We have undertaken to develop a method !or determining the resonance escape probability of homogeneous reactors containing aqueous solutions of a resonance absorber. TH(NO₃)₄ was selected as a salt since it is readily dissolvable in water. Even at the highest concentration, the atomic densities of thorium and nitrogen are small compared to hydrogen and oxygen. Only the latter two contribute significantly to the slowing down in the high energy range; therefore, a water Monte Carlo serves adequately as the neutron supply to the resonance region of thorium. In order to find an escape probability we determine the ratio of the number of neutrons that attain energies below the resonance region to the number that appear in the resonance region for the first time. The difference between the two numbers in the ratio occurs due to absorption by the resonance absorber, in this case thorium, and due to leakage. The initial source of neutrons comes from H²(d,n)He³ reaction where the deuteron beam strikes the deuterium target at the center of one face of the containing parallelepiped. Once the water Monte Carlo is run one has a set of data cards which can be used as the input data to a second Monte Carlo designed to calculate the resonance escape probability. Our prime concern is this second Monte Carlo. The resonance escape Monte Carlo is a direct analog type using known physical distributions and laws. The decisions about what events occur in the history of a neutron are based on these distributions and laws. The detailed effect of the thorium resonances on the number of neutrons absorbed is described by the cross-section as a function of energy. A Breit-Wigner Doppler broadened single level formulation gives the scattering and absorption parts of the cross-section as a function of energy. Given a neutron-thorium interaction, the neutron is either absorbed or scattered with probabilities σ_ab/(σ_ab+σ_sc) and σ_sc/(σ_ab+σ_sc). A new statistical weight is assigned the neutron by taking the latter fraction of the old weight. A new neutron is picked from the source supply only after the previous neutron has delivered a weight to low energy or to the exterior of the pile. The heart of the Monte Carlo method lies in the determination of a random variable, distributed according to some known probability function. This requires a source of random numbers which in our case is provided by the IBM-650 computer. The resonance escape probability as a function of thorium number density fits very closely to a straight line with small negative slope throughout the whole range of stable concentrations. The Monte Carlo statistics provides probable errors of less than 0.3% for all five points calculated. Each of the five points lies close enough to the straight line of least-squares fit to give an error less than 0.3%. A theoretical determination of the resonance escape probability based on resonance integral theory for the eight peaks used in the Monte Carlo calculation gives good enough agreement to make both methods plausible. Although other resonance escape calculations have been done by the Monte Carlo method, they have been for radically different systems. In all such cases the system was heterogeneous with "lumped" uranium for the absorber. Therefore comparisons between existing Monte Carlo calculations is impossible. We hope to be able to provide experimental verification for the applicability of the Monte Carlo model. / Ph. D.

