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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
221

Essays zu methodischen Herausforderungen im Large-Scale Assessment

Robitzsch, Alexander 21 January 2016 (has links)
Mit der wachsenden Verbreitung empirischer Schulleistungsleistungen im Large-Scale Assessment gehen eine Reihe methodischer Herausforderungen einher. Die vorliegende Arbeit untersucht, welche Konsequenzen Modellverletzungen in eindimensionalen Item-Response-Modellen (besonders im Rasch-Modell) besitzen. Insbesondere liegt der Fokus auf vier methodischen Herausforderungen von Modellverletzungen. Erstens, implizieren Positions- und Kontexteffekte, dass gegenüber einem eindimensionalen IRT-Modell Itemschwierigkeiten nicht unabhängig von der Position im Testheft und der Zusammenstellung des Testheftes ausgeprägt sind und Schülerfähigkeiten im Verlauf eines Tests variieren können. Zweitens, verursacht die Vorlage von Items innerhalb von Testlets lokale Abhängigkeiten, wobei unklar ist, ob und wie diese in der Skalierung berücksichtigt werden sollen. Drittens, können Itemschwierigkeiten aufgrund verschiedener Lerngelegenheiten zwischen Schulklassen variieren. Viertens, sind insbesondere in low stakes Tests nicht bearbeitete Items vorzufinden. In der Arbeit wird argumentiert, dass trotz Modellverletzungen nicht zwingend von verzerrten Schätzungen von Itemschwierigkeiten, Personenfähigkeiten und Reliabilitäten ausgegangen werden muss. Außerdem wird hervorgehoben, dass man psychometrisch häufig nicht entscheiden kann und entscheiden sollte, welches IRT-Modell vorzuziehen ist. Dies trifft auch auf die Fragestellung zu, wie nicht bearbeitete Items zu bewerten sind. Ausschließlich Validitätsüberlegungen können dafür Hinweise geben. Modellverletzungen in IRT-Modellen lassen sich konzeptuell plausibel in den Ansatz des Domain Samplings (Item Sampling; Generalisierbarkeitstheorie) einordnen. In dieser Arbeit wird gezeigt, dass die statistische Unsicherheit in der Modellierung von Kompetenzen nicht nur von der Stichprobe der Personen, sondern auch von der Stichprobe der Items und der Wahl statistischer Modelle verursacht wird. / Several methodological challenges emerge in large-scale student assessment studies like PISA and TIMSS. Item response models (IRT models) are essential for scaling student abilities within these studies. This thesis investigates the consequences of several model violations in unidimensional IRT models (especially in the Rasch model). In particular, this thesis focuses on the following four methodological challenges of model violations. First, position effects and contextual effects imply (in comparison to unidimensional IRT models) that item difficulties depend on the item position in a test booklet as well as on the composition of a test booklet. Furthermore, student abilities are allowed to vary among test positions. Second, the administration of items within testlets causes local dependencies, but it is unclear whether and how these dependencies should be taken into account for the scaling of student abilities. Third, item difficulties can vary among different school classes due to different opportunities to learn. Fourth, the amount of omitted items is in general non-negligible in low stakes tests. In this thesis it is argued that estimates of item difficulties, student abilities and reliabilities can be unbiased despite model violations. Furthermore, it is argued that the choice of an IRT model cannot and should not be made (solely) from a psychometric perspective. This also holds true for the problem of how to score omitted items. Only validity considerations provide reasons for choosing an adequate scoring procedure. Model violations in IRT models can be conceptually classified within the approach of domain sampling (item sampling; generalizability theory). In this approach, the existence of latent variables need not be posed. It is argued that statistical uncertainty in modelling competencies does not only depend on the sampling of persons, but also on the sampling of items and on the choice of statistical models.
222

Use of the Physical Classroom Environment as a Teaching and Learning Tool Including the Impact of the CCSSI in Kindergarten Through Third Grade Classrooms in Northeast Tennessee

