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Made available in DSpace on 2014-10-09T12:52:20Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:49Z (GMT). No. of bitstreams: 0 / O desenvolvimento da Medicina Nuclear, aliado à evolução dos equipamentos de diagnóstico e terapia, necessita, cada vez mais, da disponibilidade comercial de radioisótopos. Nesse contexto, o IPEN tem buscado atender e abastecer o mercado nacional. Um dos investimentos nesta área foi a aquisição de um ciclotron de 30 MeV, modelo Cyclone-30, que permitiu a produção dos radioisótopos tais como, o 18F, 67Ga, 201Tl e o 123I, sendo este último o foco do presente trabalho. Através de dados de produções rotineiras de 123I via irradiação com prótons em alvo gasoso de Xenônio com enriquecimento superior a 99,8% em 124Xe, foi realizado um estudo para identificar os fatores relevantes que influenciam diretamente o rendimento de obtenção de 123I com altíssimo grau de pureza. Embora a metodologia seja bem conhecida, quando se trata de produção comercial há uma escassez de dados sobre os parâmetros operacionais utilizados. Os parâmetros avaliados foram: pressão do gás 124Xe, intensidade de corrente de feixe de prótons, tempo de irradiação, temperatura de operação do sistema durante a irradiação, tempo de espera para formação de 123I, tempo de aquecimento do porta-alvo para recuperação do 123I formado, temperatura de aquecimento da solução de lavagem e influência do revestimento interno da câmara de irradiação com Ni. Com os resultados obtidos, foi possível alterar as condições operacionais nas produções rotineiras, conduzindo a um aumento de eficiência do processo em torno de 30%. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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Orientador: Francisco das Chagas Marques / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica "Gleb Wataghin" / Made available in DSpace on 2018-08-14T18:36:55Z (GMT). No. of bitstreams: 1 Barbieri_PauloFernando_D.pdf: 3626831 bytes, checksum: 0f15b74219acbe9e82e54ae2ce3d1ed4 (MD5) Previous issue date: 2009 / Resumo: Esta tese mostra um estudo realizado sobre implantação de xenônio (Xe) em uma matriz amorfa de silício (a-Si) utilizando a técnica de Deposição Assistida com Feixe de Íons (IBAD). Para esta finalidade, um feixe de íons de Xe com energia de 1500 eV, obtido de um canhão tipo Kaufman, foi utilizado para a realização de sputtering de um alvo de silício, produzindo um filme de a-Si em um substrato colocado a 15 cm de distância via deposição. Ao mesmo tempo outro feixe de íons de Xe, obtido por um segundo canhão de energia variável (0 a 300 eV), implantava átomos na rede do a-Si que estava em formação. Foi possível implantar concentrações de xenônio de até 5% com energia da ordem de 50 eV. Esta energia de implantação é de várias ordens de grandeza menor do que a energia utilizada no processo convencional de implantação que utiliza valores de energia de ordem de milhares de eV. Para investigar o Xe dentro da matriz amorfa de Si foram utilizadas técnicas de excitações atômicas XAS (X-Ray Absorption Spectroscopy), XPS (X-Ray Photoelectron Spectroscopy) e XAES (X-Ray Auger Excited Spectroscopy) e também simulações teóricas para XAS. Os resultados experimentais de XAS indicam que os átomos de Xe são implantados majoritariamente de forma dispersa pela matriz, isolados dentro de defeitos estruturais. Medidas de XPS mostram que os níveis eletrônicos de caroço dos átomos de xenônio são alterados em função do sítio de aprisionamento, que por sua vez, estão relacionados com a energia de implantação. Um estudo utilizando uma combinação de XPS e XAES permitiu identificar e separar as contribuições relativas às alterações eletrônicas observadas nos átomos de Xe devido a inserção deles dentro do a-Si. A análise destes resultados, XPS/XAES, forneceu indícios sobre os tamanhos das cavidades hospedeiras dos átomos de Xe. Simulações teóricas de XAS foram bastante satisfatórias e corroboram com as observações realizadas por XPS/XAES e indicam timidamente formações de aglomerados para alguns casos, ou seja, minoritariamente. Dependendo da energia de implantação, a dinâmica de crescimento do filme é alterada possibilitando estruturas diferentes na matriz. Foi também constatado que o aumento da densidade do a-Si por efeito de compactação pode ser mascarado pela introdução de elementos