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Microbeam radiation therapy (MRT) is a potential cancer therapy technique that uses an intense X-ray beam produced by a synchrotron. In MRT, an array of microplanar beams, called a microbeam, is delivered to a tumour. The dose at each centre of planar beams is extremely large (several hundred grays) while dose level in the valley between the peaks is below several tens of gray. Moreover, the width of each planar beam is typically 20 - 50 µm, and the distance from a centre of planar-beam to that of adjacent beam is 200 - 400 µm. For the latter reasons, the fundamental requirements for the dosimetry technique in MRT are (1) a micrometer-scale spatial resolution and (2) detection sensitivity at large doses (5 - 1000 Gy). No existing detectors can satisfy those two requirements together. The objective of the Ph.D. research is to develop a prototype dosimetry technique which fulfils the requirements for measuring the dose profile in the microbeam. The currently used approach relies on the indirect detection of X-rays; in which the X-ray dose is recorded on a detector plate, and then the recorded signals are digitized using a reader. Our proposed approach utilizes Sm3+-doped polycrystallites, glasses, and/or suitable glass-ceramics (though our approach is not limited to the use of Sm ion) for the detector plate, in which a valence reduction of Sm3+, that is the conversion of Sm3+ to Sm2+, takes place upon irradiation of X-rays. The extent of reduction is further read out using confocal fluorescence microscopy via the photoluminescence (PL) signals of Sm3+ and Sm2+. The work carried out throughout the course of the research includes the construction of confocal fluorescence microscopy, synthesis and characterizations of dosimeter materials, as well as application tests of our approach for measuring the dose profile of a microbeam used at synchrotron facilities -- Canadian Light Source (CLS), Saskatoon, Canada, European Synchrotron Radiation Facility (ESRF), Grenoble, France, and SPring-8, Hyogo, Japan. Further, the research has shown that 1 % Sm-doped fluoroaluminate glass is one of the best candidates for the type of dosimetric application. It has the dynamic range of ~1 to over 1000 Gy which covers the dose range used in MRT, excellent signal-to-noise ratio (large extent of Sm3+ → Sm2+ change), and excellent stability of recorded signal over time. The recorded signal in the detector is erasable by heating or exposing to light such as UV. Furthermore, with a use of confocal microscope, it has ability to measure the distribution pattern of dose over the cross-section of microbeam. Therefore, we believe that our approach is one of the most promising techniques available.

The growth of thin-film metal nitride materials from elemental metal targets by plasma-assisted pulsed laser deposition (PLD) has been explored and analysed. A new UHV PLD growth system has been installed and assembled and its system elements were calibrated. A series of GaN thin films have been grown to calibrate the system. In-situ RHEED indicated that the films were single crystal and that growth proceeded in a three-dimensional fashion. SEM images showed heavy particulation of film surfaces that was not in evidence for later refractory metal nitride films. This may be connected to the fact that Ga targets were liquid while refractory metals were solid. Most GaN films were not continuous due to insufficient laser fluence. Continuous films did not exhibit photoluminescence. HfN films have been grown by PLD for the first time. Films grown have been shown to have high reflectivity in the visible region and low resistivity. These factors, along with their crystal structure, make them suitable candidates to be used as back-contacts in GaN LEDs and could also serve as buffer layers to enable the integration of GaN and Si technologies. Growth factors affecting the films’ final properties have been investigated. Nitrogen pressure, within the operating range of the plasma source, has been shown to have little effect on HfN films. Substrate temperature has been demonstrated to have more influence on the films’ properties, with 500 °C being established as optimum. ZrN films have also been grown by PLD. Early results indicated that they exhibit reflectivities 50 % ± 5 % lower than those of HfN. However, further growth and characterisation would be required in order to establish this as a fundamental property of ZrN as nitride targets were mostly used in ZrN production. Single-crystal epitaxial GdN and SmN films have been produced by PLD. This represents an improvement in the existing quality of GdN films reported in the literature, which are mostly polycrystalline. In the case of SmN, these are the first epitaxial films of this material to be grown. Film quality has been monitored in-situ by RHEED which has allowed growth to be tailored to produce ever-higher crystal quality. Post-growth analyses by collaborators was also of assistance in improving film growth. Substrate temperatures and nitrogen plasma parameters have been adjusted to find optimum values for each. In addition, laser fluence has been altered to minimise the presence of metal particulates in the films, which interfere with magnetic measurements carried out in analyses. Capping layers of Cr, YSZ or AlN have been deposited on the GdN and SmN prior to removal from vacuum to prevent their degradation upon exposure to atmospheric water vapour. The caps have been steadily improved over the course of this work, extending the lifetime of the nitride films in ambient. However, they remain volatile and this may persist since water vapour can enter the film at the edge regardless of capping quality. Optical transmission has shown an onset of absorption at 1.3 eV for GdN and 1.0 eV for SmN.

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The majority of the metallic elements adopt simple, high-symmetry structures at ambient pressure. These consist of a highly-ordered arrangement of atoms, which can be described by a crystal lattice that is periodic in three dimensions. It may be expected that close-packed structures, such as face-centred cubic (fcc) and hexagonal close-packed (hcp), would be favoured at high pressures due to the increase in density. However, many of these elements transform to lowsymmetry, complex structures on compression. In particular, a number have been observed to adopt incommensurately-modulated structures at high pressure. In these structures, atoms are displaced from their average positions by a modulation wave, the wavelength of which is an irrational multiple of the lattice periodicity. Diamond-anvil cells (DACs) can be used to compress materials to over a million times atmospheric pressure. In these devices, a small sample is compressed between the tips of two diamond anvils. The atomic arrangement of materials at extreme pressures can then be probed using the intense x-ray beams provided by synchrotron sources. In this work, the high-pressure crystal structures of the lanthanide elements europium (Eu) and samarium (Sm) have been investigated using angle-dispersive x-ray powder dffraction techniques. The high-pressure structural behaviour of Eu has been found to be remarkably different from that of the other lanthanide elements. Two new high-pressure phases of Eu are reported, both of which have an incommensurately-modulated crystal structure.

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Made available in DSpace on 2014-10-09T12:45:37Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:41Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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