This work deals with the creation of a program for calculating the tensile losses of an in-dustrial boiler. The work is divided into four parts. The first part of the thesis deals with a theoretical introduction to the issue of pressure losses. The second part deals with all equations and procedures by which the program was created, as well as the limitations and conditions of using a given type of equation. The third part consists of a sample calculation of pressure loss-es of a real boiler. Furthermore, in the third part, the correctness of the program function is checked, where the measured losses on the real device were compared with the losses calcu-lated by the program. The fourth part deals with the influence of the boiler's parameters on its pressure losses, where the individual losses were graphically represented and compared to determine the sensitivity analysis. The output of this work is a functional program that has been optimized for ease of use and requires almost no knowledge of the user.

Radiotherapy is used in clinics to treat cancer with highly energetic ionizing particles. The radiation dose can be measured indirectly by means of radiation detectors or dosimeters. The dose deposited in a detector can be related to dose deposited in a point within the patient. In theory, however, this is only possible under charged particle equilibrium (CPE). The motivation behind the dissertation was driven by the difficult, yet crucial, dosimetry in non-CPE regions. Inaccurate dose assessment performed with standard dosimetry using ionization chambers may significantly impact the outcomes of radiotherapy treatments. Therefore, advanced dosimetry methods tailored specifically to suit non-CPE conditions must be used. This work aims to improve dosimetry in two types of non-CPE conditions that pose dosimetric challenges: regions near interfaces of tissues with low- and high- density media and in small photon fields. To achieve the main dissertation objectives, an enhanced film dosimetry protocol with a novel film calibration approach was implemented. This calibration method is based on the percent depth dose (PDD) tables and was shown to be efficient and accurate. As a result, the PDD calibration method was used for the film dosimetry process throughout the dissertation work. Monte Carlo (MC) calculations for the small field dosimetry were performed using phase-space files (PSFs) provided by Varian for TrueBeam linac. The MC statistical uncertainty in these types of calculations is limited by the number of particles (due to latent variance) in the used PSFs. This study investigated the behaviour of the latent variances (LV) with beam energy, depth in phantom, and calculation resolution (voxel size). LV was evaluated for standard 10x10 cm2 fields as well as small fields (down to 1.3 mm diameter). The results showed that in order to achieve sub-percent LV in open 10x10 cm2 field MC simulations a single PSF can be used, whereas for small SRS fields (1.3—10 mm) more PSFs (66—8 PSFs) would have to be summed. The first study in this dissertation compared the performance of several dosimetric methods in three multi-layer heterogeneous phantoms with water/air, water/lung, and water/steel interfaces irradiated with 6 and 18 MV photon beams. MC calculations were used, along with Acuros XB, anisotropic analytical algorithm (AAA), GafChromic EBT2 film, and MOSkin dosimeters. PDDs were calculated and measured in these heterogeneous phantoms. The result of this study showed that Acuros XB, AAA, and MC calculations were within 1% in the regions with CPE. At media interfaces and buildup regions, differences between Acuros XB and MC were in the range of +4.4% to -12.8%. MOSkin and EBT2 measurements agreed to MC calculations within ~ 2.5%-4.5%. AAA did not predict the backscatter dose from the high-density heterogeneity. For the third, multilayer lung phantom, 6 MV beam PDDs calculated by all treatment planning system (TPS) algorithms were within 2% of MC. 18 MV PDDs calculated by Acuros XB and AAA differed from MC by up to 3.2 and 6.8%, respectively. MOSkin and EBT2 each differed from MC by up to 3%. All dosimetric techniques, except AAA, agreed within 3% in the regions with particle equilibrium. Differences between the dosimetric techniques were larger for the 18 MV than the 6 MV beam. This study provided a comparative performance evaluation of several advanced dosimeters in heterogeneous phantoms. This combination of experimental and calculation dosimetry techniques was used for the first time to evaluate the dose near these interfaces. The second study in the dissertation aims to improve dose measurement accuracy in small radiotherapy fields. Field output factors of 