Le Phosphate Diphosphate de Thorium (β-PDT) est considéré comme une matrice<br />céramique potentielle en vue de l'immobilisation des actinides en formation géologique<br />profonde. Il s'est avéré donc impératif d'étudier les effets de l'irradiation sur la structure de<br />cette céramique et les conséquences sur sa durabilité chimique.<br />Des échantillons de β-PDT et des solutions solides associées de β-PDTU ont été irradiés<br />puis altérés en solution aqueuse. Selon la valeur du TEL électronique, le β-PDT peut<br />s'amorphiser totalement ou partiellement. Par ailleurs, la capacité de recristallisation du<br />matériau amorphe par recuit thermique a été démontrée. Les tests de lixiviation menés sur ces<br />échantillons irradiés ont montré une influence significative de la fraction amorphe sur la<br />vitesse de dissolution normalisée qui augmente d'environ un facteur 10 entre le matériau non<br />irradié et le matériau amorphe. Corrélativement, la fraction amorphe modifie le temps requis<br />pour atteindre les conditions de saturation associées aux équilibres thermodynamiques. En<br />revanche, elle ne présente aucune influence ni sur d'autres paramètres cinétiques, tels que<br />l'énergie d'activation du processus de dissolution ou l'ordre partiel par rapport aux protons, ni<br />la nature de la phase néoformée identifiée comme le Phosphate HydrogénoPhosphate de<br />Thorium Hydraté (PHPTH).<br />Des échantillons de β-PDTU ont aussi été irradiés sous rayonnements γ et α pendant les<br />tests de lixiviation afin d'étudier les effets de la radiolyse du milieu lixiviant sur la vitesse de<br />dissolution du matériau. Il est apparu que les espèces radiolytiques intervenant dans le<br />mécanisme de dissolution étaient peu stables, disparaissant rapidement dès la fin de<br />l'irradiation. Leur caractère fortement oxydant vis-à-vis de l'uranium tétravalent permet d'expliquer la différence de comportement entre les cations métalliques (U et Th)

Cette thèse est un apport à la compréhension des mécanismes d'endommagement des polymères irradiés aux ions lourds. L'étude se consacre ici exclusivement à l'influence du pouvoir d'arrêt électronique, (dE/dx)e, sur l'émission moléculaire induite par l'irradiation. Les polymères étudiés sont principalement le polyéthylène, le polypropylène et le polybutène. Les particules incidentes utilisées vont des électrons, (dE/dx)e"3,5´10-3 MeV.mg-1.cm2, aux ions 58Ni, (dE/dx)e"39 MeV.mg-1.cm2. Deux approches expérimentales différentes ont permis d'identifier la nature des gaz émis : la spectrométrie de masse et la spectroscopie infrarouge. La première technique est couramment employée pour ce genre d'étude ; elle est non sélective et permet donc la détection de H2 et des molécules lourdes. La spectrométrie de masse fournit également des informations quant aux cinétiques de diffusion des molécules produites. L'analyse des gaz émis par spectroscopie infrarouge est une nouvelle méthode développée au laboratoire. A l'aide de cette technique, nous avons identifié sans ambiguïté des molécules ayant jusqu'à trois atomes de carbone. L'analyse des spectres expérimentaux a été réalisée à partir de spectres de référence de gaz purs, réalisés au laboratoire ; nous avons ainsi pu quantifier précisément chaque gaz identifié. Nous avons suivi l'évolution des rendements gazeux avec l'augmentation du (dE/dx)e. A partir de l'ensemble de ces observations, nous avons déterminé, à différents (dE/dx)e, les rôles respectifs des différents mécanismes de formation des molécules gazeuses, comme par exemple le départ du groupe latéral et la fragmentation, à fort (dE/dx)e, de la chaîne principale due à une multi?ionisation de la macromolécule.

