L'utilisation récente des fibres optiques (FOs) à base de silice dans des environnements extrêmes, a incité les chercheurs à accélérer leurs études de vulnérabilité. De tels défis sont fortement liés à la bonne compréhension des effets à la fois macroscopiques et microscopiques des différents types de radiations sur la réponse des FOs. Cette thèse de doctorat présente une étude complémentaire aux études précédemment menées sur les différents défauts ponctuels dans les FOs à base de silice par Cathodoluminescence (CL). Cette technique offre la possibilité de détecter les centres luminescents mais aussi de suivre leurs distributions spatiales, leurs cinétiques de création et de guérison en fonction de l'irradiation électronique. Dans ce manuscrit, nous introduisons tout d'abord un résumé des connaissances actuelles sur les défauts liés à la silice pure et différemment dopée.Les détails de notre procédure expérimentale sont discutés dans le 2ème chapitre où nous montrons que les doses déposées lors des mesures CL sont très importantes. Dans le 3ème chapitre nous présentons une étude systématique de la réponse en CL des différentes classes de FOs, dans lesquelles différentes bandes d'émission sont discutées. Le 4ème chapitre traite l'impact d'une variation des conditions d'irradiation électronique sur les centres GLPC, l'un des défauts liés au Ge les plus importants. Enfin, dans le 5ème chapitre, nous avons montré la possibilité de produire des nanoparticules à base de silice par ablation laser, et la capacité de la technique CL de caractériser ce type de matériaux, ce qui ouvre la porte à d'autres utilisations de cette technique pour la caractérisation de nanoparticules. / The recent use of silica-based optical fibers (OFs) in harsh environments pushed the researchers to accelerate their vulnerability and hardening studies. Such challenges are strongly linked to the good understanding of the macroscopic as well as the microscopic effects of different types of radiations on the silica-based OF's response. This PhD thesis presents a complementary study to previous researches on the properties of different point defects in silica-based OFs by Cathodoluminescence (CL). Such technique offers the ability to both detect the luminescent centers and to follow their spatial distribution, their growth and decay kinetic as a function of the electron beam characteristics. ln the present manuscript we first summarize the current knowledge regarding point defects in pure silica or silica glass doped with Ge, P, Ce, N or Al. Details of the experimental procedure are discussed in the 2nd chapter in which we highlight that the equivalent dose deposited during the various configuration of CL measurements are very large. ln the 3rd chapter, we perform an overview study of the CL responses of different classes of OFs, in which many emission bands related to the different dopants were discussed. The 4th chapter focuses on the study of the effects of varying beam conditions on the signature of Germanium Lone Pair Center (GLPC), one of the most important Ge-related point defects. Finally, in chapter V, we demonstrate the possibility to produce silica-based nanoparticles by laser ablation process, and the ability of the CL technique to characterize such materials, which opens the door to other employments of this technique for future studies on nanoparticles.

