This is Part A of the Thesis. The link to Part B is here: / <p> A radioactive technique to determine interfacial area between solid-liquid and liquid-liquid phases has been investigated. Plastic and liquid scintillators and β-particles from tritium solution were used for this purpose.</p> <p> The tritium β-particles have a very short range (about 5 μm in water), and the surface area of contact between the tritium bearing fluid and the scintillator bearing phase is expected to be proportional to the scintillation count rate, which in turn is proportional to the number of betas crossing the interface from a very thin region.</p> <p> To test this hypothesis, two phases were placed together in a cylindrical plexiglas container, which in turn was placed in an aluminum light-tight housing. The housing also enclosed the photo-multiplier tube (PMT). Pulses corresponding to scintillations due to β absorption were taken from the PMT and fed to a suitable electronic circuit. The count rate was obtained with a multi-channel analyser.</p> <p> It was found that the count rate is a linear function of interfacial area between the tritium bearing fluid and the scintillating material. Some deviation from linearity was however noticed at very low tritium concentration. The count rate remained the same for a given interfacial area regardless of orientation of the surface(s) with respect to the photocathode face.</p> <p> This appeared to confirm the hypothesis that interfacial area could be measured in systems of this type by measuring the scintillation count rate.</p> / Thesis / Master of Engineering (MEngr)

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This work presents results on the study of the scintillation of high-pressure Xenon gas irradiated by various sources. Noble gases such as Xenon give off characteristic scintillation light when irradiated. The goal of the study was to develop a characteristic based on the scintillation time response of Xenon gas that would reliably discriminate between events from different types of primary radiation (neutron or gamma). A reliable discrimination characteristic would enable the development of room temperature, gas phase detectors for use in the search for Galactic Dark Matter. The surprising result of the present work was that a reliable discrimination characteristic existed for distinguishing x-ray, gamma ray, and alpha particle events. Results for neutrons were negative. This was due to several factors: Ionization tracks in xenon generally form two roughly cylindrical regions. A region near the center of the track, called the core, has very dense ionization. An outer region, called the penumbra, has sparse ionization. In Xenon, recombination of ions and the subsequent scintillation from the penumbra region happens slowly and can be easily distinguished from scintillation that happens in the core region. Nuclear recoils resulting from neutron collisions that give recoil energies in the same range as that predicted for WIMP-nuclear collisions are of such low energy that they do not produce a significant penumbra region in Xenon gas. As such, the scintillation time response for these events is similar to that of high-energy gamma rays. Other results of the present work include: The amount of energy deposited in the gas needed to produce a scintillation photon was measured for gamma rays and was found to be in agreement with results from other experiments. Low-energy gamma rays appeared to produce more scintillation photons for an equal amount of energy deposited than high-energy gamma rays. The decay of the singlet and triplet molecular states of xenon was observed and the lifetimes of these states were measured. The singlet state lifetime was found to be independent of pressure while the triplet state lifetime was dependent on pressure. The lifetimes were measured and compared to previous results. A better understanding of the ionization, recombination, and scintillation processes of gaseous Xenon was achieved. Argon gas has been proposed as an alternative to Xenon gas for use in a high-pressure gas scintillation detector due to its lower mass and its property of forming a core ionization region that is much less dense than the core region of xenon. This substitution may allow for a reliable discrimination characteristic to be developed. / Physics

Ce travail porte sur la simulation d’examen scanner dans le but de valider une nouvelle méthode de mesure de dose au patient en temps réel basé sur une fibre optique scintillante. Dans un premier temps nous avons étudié la réponse en énergie de notre dosimètre, et montré qu’elle était proche de celle des chambres d’ionisation utilisées actuellement. Par la suite, un scanner a été modélisé avec succès et il a été observé que lors de l’examen, la dose au patient et la dose mesurée n’était pas impactée de la même manière par certains paramètres tels que la hauteur de la table, la position du patient, ou sa morphologie. Cependant en tenant compte de ceux-ci, il a été possible de déterminer la dose reçue par le patient durant un examen à l’aide de la mesure par fibre scintillante. Nous estimons que couplée à des simulations monte-carlo adaptées, cette mesure pourrait permettre une estimation précise de la dose délivrée aux organes à risques durant un examen. / This work focuses on the simulation of CT exam, and aims to validate a new real-time patient dose measurement method that uses a scintillating optical fiber. First, we studied our dosimeter’s energy response, and we showed that it was close to the response of the ion chambers currently used. Then, we successfully modeled a scanner, and found that during the exam, patient dose and measured dose were affected differently by parameters such as the table height, the patients positioning or the patients morphology. However, when accounting for such parameters, it was possible to compute the dose delivered to the patient during an exam using the scintillating fiber measurement. We consider that when coupled with monte-carlo simulations, this measurement could be used to estimate accurately the organ dose delivered during an exam.

Single crystals of cerium doped lutetium yttrium orthosilicate scintillator (Ce:(Lu(1-X)YX)2SiO5; Ce:LYSO) were grown through Czochralski method to investigate the affection of different growth conditions on the crystal growth. The relationship between thermal field and crystal growth was investigated. Different growth parameters and insulating system were adopted and compared to decrease the temperature gradient. It is found that the new insulating bricks can decrease effectively the temperature gradient and solve the crack problem of grown crystal. The relationship between materials ratio and the impurities in crystals. It is found that the optimized material ratio is Lu2O3:Y2O3:CeO2:SiO2=1.8563:0.1397:0.008:1.99. Under this ratio, we can get the crystals with best macroscopic optical quality. The scintillation properties of grown Ce:LYSO single crystal were measured. Through comparing the data with those in published literatures, it is found that the grown Ce:LYSO crystals present outstanding optical transmission, reaching the theoretical value about 84% in the visible light region. The transmission, UV-excitation and emission spectra are almost identical with the reported ones. The luminescence efficiency (LE) of random chosen 43 Ce:LYSO¡@samples were measured on the £^-ray multi-channel energy spectrum. It is found that the Ce:LYSO samples present stable luminescence efficiency with minimum channel number 3636.37, maximum channel number 4293.78, maximum deviation of 9.91% and standard deviation of 152.24 (3.77%). Annealing treatments were carried out in the air atmosphere on the small pieces of Ce:LYSO single crystals to eliminate the oxygen vacancies in the sample. It is found that the optimized annealing condition to improve the luminescence efficiency of Ce:LYSO is 1400 ¢J, 80 h under air atmosphere.