com massas relativamente grandes, como por exemplo, Xe / Abstract: A study of xenon (Xe) implantation in amorphous silicon (a-Si) by Ion Beam Assisted Deposition (IBAD) technique is investigated in this thesis. A Xe ion beam with energy of 1500 eV, obtained from a Kaufman gun, was used for sputtering a Si target, depositing an a-Si film on the substrate holder placed 15 cm apart. Simultaneously, another Xe beam with variable energy (0 ¿ 300 eV), was used to bombard the a-Si for the implantation of Xe atoms. It was obtained Xenon concentration up to 5%, using energy as low as 50 eV, which is orders of magnitude smaller than those usually used by conventional implantation, that requires thousands of eV. For the investigation of the Xe atoms in the a-Si matrix, XAS (X-Ray Absorption Spectroscopy), XPS (X-Ray Photoelectron Spectroscopy) and XAES (X-Ray Auger Excited Spectroscopy) techniques and theoretical simulations of XAS were used. The experimental results of XAS showed that Xe atoms are dispersed in the a-Si matrix as isolated element majoritively into structural defects. XPS measurement shows that the electronic core levels of the Xe atoms are shifted as function of the trapping site, which in turn, are related to the energy implantation. The use of XPS and XAES allowed the identification and the separation of the contribution to the shift in the binding energy due to the relaxation energy. The analysis of this results, XPS/XAES, provided indications as to size of the cavity containing the trapped Xe. XAS theoretical simulations gave satisfactory results and corroborate with XPS/XAES observations and tentativelly indicated small clusters, for a minority of cases. Depending on the implantation energy, the growth dynamics of the films is changed, allowing different structures in the matrix. It was also verified that the increase in the density of the film by the packing effect is obscured by the introduction of heavier elements, for example, Xe / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Orientador: Fernando Alvarez / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-23T23:30:17Z (GMT). No. of bitstreams: 1 Cucatti_SilviaAzevedodosSantos_M.pdf: 4655575 bytes, checksum: a7bdbfd34373445d40dd8ae10065030e (MD5) Previous issue date: 2013 / Resumo: A presente dissertação aborda a nanoestruturação de ligas metálicas de ferro a partir de bombardeamento iônico com Xe+ e seu efeito nas propriedades físicas de superfície, microestrutura, e na difusão de nitrogênio após nitretação à plasma pulsado. O atual interesse em modificação de superfícies surge da possibilidade de obtenção de novas propriedades físicas, mecânicas e tribológicas dos materiais. Particularmente, o bombardeamento iônico com gases nobres tem se mostrado um método importante para modificar a cinética de difusão de nitrogênio e a eficiência do processo, além das propriedades finais do material devido a criação de defeitos criados pelo impacto iônico tanto na superfície como, por efeito cascada, no volume. Sendo assim, estudar as mudanças geradas por bombardeamento iônico em ligas metálicas é fundamental tanto do ponto de vista tecnológico como dos fenômenos físicos básicos para um melhor entendimento da difusão de nitrogênio. Duas ligas metálicas de ferro foram investigadas: aço AISI 316L (alta liga) e AISI 4140 (baixa liga). A investigação da topografia das amostras estudadas após o bombardeamento mostrou que este procedimento induz a formação de padrões nanométricos dependentes da orientação cristalina da superfície e aumenta a rugosidade dos materiais analisados. Estudos de presença de microstrain realizados por difração de raios-X a baixo ângulo mostram que o bombardeamento gera defeitos na superfície e em volume por efeito cascada, em profundidades com ordens de magnitude maiores que a distância de frenamento dos íons Xe+. Verificou-se que a microestrutura e a dureza de seção transversal das ligas nitretadas são modificadas por bombardeamento iônico prévio ao procedimento de nitretação. Amostras pré-bombardeadas da liga AISI 4140 possuem agulhas de até alguns ?m de composts de nitretos enquanto tais compostos em amostras não sujeitas ao prébombardeamento são pequenos, granulares e descontínuos. Para a liga AISI 316L, foi observada uma camada de compostos única e compacta para amostras pré-bombardeadas, diferentemente da camada dupla, tipicamente