, and examine the nature of some of the approximations employed.
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    Realistic read-out and control for Si:P based quantum computers
    Testolin, Matthew J. ( 2008)
    This thesis identifies problems with the current operation proposals for Si:P based solid-state quantum computing architectures and outlines realistic alternatives as an effective fix. The focus is qubit read-out and robust two-qubit control of the exchange interaction in the presence of both systematic and environmental errors. We develop a theoretical model of the doubly occupied D- read-out state found in Si:P based nuclear spin architectures. We test our theory by using it to determine the binding energy of the D- state, comparing to known results. Our model can be used in detailed calculations of the adiabatic read-out protocol proposed for these devices. Regarding this protocol, preliminary calculations suggest the small binding energy of the doubly occupied read-out state will result in a state dwell-time less than that required for measurement using a single electron transistor (SET). We propose and analyse an alternative approach to single-spin read-out using optically induced spin to charge transduction, showing that the top gate biases required for qubit selection are significantly less than those demanded by the adiabatic scheme, thereby increasing the D+D- lifetime. Implications for singlet-triplet discrimination for electron spin qubits are also discussed.
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    TEM and structural investigations of synthesized and modified carbon materials
    Lai, Pooi-Fun ( 1999-08)
    Due to the extreme properties of diamond, such as extreme hardness, high thermal conductivity, high electrical breakdown strength, high electron and hole mobilities and large band gap, it is of interest to study this material in detail. Before advantage can be taken of diamond’s properties for high-temperature, high-power electronic applications successful doping/ion implantation of diamond must be achieved. This requires an understanding of the types of defects produced during ion irradiation. In the present work, type IIa diamond has been irradiated with various doses of 320keV Xe ions at room temperature. Analytical techniques used are electron spin resonance spectroscopy, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. Previous models have suggested that upon ion impact, amorphous and/or graphitized clusters are formed in diamond, which will overlap at a critical dose to form a semi-continuous graphitized layer. (For complete abstract open document)
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    Star formation and galaxy evolution of the Local Universe based on HIPASS
    Wong, Oiwei Ivy ( 2007-12)
    This thesis investigates the star formation and galaxy evolution of the nearby Local Volume based on Neutral Hydrogen (HI) studies. A large portion of this thesis consists of work with the Northern extension of the HI Parkes All Sky Survey (HIPASS). HIPASS is an HI survey of the entire Southern sky up to a declination of +25.5 degrees (including the Northern extension) using the Parkes 64-metre radio telescope. I have also produced a catalogue of the optical counterparts corresponding to the galaxies found in Northern HIPASS. From this optical catalogue, we also conclude that we did not find any isolated dark galaxies. The other half of my thesis consists of work with the SINGG and SUNGG projects. SINGG is the Survey for Ionization in Neutral Gas Galaxies and SUNGG is the Survey of Ultraviolet emission in Neutral Gas Galaxies. Both SINGG and SUNGG are selected from HIPASS and are star formation studies in the H-alpha and ultraviolet (UV), respectively. My work in the SINGG-SUNGG collaboration is mostly based on SUNGG. Using the results of SUNGG, I measured the local luminosity density and the cosmic star formation rate density (SFRD) of the Local Universe. Using far-infrared (FIR) observations from IRAS, the FIR luminosity density was also calculated. Combining the FUV luminosity density and the FIR luminosity density, the bolometric SFRD of the Local Universe was estimated. This thesis also includes the discovery of one of the nearest drop-through ring galaxies, NGC 922, which is a factor of three closer than the infamous Cartwheel galaxy.