La demande pour les éléments de terres rares (ETR) ne cesse d'augmenter avec les années ce qui ne devrait pas s'essouffler à moyen long terme puisque leurs propriétés chimiques sont essentielles pour de nombreuses technologies vertes et durables qui réduisent la consommation d'énergie fossile et les gaz à effet de serre. Comme le thorium et l'uranium possèdent des caractéristiques similaires aux ETR, ceux-ci sont aussi présents dans les réseaux cristallins des minerais d'ETR. Par le fait même, le lixiviat résultant de l'extraction des ETR des minerais peut contenir ces actinides. Leur présence n'est pas souhaitable lors de la production d'ETR dû au caractère radioactif de ces éléments. Le travail décrit dans le cadre de ce mémoire traite de la séparation du thorium des éléments de terres rares au sein de lixiviats miniers. Pour retirer celui-ci du lixiviat, la stratégie de l'extraction au point trouble a été envisagée. L'étude s'est basée sur trois principaux lixiviats miniers. Search Minerals a mandaté SGS Canada inc. pour faire des essais pilotes "acid baking/water leach". De ceux-ci en est résulté, le lixiviat en milieu sulfurique. Un lixiviat en milieu nitrique ainsi qu'un lixiviat en milieu chlorhydrique issu d'essais en laboratoire à CanmetMINES ont également été utilisés. Les extractions au point trouble classiques traitent des liquides ayant des matrices moins chargées en ions et des concentrations en analyte beaucoup plus faibles que dans les lixiviats miniers. L'objectif du développement de la méthode d'extraction au point trouble était d'adapter la méthode à une matrice de lixiviats miniers en plus de concentrer le thorium dans la phase riche en surfactant. Des huit ligands testés, deux se sont démarqués. Le P,P'-di-(2-éthylhexyle) méthane diphosphonique acide (H₂DEH[MDP]) est celui possédant le potentiel d'extraction le plus important permettant sélectivement le transfert du Th dans les micelles mixtes dans un lixiviat sulfurique. L'acide éthylènediaminetétraméthylène phosphonique (EDTMP) a aussi prouvé son potentiel à séparer le Th des ETR dans un lixiviat chlorhydrique. L'optimisation des systèmes en changeant les concentrations en réactifs utilisés a été effectuée. Les analyses élémentaires ont montré que cette technique était prometteuse pour une application pour le domaine minier. L'exploration de l'extraction séquentielle du Th suivie de la séparation et préconcentration des ETR lourds des ETR légers avec l'isopropyl diglycolamide (i(pr)-DGA) a été réalisée. Les résultats préliminaires ont montré la capacité potentielle de la technique à réaliser la séparation. / The demand for rare earth elements (REE) continues to increase over the years and should not slow down in the medium to long-term horizon since their chemical properties are essential for many green and sustainable technologies which mitigate the consumption of fossil energy and greenhouse gases. Since thorium and uranium exhibit similar characteristics to REE, these are also present in the mineral lattice of REE ores. As a result, the leachate from the extraction of REE from minerals may contain actinides. Their presence is undesirable during the production of REE due to the radioactive nature of these elements. The work described in this thesis deals with the separation of thorium from the rare earth elements of mining leachates. To remove them from the leachate, the cloud point extraction strategy was considered. The study was based on three main mining leachates. Search Minerals commissioned SGS Canada inc. to carry out acid baking/water leach pilot tests. From these resulted the sulfuric medium leachate. A nitric medium leachate as well as a hydrochloric medium leachate from laboratory testing at CanmetMINING have also been used. Classic cloud point extractions are typically applied to solutions with less ionic content and much lower analyte concentrations than in mining leachates. The goal of developing this new cloud point extraction method was to concentrate the thorium in the surfactant-rich phase. Of all the ligands tested, two stood out. The P, P - di- (2-ethylhexyl) methane diphosphonic acid (H₂DEH [MDP]) is the one with the greatest extraction potential, allowing the selective transfer of Th into mixed micelles in a sulfuric leachate. The ethylene diamine tetra (methylene phosphonic acid) (EDTMP) has also demonstrated its potential to separate Th from REE in hydrochloric leachate. The optimization of the systems by changing the concentrations of reagents used was carried out. The elemental analyses showed that this technique was promising for an application in mineral processing. The exploration of the sequential extraction of Th followed by the separation and preconcentration of heavy REE from light REE with isopropyl diglycolamide (i(pr)-DGA) was carried out. Preliminary results showed its potential ability to perform this task.

Uranium and thorium radionuclides were measured on two gravity cores (T17G and T18G) and one box core (T19B) collected from the western South Okinawa Trough (SOT), one gravity core from off shore Southwest Taiwan (N3) and three box cores (C, D and E) from the northern South China Sea (SCS) in order to examine the variations of these radionuclides and their activity ratios in the sediments of the areas and to characterize the source function of the sediments and their geochemical implication based on these nuclides. For long half-life radionuclides such as 238U, 234U, 232Th and 230Th, the activities in the cores of the SOT and Southwest Taiwan areas show no significant vertical or areal variations, implying no significant variation in sediment supply or depositional environment within the past 100 years. The average activity of 238U is 1.65 dpm/g and 1.33 dpm/g in the SOT and Southwest Taiwan areas, respectively, and that of 232Th is 3.57 dpm/g and 3.34 dpm/g, respectively. The average activities of 238U and 232Th are, respectively, 1.37 dpm/g and 2.37 dpm/g in the SCS. The mean 232Th activity is lower in the SCS than in the SOT and Southwest Taiwan. The mean 232Th activity of the sediments in the SOT and Southwest Taiwan is quite comparable to that of the shale, slate and black schist in Taiwan, suggesting that these sediments are the terrigenous detrial materials from Taiwan. The 238U and 234U activities in the cores of these two areas show no significant vertical nor areal variations with activities ranging between 1.3 and 2 dpm/g, and their 234U /238U activity ratios being about 1.1, quite close to that of seawater (1.14). Since 238U and 234U are quite comparable among the three areas, the higher activity of 230Th in excess over 234U in the northern SCS may be due to greater water depth that allows more 234U produced 230Th to be scavenged from the water column. The uranium and thorium radionuclides and their activity ratios in the SOT and Southwest Taiwan sediments suggest that these sediments are the terrigenous detrial materials from Taiwan. The source function of the SCS sediments is more complex than that of the above-mentioned sediments.