encontrada em amostras sem tratamento prévio. Além disso, para esta liga, um perfil de dureza maior e mais suave foi encontrado para amostras pré-bombardeadas. Relacionamos as modificações da difusão de nitrogênio por bombardeamento iônico com o aumento do sticking factor do nitrogênio devido a múltiplas colisões resultantes da nanoestruturação e presença de canais extras de difusão / Abstract: This dissertation deals with the nanostructuration by low energy ion beam Xe+ (< 1keV) bombardment on the surface properties, microstructure and nitrogen diffusion after pulsed plasma nitridation of iron alloys. The current interest in surface modification arises from the possibility of obtaining new physical, mechanical and tribological properties of materials. In particular, noble gases ion bombardment is an important method for improve nitrogen diffusion modification, since the efficiency of the nitriding process and the material final properties are influenced by defects created by the ionic impact on both surface and bulk, the later by cascade collision effects. Therefore, the understanding of the modifications generated by ionic bombardment on metallic alloys is essential for better understanding nitrogen diffusion. Two type of alloys were investigated in this study: steels AISI 316L (high alloy) and AISI 4140 (low alloy). Investigations of the topography after bombardment showed that this procedure induces the formation of nanometric patterns within crystalline grains, increasing the roughness of the analyzed materials. Studies of the microstrain created by the ion bombardment by X-ray diffraction show that the bombardment generates bulk defects at distances orders of magnitude larger than the Xe+ ion stopping distance by cascade collisions. It was found that microstructure and cross-section hardness of nitrided alloys are modified by pre- ionic bombardment. Pre-bombarded AISI 4140 samples form long and needle-shaped nitrides compounds while the ones found in standard samples are small and discontinuous. For the AISI 316L alloy, differently from the double nitride layer normally found in nitrided standard samples, the pre-bombardment treatment leads to a single thick compact case, higher hardness and a smoother hardness gradient. We can correlate the nitrogen diffusion modifications by ionic bombardment with the increasing of the surface sticking factor and the presence of extra diffusion channels, due to the increasing roughness and defects in bulk after Xe+ bombardment, respectively / Mestrado / Física / Mestra em Física

Abstract Xenon atoms and sulfur hexafluoride (SF6) molecules can be dissolved in liquids and liquid crystals or adsorbed in porous materials. Nuclear magnetic resonance (NMR) spectra of 129Xe or 19F nuclei reveal information about their surroundings. This means that xenon atoms and SF6 molecules can be used as probes to indirectly study materials by NMR spectroscopy. The change in the spectra arises from a NMR interaction called shielding. Especially in the case of xenon, shielding reveals even the slightest changes, for example, in the density of a liquid it is dissolved in. Because a change in temperature leads to a change in the density of the liquid as well, temperature change is observed as a shift of the resonance line in the 129Xe NMR spectrum. This property can be utilized in the accurate determination of the sample temperature. Self-diffusion measurements of the gases provide additional information on a larger scale rather than just the immediate surroundings of atoms or molecules. Various liquid crystals were studied using xenon and SF6 as probes proving their applicability. It is often considered that the signal observed in NMR experiments is very weak and limits the full potential of the method. This is true especially with the samples in gaseous form. The Spin-Exchange Optical Pumping (SEOP) hyperpolarization method solves this problem in the case of xenon. A 129Xe NMR signal can be enhanced by a factor of 104–105 by SEOP and this opens access to techniques that are not otherwise possible. The remote detection technique, which separates the encoding and detection steps of the typical NMR experiment both temporally and spatially, is one of these techniques. The potential of the combination of SEOP and remote detection techniques was shown in studies of thermally modified Pinus Sylvestris.