Nous avons étudié par spectroscopie de Résonance Paramagnétique Électronique (RPE), par simulation de spectre RPE, par spectroscopie Raman et par photoluminescence les ions Ti3+ formés par irradiation aux β de 3,37 x 10^5 à 1,68 x 10^9 Gy dans les verres SiO2 - Na2O - TiO2 et SiO2 - Al2O3 - B2O3 - Na2O - TiO2. Nous montrons que ces ions existent dans les verres silicatés dans trois sites différents correspondant à trois environnements différents : [VI]Ti3+, [V]Ti3+ sous forme de pyramide à base carrée et peut être sous forme de bi-pyramide. Il existe de même trois sites des ions Ti3+ dans les verres alumino-borosilicatés et le site des ions [VI]Ti3+ est commun aux deux classes de verres. L'étude de l'évolution des proportions d'ions Ti3+ dans chacun des sites ainsi que l'étude des paramètres RPE des sites montre que leur existence est déterminée par la dose intégrée et la valeur du rapport [Na]/[Ti] que nous redéfinissons dans le cas des verres alumino-borosilicatés. Enfin, sous une excitation à 266 nm nous observons à température ambiante une bande d'émission à 500 nm attribuée aux ions Ti4+. Sous une excitation à 532 nm, nous observons une bande d'émission à 590 nm avec des temps de vie courts d'une centaine de ns que nous attribuons soit aux ions Ti3+ soit à des défauts formés près des ions Ti4+. Dans l'ensemble, cette étude montre donc qu'il est possible de contrôler par la chimie et la dose intégrée l'environnement des ions Ti3+ et donc les propriétés d'émission de cet ion dans un verre.

Graduation date: 1997

This study was conducted to examine the antioxidative effect of conjugated linoleic acid (CLA) in irradiated, cooked ground beef patties. The hypothesis was that CLA would be retained during irradiation and would reduce lipid oxidation that is caused by irradiation. The objective was to evaluate the effects of CLA alone and in combination with irradiation on lipid oxidation, fatty acid composition, cooking loss, moisture and fat content, and trained panel sensory evaluations of beef patties. CLA was added at 0, 1, 2, or 4% level during the grinding process. Addition of CLA during the grinding process increased CLA cis-9,trans-11 and CLA trans-10,cis-12 isomers in both irradiated and non-irradiated cooked ground beef patties (irradiated at 1.6 kGy) (P = 0.0001). Weight loss during cooking was greater in irradiated beef patties than in non-irradiated patties (P = 0.004). Irradiation reduced the serumy/bloody aromatic attribute and increased browned aromatic attribute, browned aftertaste, and wet dog/hairy aromatic attribute (P < 0.05). There was no significant main effect of irradiation on the basic tastes. The linoleic acid, CLA cis-9,trans-11, and CLA trans-10,cis-12 were decreased by irradiation (P < 0.05). Although irradiation decreased the CLA isomers, higher percentages of CLA isomers were retained in irradiated patties containing a 4% free fatty acid preparation of CLA (FFA-CLA), reflecting the ability of the FFA preparation to reduce lipid oxidation that is caused by irradiation. The thiobarbituric acid reactive substances (TBARS) values were significantly higher in irradiated, cooked ground beef patties than in non-irradiated ground beef patties (P = 0.004). Although the FFA-CLA was effective in reducing lipid oxidation that is caused by irradiation, it increased painty aromatic attribute, bitter taste, and astringent aftertaste due to the soapy flavor of the free fatty acid (all P < 0.05). The FFA-CLA decreased cooked beef/brothy and serumy/bloody aromatic attribute and browned aftertaste (all P < 0.05). The 1% triacylglycerol (TAG) preparation of CLA reduced TBARS in irradiated, cooked patties to levels seen in control, non-irradiated patties. The 1% TAG concentration also provided good retention of CLA in the cooked ground beef.

The trapping behavior of deuterium and helium in polycrystalline tungsten (PCW) under D+-only, He+-only, sequential and simultaneous irradiation was studied as a function of incident ion fluences and irradiation temperature. Deuterium implanted at 300 and 500 K gets trapped at surface adsorption sites, vacancy-related traps, or extended defects. No deuterium was trapped for 700 K implantations. Results were affected by tungsten-carbide impurities in PCW specimens. It is suggested that He trapping occurs via the formation of He clusters, at impurity sites, or as part of He-vacancy complexes. For sequential implantations, D and He were found to de-trap each other, with He impeding the trapping of D when implanted first at 300 K. Under simultaneous irradiation a decrease in D inventories was observed for all cases, and a re-distribution of He to higher energy traps (associated with He-vacancy complex formation) was observed for higher fluences and temperatures.

The trapping behavior of deuterium and helium in polycrystalline tungsten (PCW) under D+-only, He+-only, sequential and simultaneous irradiation was studied as a function of incident ion fluences and irradiation temperature. Deuterium implanted at 300 and 500 K gets trapped at surface adsorption sites, vacancy-related traps, or extended defects. No deuterium was trapped for 700 K implantations. Results were affected by tungsten-carbide impurities in PCW specimens. It is suggested that He trapping occurs via the formation of He clusters, at impurity sites, or as part of He-vacancy complexes. For sequential implantations, D and He were found to de-trap each other, with He impeding the trapping of D when implanted first at 300 K. Under simultaneous irradiation a decrease in D inventories was observed for all cases, and a re-distribution of He to higher energy traps (associated with He-vacancy complex formation) was observed for higher fluences and temperatures.

