School of Physics - Theses
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ItemDetecting and characterising extrasolar planets in reflected light(University of Melbourne, 2009)
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ItemFault-tolerant quantum computation with local interactions(University of Melbourne, 2009)
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ItemMonte Carlo characterisation of narrow photon fields for intensity modulated radiotherapy(University of Melbourne, 2006)
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ItemFocusing of an atomic beam using a TEM01 mode lens(University of Melbourne, 2006)
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ItemThe Panoramic Deep Fields( 2001)The Panoramic Deep Fields are a deep multicolour survey of two ~ 25 ° fields at high galactic latitude. The survey images have been constructed by digitally stacking scans UK Schmidt plates. Deep images (Bj ~23.5) with low contamination have been obtained by subtracting the background from the individual plates scans and using bad pixel rejection during the stacking. The size and depth of the fields allow the accurate statistical measurement of the environments and evolution of galaxies and AGN. The clustering of galaxies and galaxy clusters has been measured from z ~0.4 until the current epoch. The clustering properties of galaxies are strongly correlated with colour and blue U – Bj selected galaxies exhibit weaker clustering than any morphologically selected sample. The weak clustering (ro ≤ 3h -1 Mpc) of blue galaxies implies galaxy colour and stellar population are more strongly correlated with environment than galaxy morphology. Despite the large fields-of-view, the clustering of red galaxies and clusters varies significantly between the two fields and the distribution of clusters is consistent with this being due to large-scale-structure at z ~0.4. The evolution and environments of AGN have been measured at intermediate redshifts with the Panoramic Deep Fields. Photometric redshifts, colour selection and the NVSS have been used to compile a catalogue of ~ 180 0.10 < z< 0.55 radio galaxies. The evolution of the radio galaxy luminosity function is consistent with luminosity evolution parameterised by L (z) ~ L(0) (1+z)3.4. The environments of UBR selected AGN and radio galaxies have been measured at z~0.5 using the Panoramic Deep Field galaxy catalogue. By applying photometric red-shifts and colour selection criteria to the galaxy catalogue, it has been possible to increase the signal-to-noise of the angular correlation function and measure the cross-correlation with specific galaxy types. Most AGN host environments are comparable to the environments of galaxies with the same morphology. However, ~6% of UBR selected AGN are in significantly richer environments. No significant correlation between AGN luminosity and environment was detected in the Panoramic Deep Fields. The richness of AGN environments is not strongly correlated with redshift and the rapid evolution of the AGN luminosity function is not caused by evolution of AGN host environments.
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ItemA spectroscopic and chromatographic study of the photochemical properties of daylight fluorescent paint( 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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ItemAstigmatic phase retrieval of lightfields with helical wavefronts( 2012)The controlled use of coherent radiation has led to the development of a wide range of imaging methods in which aspects of the phase are enhanced through diffraction and propagation. A mathematical description of the propagation of light allows us to determine the properties of an optical wavefield in any plane. When a sample is illuminated with coherent planar illumination and its diffracted wavefield is recorded in the far-field of propagation, a direct inverse calculation of the phase can be quickly performed through computational means – the fast Fourier transform. Algorithmic processing is required, however, because only the intensity of the diffracted wavefield can be recorded. To determine structural information about the sample, some other information must be known about the experimental system. What is known, and how it is processed computationally, has led to the development and successful application of a broad spectrum of phase reconstruction iterative algorithms. Vortices in lightfields have a helical structure to their wavefront, at the core of which exists, necessarily, a screw-discontinuity to their phase. They have a characteristic intensity distribution comprising a radially symmetric bright ring around a dark core which, for either handedness of the rotation of the vortex, appears identical. Observation of a vortex is, therefore, ambiguous in its ability to determine its true direction of rotation. The ubiquitous presence of vortices in all lightfields hinder the success of phase reconstruction methods based on planar illumination and, if successful, render any reconstruction of the phase non-unique, due to the ambiguity associated to their helicity. The presence of a controlled spherical phase distortion can break the symmetry of the appearance of the vortices and, hence, remove the ambiguity from the system and drive algorithms to a solution. For the pathological case of an on-axis vortex, however, spherical distortion will not break the radial symmetry. The astigmatic phase retrieval method separates the spherical distortion into cylindrical distortion in two orthogonal directions. This form of phase distortion breaks the symmetry of a vortex allowing a unique determination of the phase. The incorporation of such use of cylindrical distortion into an iterative phase reconstruction algorithm forms the basis for the astigmatic phase retrieval (APR) method. Presented in this thesis is the creation and propagation of lightfields with helical wavefronts, produced through simulation and experiment. Observation of the effects of cylindrical distortion on vortices is explored in detail, particularly for split high-charge vortices where their positions can inform the type and strength of the applied phase distortion. Experimentally, onaxis vortices are created and distorted for the purposes of astigmatic phase retrieval in both visible light and X-ray wavefields. This thesis presents the first experimental demonstration of the astigmatic phase retrieval (APR) method, successfully applied optically with a simple test sample. The method is also applied to lightfields with helical wavefronts. The successful unambiguous reconstruction of on-axis chargeone and charge-two visible light vortices are presented, which is the first experimental demonstration on the unique phase reconstruction of an on-axis vortex from intensity measurements alone. Experiments are then performed to apply the method to vortices created in X-ray wavefields. The parameters of the experiment and the data have not, however, allowed for a successful reconstruction in this case. It is demonstrated through extensive simulation analysis that the APR method is a fast and robust imaging method. It is also shown that, through observation of the error metric, experimental parameters can be corrected or even determined, making the method successful even if there is no a priori knowledge of the experimental system. The application of the APR method as a general imaging technique for use in high-resolution X-ray diffraction experiments is, therefore, is a logical extension of the work of this thesis.
