School of Physics - Theses
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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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ItemStudies for the adaptation of a field ionization ion source for a proton microprobe( 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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ItemApplication of a scanning proton microprobe as a diagnostic tool and the development of a high brightness ion source( 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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ItemStudies in phase and inversion problems for dynamical electron diffraction( 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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ItemNeutrino oscillations and the early universe( 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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ItemTEM and structural investigations of synthesized and modified carbon materials( 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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ItemSuperfluid spherical Couette flow and rotational irregularities in pulsars( 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.
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ItemControllable few state quantum systems for information processing( 2006-10)This thesis investigates several different aspects of the physics of few state quantum systems and their use in information processing applications. The main focus is performing high precision computations or experiments using imperfect quantum systems. Specifically looking at methods to calibrate a quantum system once it has been manufactured or performing useful tasks, using a quantum system with only limited spatial or temporal coherence. A novel method for characterising an unknown two-state Hamiltonian is presented which is based on the measurement of coherent oscillations. The method is subsequently extended to include the effects of decoherence and enable the estimation of uncertainties. Using the uncertainty estimates, the achievable precision for a given number of measurements is computed. This method is tested experimentally using the nitrogen-vacancy defect in diamond as an example of a two-state quantum system of interest for quantum information processing. The method of characterisation is extended to higher dimensional systems and this is illustrated using the Heisenberg interaction between spins as an example. The use of buried donors in silicon is investigated as an architecture for realising quantum-dot cellular automata as an example of quantum systems used for classical information processing. The interaction strengths and time scales are calculated and both coherent and incoherent evolution are assessed as possible switching mechanisms. The effects of decoherence on the operation of a single cell and the scaling behaviour of a line of cells is investigated. The use of type-II quantum computers for simulating classical systems is studied as an application of small scale quantum computing. An algorithm is developed for simulating the classical Ising model using Metropolis Monte-Carlo where random number generation is incorporated using quantum superposition. This suggests that several new algorithms could be developed for a type-II quantum computer based on probabilistic cellular automata.