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.