In modern high temperature nuclear reactors, silicon carbide (SiC) is used as the main diffusion barrier for the fission products in coated fuel spheres called TRISO particles. In the TRISO particle, pyrolytic carbon and SiC layers retain most of the important fission products like xenon, krypton and cesium effectively at temperatures up to 1000 oC. Previous studies have shown that 400 oC to 600 oC implantation of heavy ions into single crystal 6H-SiC causes the SiC to remain crystalline with many point defects and dislocation loops (damage). The release of Xe at annealing temperatures above 1400 oC is governed by the normal volume diffusion without any hindrance of trapping effects. In this study two phenomena in single crystal 6H-SiC implanted by 360 keV Xenon ions were studied using Rutherford Backscattering Spectroscopy (RBS) and channeling. Radiation damage and its annealing behavior at annealing temperatures ranging from 1000 oC to 1500 oC, and the diffusion of xenon in 6H-SiC at these annealing temperatures were investigated. 360keV xenon ions were implanted into a single crystalline wafer (6H-SiC) at 600 oC with a fluence of 1 × 1016 cm-2. The sample was vacuum annealed in a computer control Webb 77 graphite furnace. Depth profiles were obtained by Rutherford backscattering spectrometry (RBS). The same set-up was used to investigate radiation damage of the 6H-SiC sample by channeling spectroscopy. Isochronal annealing was performed at temperatures ranging from 1000 to 1500 °C in steps of 100 oC for 5 hours. Channeling revealed that the 6H-SiC sample retained most of its crystal structure when xenon was implanted at 600 °C. Annealing of the radiation damage took place when the sample was heat treated at temperatures ranging from 1000 oC to 1500 oC. The damage peak almost disappears at 1500 oC but the virgin spectrum was not achieved. This happened because of dechanneling due to extended defects like dislocations remaining in the implanted region. RBS profiles showed that no diffusion of the Xe occurred when the sample was annealed at temperatures from 1000 oC to 1400 oC. A slight shift of the xenon peak position towards the surface after annealing at 1400 °C was observed for 600 oC implantation. After annealing at 1500o C, a shift toward the surface accompanied by a broadening of the Xe peak indicating that diffusion took place. This diffusion was not accompanied by a loss of xenon from the SiC surface. The shift towards the surface is due to thermal etching of the SiC at 1400-1500 °C. Modern high temperature gas-cooled reactors operate at temperatures above 600 oC in the range of 750 oC to 950 oC. Consequently, our results indicate that the volume diffusion of Xenon in SiC is not significant in SiC coated fuel particles. / Dissertation (MSc)--University of Pretoria, 2014. / gm2014 / Physics / unrestricted

Le xénon est utilisé par la plupart des systèmes de propulsion électrique à grilles. Cependant sa rareté, son coût de production important ainsi que son usage dans de nombreuses applications industrielles font apparaître la nécessité de trouver une alternative à ce propergol. Il est apparu que l'iode était un candidat potentiel pour cela, étant beaucoup moins cher à produire et beaucoup moins rare. Il se présente sous forme de cristaux violacés dans les conditions standards de pression et de température et possède une pression de vaporisation peu élevée ainsi qu'un potentiel d'ionisation plus bas que celui du xénon. Un modèle global d'un plasma d'iode dans un propulseur électrique à grilles a donc été développé afin d'étudier le comportement et les performances d'un tel dispositif. Ces résultats de l'iode sont comparés à ceux du même dispositif obtenus par l'utilisation du xénon, les conditions d'opération étant bien évidemment similaires. Le modèle prédit une efficacité globale du propulseur 15% plus grande pour l'iode. Les résultats du modèle global en iode sont également comparés avec des résultats expérimentaux obtenus dans un propulseur électrique à grilles, sous des conditions d'opération et paramètres d'entrée similaires. Un band d'essai expérimental entièrement dédié à l'étude de l'iode comme nouveau propergol pour la propulsion à grilles a en effet été assemblé avec toutes les précautions nécessaires, l'iode étant un élément corrosif et chimiquement actif avec certains matériaux. Le banc d'essai en iode fut également utilisé pour effectuer la preuve de concept en iode du propulseur PEGASES. / Most state-of-the-art electric space propulsion systems such as gridded and Hall thrusters use xenon as the propellant gas. However, xenon is rare, expensive to produce and used in a number of competing industrial applications. Alternatives to xenon are currently being investigated, and iodine has emerged as a potential candidate. Its lower cost, larger availability, its solid state at standard temperature and pressure, its low vapour pressure and its low ionization potential makes it an attractive option. A global model of iodine plasma inside an ion gridded thruster has therefore been developed to study behaviour and performances of this propellant. We compared the iodine results with ones obtained in xenon under otherwise similar conditions. The model predicts a thruster efficiency 15% higher for iodine compared to xenon. Results of the iodine global model were compared with experimental data obtained under similar operating conditions and input parameters in a gridded ion thruster. An experimental test bench dedicated to iodine plasma study, inside a classic ion gridded thruster and PEGASES thruster, has been assembled with all precautions needed. Iodine is a corrosive gas and chemically active with certain metals and the right choice of materials is therefore important. The positive ion and electron densities obtained by the model and in experiments appeared to show close values, indicating that the iodine chemistry and reaction set used in the global model seem relevant to a first order approximation.