Cold trapped ions have many applications in quantum information science and precision metrology. In this thesis, we present progress toward two objectives involving ions confined to linear RF traps: the strong coupling of Ba+ ions with a high finesse optical cavity, and the observation of an optical nuclear transition in 229Th3+. In pursuit of the first objective, a novel high-temperture vapor cell for the spectroscopy of neutral barium was constructed. Using this vapor cell, a new technique for isotope-selective photoionization loading of Ba+ in an ion trap was developed. In pursuit of the second objective, techniques ultimately to be used in creating, trapping, and observing 229Th3+ are studied using 232Th3+. Ion traps are loaded with 232Th3+ via laser ablation of thorium targets. 232Th3+ is detected optically using laser-induced fluorescence and electronically using a channel electron multiplier. A technique for creating ablation targets from trace quantities of thorium nitrate is presented. The primary loss mechanisms of Th3+, charge exchange and chemical reactions, are studied.

Trapped cold ions are tools which we used to approach two very disparate areas of physics, strong coupling between Ba+ ions and optical resonators, and investigations of a low-energy nuclear isomer of 229-Th. The first part of this thesis describes our progress towards the integration of a miniature Paul (rf) ion trap with a high finesse (F=30000) optical cavity. Ba+ ions were trapped and cooled for long periods and a new scheme for isotope selective photoionization was developed. The second part of this thesis describes our progress towards controlled excitation of the low energy nuclear isomer of 229-Th, which may provide a bridge between the techniques of cold atomic and nuclear physics. As a step towards this goal, 232-Th3+ ions were confined in rf ions traps and cooled via collisions with a buffer gas of helium. A sophisticated scanning program was developed for controlling ion trap loading, tuning lasers, and running a CCD camera to look for fluorescence. The low-lying electronic transitions of Th3+ at 984 nm, 690 nm and 1087 nm were observed via laser fluorescence.

L’uranium, le thorium et leurs descendants se trouvent a l’état de traces dans toutes les roches sur Terre. Sous l’action de l’érosion, des mécanismes de formation des sols et des transferts sol-plante, ces radionucléides sont disséminés dans toutes les composantes de l’environnement et via les chaînes alimentaires, ils sont transmis aux animaux et à l’homme. L’objectif de cette étude consiste à améliorer les connaissances sur les niveaux et la variabilité des activités de ces radionucléides dans différentes denrées et à faire progresser les connaissances sur leurs sources et leurs transferts. Cette étude s’intéresse à la variabilité géologique des sites étudiés (substrat granitique, volcanique et alluvial) sur lesquels sont prélevés différentes denrées (légumes, céréales, viandes, oeufs et produits laitiers). Les sources potentielles de radionucléides (eau d’irrigation et sol pour les végétaux ; eau d’abreuvage, aliments et sol pour les produits animaux) sont prélevées afin d’étudier leur contribution aux activités mesurées dans les denrées. Les résultats obtenus montrent une grande variabilité des activités dans les végétaux, moins prononcée dans les produits animaux. La source principale de radionucléides aux végétaux semble être le sol de culture. La remise en suspension de particules de sol et leur adhésion à la surface des végétaux ainsi que l'eau d’irrigation semblent important dans certains cas. Concernant les activités dans les produits animaux, une contribution significative du sol a l’activité en thorium a été mise en évidence. La contribution de l’eau à l’activité en uranium dans les volailles constitue une piste à approfondir. / Uranium, thorium and their decay products are present in trace amounts in all rocks on Earth. Weathering, Mechanisms of soil formation and soil-plant transfers lead to the presence of these radionuclides in all the components of the environment and, through the food-chain transfers, they are also present in animals and men. The objective of this study consists in improving the knowledge on the levels and the variability of the activities of these radionuclides in various foodstuffs and on their sources and transfers. This study is based on the geological variability of the studied sites (granitic, volcanic and alluvial areas) where various foodstuffs are sampled (vegetables, cereals, meat, eggs and dairy products). The possible sources of radionuclides (irrigation waters and soils for plants; water, food and soils for animals) are also sampled in order to study their contribution to the measured activities in the foodstuffs. The results obtained present high variability of the activities in plants, less pronounced in animal products. For plants, the main radionuclide source seems to be the crop soils. Irrigation water, soil particle resuspension and their adhesion to plant surface seems to be important in some cases. For the activities in animal products, a significant contribution of the soil to thorium activity was highlighted. Water contribution to uranium activity in meat and eggs is an area worth further researches. Thus, this study of the possible sources of radionuclides highlights the importance of their role in the understanding of the radionuclide transfers to foodstuffs.

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