Pecan nuts might become contaminated with foodborne pathogens, such as Salmonella and E. coli., through birds and other potential sources of contamination that can lead to serious illness or even death, as well as financial losses. For example, the outbreak of Salmonella in pecan products in Texas caused major product recalls in 2010. Irradiation with electron beams could be an effective method of preventing potential outbreaks without changing the pecans' taste, color and flavor and without causing any risk of recontamination before the product reaches the consumer. However, when irradiation is applied alone, the shelf life of the product is decelerated because of the detrimental effect of lipid oxidation. Therefore, to extend the shelf of the pecans while assuring their safety, irradiation of pecans under modified atmosphere packing (MAP) conditions could be a viable option. This research showed that when treated with electron beams, surrogates of Escherichia coli (a cocktail of BAA-1427, BAA-1428, and BAA-1430), and Salmonella (S. Typhimurium LT2) were more resistant to ionizing radiation (higher D10 values) when packed under vacuum (VP) than under air or other MAP conditions. This research also showed that lipid oxidation in pecans (due to exposure to ionizing radiation) shows a lag phase, probably due to the antioxidants present in pecan nuts. The lag phase represents a delay period before the pecan nuts start to get rancid (increase in PV formation), and it is best described by a modified Gompertz model. Kinetic evaluation of the lipid oxidation reaction suggests that the dose level has a more drastic effect in PV formation than the type of package used during the irradiation treatment, e.g., vacuum packed versus nitrogen-packed. Moreover, accelerated shelf life studies (4 weeks at 48.9oC) showed that vacuum-packed (VP) pecans can be stored at -25 degrees C up to three years, while irradiated (at 3.0 kGy) VP pecans can be stored only for eight months, without the detrimental effects of lipid oxidation. Therefore, irradiation of pecans under air at 3.0 kGy reduces the shelf life of the nuts in terms of rancidity, but vacuum-packaging can be used to extend their shelf-life. Irradiation in oxygen packaging increases rancidity and the oxidation reaction rate accelerates with increasing dose. Irradiation under nitrogen packaging requires lower doses to achieve the almost same number of log reductions in microbial population. The use of nitrogen packaging also inhibits the oxidative reaction leading to rancidity in pecans. Although there are some drawbacks to the application of nitrogen packaging in an irradiation plant (special machinery and packaging films (permeability specifications for N2 gas)), the savings induced by avoiding recalls may make this technology worthy of consideration.

Application of carbon nanotubes (CNTs) in various fields demands a thorough investigation of their stability under irradiation. Open structure, ability to reorganize and heal defects, and large surface-to-volume ratio of carbon nanotubes affect materials' radiation response so that it differs from their bulk counterparts. Despite the work conducted to this date, radiation damage and mechanisms governing the evolution of CNTs under irradiation are still deficient in fundamental understanding. This dissertation is aimed to comprehend and characterize radiation response and crystalline-to-amorphous transition in ion and electron irradiated carbon nanotubes using various techniques, including but not limited to, transmission electron microscopy (TEM) and Raman spectroscopy. It shows that ion irradiation can be used to engineer properties of nanotubes in a controllable manner and significantly improve thermal diffusivity and conductivity of the material. This work also establishes the role of nuclear and electronic stopping powers in thermal diffusivity enhancement: thermal properties of irradiated CNTs are governed by nuclear stopping power of bombarding species. The change of thermal properties with irradiation is driven by two competing mechanisms: inter-tube displacement-mediated phonon transport and defect-induced phonon scattering. In addition to experiments, molecular dynamic simulations are used to confirm validity of the obtained results. Radiation damage in CNTs at various temperatures as a function of ion energy, flux and fluence is examined. Mechanisms governing crystalline-to-amorphous transition under electron and ion irradiations are explored, applicability of previously suggested models discussed, and new models introduced. The results show enhanced defect annealing at elevated irradiation temperatures, which delays the formation of amorphous regions. Investigation of nanotube stability after various processing techniques and irradiation indicated that radiation response of CNTs in a composite is similar to that of individual nanotubes.

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