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ItemSoft supersymmetry breaking from stochastic superspace( 2013)A consequence of exact supersymmetry is the prediction of a mass degenerate superpartner for each standard model particle. The non-observation of such particles demands that supersymmetry manifests at low energies in softly broken form, such that new types of divergences (i.e. higher than logarithmic) are avoided. We propose a new realisation of softly broken supersymmetric theories as theories defined on stochastic superspace. With a suitably chosen probability distribution, the soft supersymmetry breaking parameters emerge upon averaging over the fluctuating superspace coordinates. At the classical level, the supersymmetry breaking is parametrised by a single mass parameter, $\xi$, which describes the stochasticity of the Grassmannian coordinates. It is therefore highly predictive, and by virtue of its characteristic structure of soft breaking terms, within reach for detection or falsification at the Large Hadron Collider. In the context of the standard model with stochastic supersymmetry, the $B_{\mu}$ parameter, the universal soft trilinear coupling $A_0$, the universal gaugino mass $m_{1/2}$ and the universal scalar mass $m_0$ are all given solely in terms of $\xi$. The relations are $B_{\mu} = \xi^*$, $A_0 = 2 \xi^*$, $m_{1/2} = |\xi|/2$ and $m_0 = 0$. At the quantum level, these relations hold at a certain scale, $\Lambda$, which is a second free parameter. The soft scalar masses, zero at tree-level, are induced radiatively through the renormalisation group equations at one-loop. Employing an analytical solution to an approximation of the one-loop renormalisation group equations, we find the parameter space of minimal stochastic supersymmetry to be highly constrained by the nature of its lightest supersymmetric particle (LSP). A viable neutralino LSP only emerges when the cutoff scale is taken to be greater than $\mgut$. In a more detailed analysis using sparticle spectrum calculator software to determine the mass and decay spectra, each point in parameter space is checked against known limits on relic density and rare decay processes. We then use a fast simulation of the ATLAS detector to determine which points in its parameter space are excluded by ATLAS zero lepton searches, which are amongst the most constraining limits on direct sparticle production. We find that the minimal model is definitively excluded by the recent discovery of a Higgs with mass approximately $10 \, \GeV$ heavier than that predicted by stochastic superspace. To address the observation of nonzero neutrino masses, we separately consider R-parity violation and the type-I seesaw mechanism as extensions to minimal model. In the former case, we are able to introduce neutrino masses and mixings consistent with experiment by including purely trilinear R-parity violating superpotential terms and assuming the less constrictive baryon triality symmetry. The latter case is found only to be viable when the neutrino Yukawa coupling is small relative to the top Yukawa, and the cutoff scale is large. However, as these models do not affect the Higgs mass prediction, they are excluded for the same reason as the minimal model. Finally, we consider the next-to-minimal supersymmetric standard model in stochastic superspace. The introduction of a gauge singlet superfield offers the possibility of increasing the mass prediction for the Higgs relative to the minimal model. Indeed, we observe a global increase such that $m_h = 116.6 \div 121.0 \, \GeV$. However, this is insufficient to achieve overlap with the allowed mass range from CMS and ATLAS searches.
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ItemUHE neutrino detection using the lunar Čerenkov technique( 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.