School of Physics - Theses
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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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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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ItemThe physics of faster than light objects( 1990)A new formulation of the theory of tachyons is developed using the same two postulates as in Special Relativity. Use is made of a "switching principle" to show how tachyons automatically obey the laws of conservation of energy, momentum and electric charge. Tachyonic transformations of spacetime position, velocity, momentum, energy, force and acceleration are rigorously derived from first principles. Further tachyonic transformations are derived from Maxwell's equations for the electromagnetic field components E, B, D, H, P and M, current and charge densities, scalar and vector potentials and the electromagnetic field tensor. It turns out that there are two main types of transformations which apply when going from a bradyonic inertial reference frame to a tachyonic inertial reference frame and vice versa. The first type involves the same transformations as apply for subluminal frames, but with the inclusion of factors of either +i or -i when crossing the light barrier. Examples of this type of transformation include spacetime position, energy, momentum, acceleration, electromagnetic scalar and vector potentials, electric charge density and current density. The second general type of transformation has exactly the same form for u 2 < c2 and u2 > c2 , examples of which include velocity, force and the electromagnetic field vectors E, B, D, H, P and M. Here u is the boost speed between the two inertial reference frames. Several examples are given to show how tachyons can produce real and measurable effects. It turns out that the electromagnetic field produced by a charged tachyon takes the form of a "Mach cone", inside which the electromagnetic field is real and detectable. The field outside the cone is imaginary and undetectable because the field has not yet arrived from the source. Tachyons also exhibit a Doppler effect, which is a blueshift for approach and a redshift for recession, just as for bradyons. The transverse Doppler effect is a redshift for c2 < u2 < 2c2 but is a blueshift for 2c2 < u2. Tachyons also produce an "optic boom" effect when the observer contacts the edge of the Mach cone, while inside the cone the observer experiences the "two source effect" t whereby the tachyon appears to be in two separate and mutually receding places. This means that electromagnetic fields and potentials are given by a superposition due to an “earlier” and a “later” source. Further examples include calculations of the magnetic dipole moment of a tachyonic current loop and of the speed of light in a tachyonic dielectric. (Part Abstract)
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ItemDirect observation of charged particle tracks in crystals( 1995)From a general point of view in material science, it is interesting to know, how swiftly moving, heavy ions (such as those ejected from spontaneously occuring nuclear fission events inside natural minerals) interact with solid bulk matter. For example, in geological mineral deposits (such as zircon, apatite, and sphene) one often finds that naturally radioactive isotopes (uranium, thorium etc.) are included in varying degrees of abundance. In particular, single 238 U atoms have a small but finite probability to disintegrate through a spontaneous nuclear fission event, thereby ejecting two highly charged and comparably heavy nuclear fragments with about 250Me V kinetic energy. This fact became known only about 30 years ago when straight-line, needle-like contrast features were observed in electron micrographs of mica (S&B 59, P&W 62). These so called "fission tracks" originate through the damage incurred on the host material containing naturally radioactive isotopes by the fast moving, charged and heavy fission-fragments penetrating the matrix in two oppositely directed straight lines. The length of the tracks is related to the penetration range of the ions, which in turn is determined through the initially available kinetic energy, the atomic number, and charge-state of the projectile as well as the mean density of the target material. (Because the energy level difference between the initial and final state of the nuclear decay event is constant on average, a fixed amount of kinetic energy is available for the two fragments. This fact requires the fission tracks to have a more or less constant longitudinal dimension of around 50µm.) Their thickness, however, could not be determined accurately up to very recently because the image quality remained poor due to the technical limitations in electron-optical engineering in the first few generations of electronrnicroscopes which were commercially marketed after their pioneering development by Ruska some sixty years ago (Rus 49). With the improved performance of the recent generation of high-voltage microscopes a renewed attempt was made to determine a) the electron optical conditions of making fission tracks (or at least short sections thereof) visible in their latent form through transmission electron microscopy, employing a variety of contrast mechanisms, b) the geometrical parameters of the track morphology in the particular instance of 208Pb20+ ions penetrating slabs of zircon crystals at an initial kinetic energy of 14MeV/u, c) the structural conditions as well as the nature and mechanism of the damage formation. Lastly, it was also hoped to be able to image the damaged regions showing atomic-size detail in High Resolution Electron Microscopy (HRTEM), thus making the native structure of the damaging effects of nuclear radiation penetrating bulk matter directly visible for the first time.