6 MV beams from TrueBeam linear accelerator (linac) collimated with 1.27-40 mm diameter cones were calculated and measured using MC and EBT3 films. A set of detector specific correction factors for two widely used dosimeters (EFD-3G diode and PTW-60019 microDiamond detectors) were determined based on GafChromic EBT3 film measurements and calculated using MC methods. MC calculations were performed for microDiamond detector in parallel and perpendicular orientations relative to the beam axis. The result of this study showed that the measured OFs agreed within 2.4% for fields ≥10 mm. For the cones of 1.27, 2.46, and 3.77 mm diameter maximum differences were 17.9%, 1.8% and 9.0%, respectively. MC calculated OF in water agreed with those obtained using EBT3 film within 2.2% for all fields. MC calculated output correction factors for microDiamond detector in fields ≥10 mm ranged within 0.975-1.020 for perpendicular and parallel orientations. MicroDiamond detector correction factors calculated for the 1.27, 2.46 and 3.77 mm fields were 1.974, 1.139 and 0.982 with detector in parallel orientation, and these factors were 1.150, 0.925 and 0.914 in perpendicular orientation. EBT3 and MC obtained correction factors agreed within 3.7% for fields of ≥3.77 mm and within 5.9% for smaller cones. This work provided output correction factors for microDiamond and EFD-3G detectors in very small fields of 1.27 – 3.77 mm diameter and demonstrated over and under-response of these detectors in such fields. These correction factors allow improve the accuracy of dose measurements in small photon fields using these detectors. / Graduate / 2019-08-30

La mesure de la radioactivité dans les filtres utilisés pour la surveillance de l’aérocontamination de l’air présente une difficulté métrologique majeure. En effet, l’absorption des rayonnements a dans le médium filtrant et la masse d’aérosols accumulés biaisent la réponse nucléaire. Ce travail de thèse porte sur la détermination de facteurs de correction de la dégradation de la radioactivité mesurée dans les filtres de surveillance. Dans un premier temps, des filtres radioactifs représentatifs des prélèvements atmosphériques ont été préparés à l’aide du banc d’essais nucléaire ICARE. L’étude expérimentale sur les filtres de référence a permis d’avoir une base de données pout la détermination des facteurs de correction dans diverses conditions de filtration. Dans un second temps, ce travail a conduit une nouvelle méthode numérique mise au point pour déterminer les facteurs de correction. Il s’agit de coupler des simulations de filtration des particules d’aérosol à l’aide de GeoDict, permettant de calculer des écoulements dans les milieux poreux et des simulations de parcours de particules a dans la matière à l’aide de MCNPX. Le bon accord obtenu, en comparant les réponses des spectres en énergie et des facteurs de correction numériques et expérimentaux, a permis de valider le modèle numérique / The measurement of radioactivity in the filters of airborne radioactive surveillance is a major difficulty metrology. Indeed, the absorption of a radiation in the filter media and the mass of aerosols accumulated distort the nuclear counters response. This thesis work focuses on the determination of correction factors for the radioactivity loss in the survey filters. In a first step, radioactive filters representing the atmospheric samples have been prepared using the nuclear test bench ICARE. The experimental study on reference filters provided a database to determine correction factors for various filtration conditions. The second step of the work proposed a new numerical method developed to determine the correction factors. It consists of coupling GeoDict for particles filtration simulations and MCNPX simulations for a transport in matter. The good agreement obtained by comparing the numerical and experimental correction factors has permitted to validate the numerical model

La radiothérapie guidée par résonance magnétique promet une administration de dose plus précise que les techniques conventionnelles puisqu’elle permet une visualisation en temps réel des structures internes avant et pendant le traitement. Cependant, la dosimétrie doit être réalisée en présence de champs magnétiques. Alors que le champ magnétique n’affecte pas le transport des particules neutres, il affecte le transport des particules chargées secondaires en raison de la force de Lorentz, qui modifie le champ de rayonnement et la réponse de dose du détecteur. Cette thèse vise à comprendre l’effet du champ magnétique sur la réponse de dose du détecteur, à la caractériser et à fournir des facteurs de correction de qualité prenant en compte l’impact du champ magnétique. Dans