Results of I-Xe analyses have been obtained from meteorite samples that experienced different extents of thermal processing in the early Solar System in order to help characterise the movements of iodine and xenon in the early Solar System and constrain the timing of these movements using the I-Xe chronometer. Samples were irradiated to convert 127I to 128Xe* and allow simultaneous measurements of iodine and xenon isotopes. Xe isotopes were measuring using the RELAX mass spectrometer.I-Xe ages of material of different metamorphic grade from R-chondrites NWA 6492, NWA 830 and NWA 3364 suggest a link between the time of closure to Xe-loss and extent of metamorphism on the R-chondrite parent body. However, further I-Xe analyses of R5 material from NWA 6492 and R4 and R6 material from other R-chondrites are needed to confirm this. The most primitive material analysed give I– Xe ages between 4559 – 4554 Myr, slightly later than reported Mn-Cr ages. This may support the ideal of radial heterogeneity of 53Mn in the early Solar System. However differences could also be due to variations in the samples analysed. Future analyses of I-Xe and Mn-Cr ages in mineral separates from the same R-chondrite are recommended in order to investigate this hypothesis. Closure to Xe-loss in chondrules on the R-chondrite parent body appears to have occurred ~5 – 10 Myr later than on the ordinary and enstatite parent bodies. This implies either later accumulation of material or slower cooling in a larger body.Comparisons of I-Xe systematics in anomalous eucrites Bunburra Rockhole and Ibitira and “nomalous” eucrites Juvinas and Béréba show lower 129I/244Pu ratios in the “nomalous” eucrites. This is not due to formation on a less volatile-rich body but instead reflects extended loss of Xe on 4 Vesta. 129I/244Pu ratios indicate igneous processing continued on 4 Vesta for ~50-100 Myr after geological activity had ceased on the anomalous eucrites parent bodies. The extended processing seen in Juvinas and Béréba is attributed to formation on a larger body that retained heat for longer. If, as the data suggest, the anomalous eucrites formed on a separate parent body it must have been catastrophically disrupted as Vesta is thought to be the only remaining differentiated asteroid. The larger size of Vesta may explain why it has uniquely survived the impacts that destroyed its siblings. Analyses of the unique achondrite GRA 06129 show that the I-Xe system in this meteorite has no chronological significance. The data instead suggest that iodine-bearing plagioclase formed early but thermal metamorphism resulted in loss of 129Xe* from iodine bearing sites. Uranium-bearing apatite appears to be a secondary mineral that incorporated parentless 129Xe* and 129Xe*that had been redistributed during earlier metamorphism. A trapped-Xe component released at high-temperatures may be a primitive component such as Q-Xe, though terrestrial–Xe acquired during weathering cannot be ruled out by this study. If Q-Xe is present, it is most likely hosted in a primary phase other than plagioclase. During its terrestrial residence time GRA 06129 acquired iodine via Antarctic weathering. I-Xe analyses on Antarctic meteorites should therefore be carried out with caution. Further Xe analyses of mineral separates from GRA 06129 would help constrain the host phase of the trapped Xe. That the I-Xe system of the plagioclase has been completely reset make it a good candidate mineral for I-Xe dating of primary processes whereas I-Xe dating of apatite appears more problematic.