Sous réserve d’une énergie cinétique suffisamment faible, un neutron libre peut être piégé matériellement ou magnétiquement de sorte à garantir son confinement au sein d’un volume défini. Cette caractéristique permet l’étude de plusieurs paramètres, notamment de son temps de vie moyen. L’expérience HOPE, piège magnétique de neutrons ultra-froids mis en œuvre à l’Institut Laue Langevin à Grenoble, vise à fournir une valeur précise de ce temps de vie au travers de différentes méthodes. L’une d’entre elles consiste à observer les électrons émis par la décroissance bêta du neutron. Le détecteur SECOND a été spécifiquement conçu pour permettre le comptage de ces électrons au sein de l’expérience HOPE. La grande difficulté de ce projet réside dans le faible taux de comptage des électrons attendu, qui nécessite la discrimination des rayonnements parasites. Dans ce but, SECOND est constitué de deux étages de détection, dont le principal, un phoswich de scintillateurs plastiques, a donné des résultats probants lors de premiers tests fonctionnels à basse température ; la différenciation des événements induits par des muons cosmiques est efficace dans 98 % des cas, et tout porte à croire qu’elle sera considérablement améliorée par l’utilisation d’un système d’acquisition adapté à l’application souhaitée. / Considering a low enough kinetic energy, a free neutron can be materially or magnetically trapped in a defined volume. This trapping allows experimenters to study the neutron and its characteristics, and in this case, to measure its mean lifetime. The HOPE experiment commissioned at Laue Langevin Institute in Grenoble is aimed at providing a 1 %- accuracy value. One way to measure lifetime is to record every single neutron beta decay occurring in the trap by counting the emitted electrons. The detector SECOND has been specifically designed to fulfill this goal within HOPE but also to discriminate other types of particles that induce false events. The latter argument is the reason for the two detection stages SECOND is composed of. The plastic scintillators phoswich constitutes the main part of the detector and has been successfully operated during preliminary tests at low temperature. The rejection rate of cosmic muons events is about 98 %, and this value can be drastically enhanced using a more suitable data acquisition system.

L'amélioration de la résolution en énergie des calorimètres hadroniques est adressée dans cette thèse. L'approche envisagée se base sur la technique du dual-readout qui consiste à détecter simultanément les radiations Cherenkov et la scintillation. La comparaison de ces deux signaux permet en effet de compenser les fluctuations observées dans la détection de gerbes hadroniques. Les grenats d'Aluminium et de Lutetium (LuAG), qui sont d'efficaces scintillateurs une fois activés avec des terres rares, peuvent aussi jouer le rôle de radiateur Cherenkov sous leur forme non-dopée. Les deux types de matériaux peuvent alors être assemblés pour former un calorimètre dual-readout performant. Dans l'objectif d'étudier la faisabilité de ce concept, les effets de la concentration en dopant et de l'addition de divers codopants sur le rendement lumineux et les propriétés temporelles ont été étudiés. Nous avons montré le rôle important de la technique de croissance choisie sur la nature et la concentration des défauts structuraux. La géométrie optimale, qui se base sur des monocristaux en forme de fibres, donne l'avantage à la technique de micro-pulling down. Cette technologie ne montre pas de meilleurs résultats que les techniques de Bridgman et de Czochralski mais a été retenue pour des raisons de coût et d'adaptabilité pour une production à grande échelle. L'optimisation des paramètres de croissance a permis la production de fibres monocristallines de LuAG dopées avec du Cérium présentant un rendement lumineux de 8000 photons par MeV et un bon comportement en tant que guide de lumière grâce à une qualité optique bien maitrisée. Des tests avec des faisceaux d'électrons et de pions, en conditions de calorimétrie à haute énergie, permettent désormais d'envisager la production d'un prototype à plus grande échelle / This thesis focuses on the improvement of the energy resolution of hadron calorimeters. The approach is based on dual-readout, which consists in the simultaneous detection of both scintillation and Cherenkov light. The comparison of these two signals allows a compensation of the energy fluctuations, which are inherent to the detection of hadronic showers. Lutetium Aluminium garnets (LuAG), which are efficient scintillators when activated with rare-earth dopants (i.e. Cerium), can also act as Cherenkov radiators when undoped. Both undoped and doped crystals can then be assembled to build an efficient dual-readout calorimeter. With the objective to investigate the feasibility of this concept, the effects of the doping concentration and the use of various co-dopant on the light output and the timing properties of LuAG were studied. The growth method was demonstrated to induce significant differences in the nature and concentration of structural defects. The optimum geometry, which is based on singlecrystals shaped into fibers, favors the micro-pulling down technique. This technology does not outperform Bridgman and Czochralski techniques but was chosen on bases of cost considerations and large scale productions abilities. The optimization of the growth parameters led to the production of single-crystalline fibers of Cerium-doped LuAG with a light output of 8000 photons per MeV and an adequate behavior as light guide due to a well-controlled optical quality. Test with electrons and pions in high energy calorimetry conditions allow to engage a future production of a larger-scale prototype

Call number: LD2668 .T4 1956 B87 / Master of Science