School of Physics - Theses

Permanent URI for this collection

Search Results

Now showing 1 - 10 of 29
  • Item
    Thumbnail Image
    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.
  • Item
    Thumbnail Image
    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.
  • Item
    Thumbnail Image
    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.
  • Item
    Thumbnail Image
    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.
  • Item
    Thumbnail Image
    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.
  • Item
    Thumbnail Image
    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.
  • Item
    Thumbnail Image
    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.
  • Item
    Thumbnail Image
    Quantum information engineering: concepts to quantum technologies
    Devitt, Simon ( 2007-11)
    This thesis investigates several broad areas related to the effective implementation of quantum information processing, from large scale quantum algorithms and error correction, through to system identification and characterization techniques, efficient designs for quantum computing architectures and the design of small devices which utilize quantum effects. The discussion begins with the introduction of a quantum circuit appropriate for implementing Shor’s factoring algorithm on Linear Nearest Neighbor qubit arrays such as the Kane phosphorus in silicon system. Detailed numerical sim- ulations are then presented, demonstrating the sensitivity of the circuit under coherent quantum errors. The concepts of Quantum Error Correction and Fault-tolerant computation are reviewed with original work carried out to show the relative robustness and practicality of Fault-tolerant computation for logical state preparation. Methods of intrinsic system identification and characterization are proposed. Protocols for characterizing both the confinement of a multi-level system to the qubit subspace and the Hamiltonian dynamics governing two-qubit interactions are presented as well as a brief review of characterization techniques already developed for single qubit dynamics. (For complete abstract open document)
  • Item
    Thumbnail Image
    On the one-dimensional bose gas
    Makin, Melissa I. ( 2007-02)
    The main work of this thesis involves the calculation, using the Bethe ansatz, of two of the signature quantities of the one-dimensional delta-function Bose gas. These are the density matrix and concomitantly its Fourier transform the occupation numbers, and the correlation function and concomitantly its Fourier transform the structure factor. The coefficient of the delta-function is called the coupling constant; these quantities are calculated in the finite-coupling regime, both expansions around zero coupling and infinite coupling are considered. Further to this, the density matrix in the infinite coupling limit, and its first order correction, is recast into Toeplitz determinant form. From this the occupation numbers are calculated up to 36 particles for the ground state and up to 26 particles for the first and second excited states. This data is used to fit the coefficients of an ansatz for the occupation numbers. The correlation function in the infinite coupling limit, and its first order correction, is recast into a form which is easy to calculate for any N, and is determined explicitly in the thermodynamic limit.
  • Item
    Thumbnail Image
    Superfluid spherical Couette flow and rotational irregularities in pulsars
    PERALTA, CARLOS ANDRES ( 2006-10)
    Small amplitude rotational irregularities are observed in a number of rotation-powered pulsars. They fall into two classes: (i) glitches, defined as abrupt increases in the angular velocity of a pulsar (accompanied sometimes by changes in the angular acceleration Ω), of which 286 have been observed in 101 objects; and (ii) timing noise, a continuous stochastic fluctuation in phase, or, which is observed mostly in young and adolescent pulsars (with ages ≥ 10 4 yr). Both classes of irregularity seem to arise from some mechanism that couples the angular momentum of the solid crust and superfluid core of the star, which is activated suddenly when differential rotation exceeds a threshold. Coupling mechanisms proposed to date include catastrophic vortex unpinning in the inner crust, triggered by starquakes; vortex creep, due to thermally activated quantum tunnelling; superfluid-superconductor interactions in the core; and superfluid instabilities. The associated theories are phenomenological, not predictive.