This work establishes a framework for Monte Carlo simulations of complex, modulated electron fields produced by Varian's TrueBeam medical linear accelerator for investigations into modulated electron radiation therapy (MERT) and combined modulated photon and electron radiation therapy (MPERT). Both MERT and MPERT have shown potential for reduced low dose to normal tissue without compromising target coverage in the external beam radiation therapy of some breast, chest wall, head and neck, and scalp cancers. This reduction in low dose could translate into the reduction of immediate radiation side effects as well as long term morbidities and incidence of secondary cancers. Monte Carlo dose calculations are widely accepted as the gold standard for complex radiation therapy dose modelling, and are used almost exclusively for modelling the complex electron fields involved in MERT and MPERT. The introduction of Varian's newest linear accelerator, the TrueBeam, necessitated the development of new Monte Carlo models in order to further research into the potential role of MERT and MPERT in radiation therapy. This was complicated by the fact that the field independent internal schematics of TrueBeam were kept proprietary, unlike in previous generations of Varian accelerators. Two approaches are presented for performing Monte Carlo simulations of complex electron fields produced by TrueBeam. In the first approach, the dosimetric characteristics of electron fields produced by the TrueBeam were first compared with those produced by an older Varian accelerator, the Clinac 21EX. Differences in depth and profile characteristics of fields produced by the TrueBeam and those produced by the Clianc 21EX were found to be within 3%/3 mm. Given this information, complete accelerator models of the Clinac 21EX, based on its known internal geometry, were then successfully modified in order to simulate 12 and 20 MeV electron fields produced by the TrueBeam to within 2%/2 mm of measured depth and profile curves and to within 3.7% of measured relative output. While the 6 MeV TrueBeam model agreed with measured depth and profile data to within 3%/3 mm, the modified Clinac 21EX model was unable to reproduce trends in relative output as a function of field size with acceptable accuracy. The second approach to modelling TrueBeam electron fields used phase-space source files provided by Varian that were scored below the field-independent portions of the accelerator head geometry. These phase-spaces were first validated for use in MERT and MPERT applications, in which simulations using the phase-space source files were shown to model depth dose curves that agreed with measurement within 2%/2 mm and profile curves that agreed with measurement within 3%/3 mm. Simulated changes in output as a function of field size fell within 2.7%, for the most part. In order to inform the positioning of jaws in MLC-shaped electron field delivery, the change in output as a function of jaw position for fixed MLC-apertures was investigated using the phase-space source files. In order to achieve maximum output and minimize treatment time, a jaw setting between 5 and 10 cm beyond the MLC- field setting is recommended at 6 MeV, while 5 cm or closer is recommended for 12 and 20 MeV with the caveat that output is most sensitive to jaw position when the jaws are very close to the MLC-field periphery. Additionally, output was found to be highly sensitive to jaw model. A change in divergence of the jaw faces from a point on the source plane to a 3x3 mm^2 square in the source plane changed the shape of the output curve dramatically. Finally, electron backscatter from the jaws into the monitor ionization chamber of the TrueBeam was measured and simulated to enable accurate absolute dose calculations. Two approaches were presented for measuring backscatter into the monitor ionization chamber without specialized electronics by turning o the dose and pulse forming network servos. Next, a technique was applied for simulating backscatter factors for the TrueBeam phase-space source models without the exact specifications of the monitor ionization chamber. By using measured backscatter factors, the forward dose component in a virtual chamber was determined and then used to calculate backscatter factors for arbitrary fields to within 0.21%. Backscatter from the jaws was found to contribute up to 2.6% of the overall monitor chamber signal. The measurement techniques employed were not sensitive enough to quantify backscatter from the MLC, however, Monte Carlo simulations predicted this contribution to be 0.3%, at most, verifying that this component can be neglected. / Graduate / 0756 / lloyd.samantha@gmail.com

There are at least two common models for calculating the photoemission of accelerated electrons. The 'extended-charge-distribution' method uses the quantum probability current (multiplied by the electron charge) as a source current for Maxwell's equations. The 'point-like-emitter' method treats the electron like a point particle instead of like a diffuse body of charge. Our goal is to differentiate between these two viewpoints empirically. To do this, we consider a large electron wave packet in a high-intensity laser field, in which case the two viewpoints predict measurable photoemission rates that differ by orders of magnitude. Under the treatment of the 'extended-charge-distribution' model, the strength of the radiated field is significantly limited by interferences between different portions of the oscillating charge density. Alternatively, no suppression of photoemission occurs under the 'point-like-emitter' model because the electron is depicted as having no spatial extent. We designed an experiment to characterize the photoemission rates of electrons accelerated in a relativistic laser focus. Free electron wave packets are produced through ionization by an intense laser pulse at the center of a large vacuum chamber. These quantum wave packets can become comparable in size to the laser wavelength through natural spreading and interactions with the sharp ponderomotive gradients of the laser focus. Electron radiation emitted out the side of the focus is collected by one-to-one imaging into a 105-micron gold-jacketed fiber, which carries the light to a single photon detector located outside the chamber. The electron radiation is red-shifted due to mild relativistic acceleration, and we use this signature to spectrally filter the outgoing light to discriminate against background. In addition, the temporal resolution of the electronics allows distinction between light that travels directly from the focus into the collection system and laser light that may scatter from the chamber wall.