le premier article, quatre chambres d’ionisation à petite cavité ont été caractérisées via des simulations de Monte Carlo et des mesures expérimentales. Il a été constaté que le champ magnétique accentuait tous les détails géométriques. Une description précise du volume sensible effectif est cruciale dans les simulations. De plus, la géométrie modélisée doit être aussi proche que possible de la géométrie réelle, y compris les couches d’air internes. Des facteurs de correction de qualité tenant compte du champ magnétique et de son incertitude du budget d’incertitude sont présentés pour différentes configurations. Le deuxième article a évalué l’effet du champ magnétique sur les facteurs de perturbation de cinq détecteurs à petite cavité, dont trois détecteurs à petite cavité et deux détecteurs à semi-conducteurs. Les facteurs de perturbation des composants structurels, les facteurs de moyenne de densité et de volume ont été déterminés pour différentes tailles de champ et orientations. De plus, des facteurs de correction de qualité ont été calculés dans les mêmes conditions. Les résultats montrent que le champ magnétique a un impact significatif sur le facteur de perturbation de la densité dans les chambres d’ionisation. En revanche, son impact est plus prononcé dans les composants structurels des détecteurs semi-conducteurs. L’objectif du troisième article était de fournir plus d’informations sur la compréhension de la relation dose-réponse des détecteurs dans les champs magnétiques via des calculs de spectres de fluence électronique. La fluence des électrons différentiel en énergie dans la cavité du détecteur peut être fortement modifiée dans les champs magnétiques, et les perturbations de fluence sont généralement plus évidentes pour les électrons de faible énergie. Ces calculs ont montré l’interaction entre plusieurs facteurs qui rendent les effets de perturbation imprévisibles dans le faisceau de photons couplé aux champs magnétiques : 1) orientation du détecteur et du champ magnétique, 2) taille et forme de la cavité, 3) composants structurels, 4) couche d’air entre le détecteur et le milieu et leur asymétrie, et 5) l’énergie. / Magnetic resonance-guided radiation therapy promises more accurate dose delivery than conventional techniques by allowing real-time visualization of internal structures before and during treatment. However, the dosimetry must be performed in the presence of magnetic fields. While the magnetic field does not affect the transport of uncharged particles, it affects the transport of secondary charged particles due to the Lorentz force, which modifies the radiation field and the detector dose-response. This thesis aims to understand the effect of the magnetic field on detector dose-response, characterize it, and provide quality correction factors accounting for the impact of the magnetic field. In the first article, four small-cavity ionization chambers were characterized via Monte Carlo simulations and experimental measurements. It was found that the magnetic field emphasized all the geometrical details. An accurate description of the effective sensitive volume is crucial in the simulations. Also, the modelled geometry must be as close as possible to the actual geometry, including the internal air layers. Quality correction factors accounting for the magnetic field and its uncertainty budget uncertainty are presented for different configurations. The second article evaluated the magnetic field effect on perturbation factors of five small volume detectors, including three ionization chambers and two solid-state detectors. The perturbation factors from extracameral components, density and volume averaging factors were determined for different field sizes and orientation setups. Additionally, quality correction factors were calculated in the same conditions. Results show that the magnetic field significantly impacts the density perturbation factor in the ionization chambers. In contrast, its impact is more pronounced in the extracameral components in the solid-state detectors. The purpose of the third article was to provide more insight into the understanding of detector dose-response in magnetic fields via calculations of electron fluence spectra. The electron fluence differential in energy in the detector cavity can be severely modified in magnetic fields, and fluence perturbations are generally more evident for low-energy electrons. These calculations showed the interplay between multiple factors that make the perturbation effects unpredictable in photon beams coupled to magnetic fields: 1) detector and magnetic field orientation, 2) cavity size and shape, 3) extracameral components, 4) air gaps and their asymmetry, and 5) energy.