Hensley-Pipkin, Charity 01 August 2015 (has links)
The specific goal of this study was to determine the use of the physical classroom environment as a teaching and learning tool in an era of the implementation of the Common Core State Standards Initiative (CCSSI). This qualitative multi-case study focused on the learning principles and epistemological beliefs of primary teachers with reference to the physical classroom environment and the teaching process in regard to meeting the expectations set forth by the CCSSI. The researcher sought participation from a city school district in Northeast Tennessee which included a total of 8 participating teachers consisting of 2 each of grades kindergarten, first, second, and third. The Teacher Beliefs Survey (Woolley, Benjamin, & Woolley, 2004) was administered to determine teachers’ philosophical position regarding constructivist and traditional beliefs. Based upon responses, 8 teachers representing the most constructivist and most traditional teachers in each grade were selected for further participation. Teachers’ practices and perceptions of the role of the physical environment in the teaching and learning process including consideration of the CCSSI were further explored through interview. Each physical classroom environment was evaluated using the Primary Educators Environment Rating Scale (PEERS), a rubric designed to assess the use of the physical classroom environment on a continuum from traditional to constructivist practices (Evanshen & Faulk, under review). Observational field notes and photographs were collected in order to document environmental components of the physical classroom environment of each participant. Data was collected and triangulated through the use of the aforementioned methods. Through the data analysis process, the researcher found all participants to demonstrate support for the role of the physical environment in the teaching and learning process which was determined based on results of the interview in conjunction with findings of the PEERS and supporting photographic evidence. Each teacher’s personal experiences and philosophy of education was found to guide the physical classroom environment design and layout in various ways. While most teachers felt the CCSSI had little or no impact on their physical classroom environment, all shared in varying degrees the use of the physical environment as a tool to support students in developing 21st century skills.
223

Advanced BaZrO3-BaCeO3 Based Proton Conductors Used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs)

Bu, Junfu January 2015 (has links)
In this thesis, the focus is on studying BaZrO3-BaCeO3 based proton conductors due to that they represent very promising proton conductors to be used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs). Here, dense BaZr0.5Ce0.3Y0.2O3-δ (BZCY532) ceramics were selected as the major studied materials. These ceramics were prepared by different sintering methods and doping strategies. Based on achieved results, the thesis work can simply be divided into the following parts: 1) An improved synthesis method, which included a water-based milling procedure followed by a freeze-drying post-processing, was presented. A lowered calcination and sintering temperature for a Hf0.7Y0.3O2-δ (YSH) compound was achieved. The value of the relative density in this work was higher than previously reported data. It is also concluded that this improved method can be used for mass-production of ceramics. 2) As the solid-state reactive sintering (SSRS) represent a cost-effective sintering method, the sintering behaviors of proton conductors BaZrxCe0.8-xLn0.2O3-δ (x = 0.8, 0.5, 0.1; Ln = Y, Sm, Gd, Dy) during the SSRS process were investigated. According to the obtained results, it was found that the sintering temperature will decrease, when the Ce content increases from 0 (BZCLn802) to 0.3 (BZCLn532) and 0.7 (BZCLn172). Moreover, the radii of the dopant ions similar to the radii of Zr4+ or Ce4+ ions show a better sinterability. This means that it is possible to obtain dense ceramics at a lower temperature. Moreover, the conductivities of dense BZCLn532 ceramics were determined. The conductivity data indicate that dense BZCY532 ceramics are good candidates as either oxygen ion conductors or proton conductors used for ITSOFCs. 3) The effect of NiO on the sintering behaviors, morphologies and conductivities of BZCY532 based electrolytes were systematically investigated. According to the achieved results, it can be concluded that the dense BZCY532B ceramics (NiO was added during ball-milling before a powder mixture calcination) show an enhanced oxygen and proton conductivity. Also, that BZCY532A (NiO was added after a powder mixture calcination) and BZCY532N (No NiO was added in the whole preparation procedures) showed lower values. In addition, dense BZCY532B and BZCY532N ceramics showed only small electronic conductivities, when the testing temperature was lower than 800 ℃. However, the BZCY532A ceramics revealed an obvious electronic conduction, when they were tested in the range of 600 ℃ to 800 ℃. Therefore, it is preferable to add the NiO powder during the BZCY532 powder preparation, which can lower the sintering temperature and also increase the conductivity. 4) Dense BZCY532 ceramics were successfully prepared by using the Spark Plasma Sintering (SPS) method at a temperature of 1350 ℃ with a holding time of 5 min. It was found that a lower sintering temperature (&lt; 1400 ℃) and a very fast cooling rate (&gt; 200 ℃/min) are two key parameters to prepare dense BZCY532 ceramics. These results confirm that the SPS technique represents a feasible and cost-effective sintering method to prepare dense Ce-containing BaZrO3-BaCeO3 based proton conductors. 5) Finally, a preliminary study for preparation of Ce0.8Sm0.2O2-δ (SDC) and BZCY532 basedcomposite electrolytes was carried out. The novel SDC-BZCY532 based composite electrolytes were prepared by using the powder mixing and co-sintering method. The sintering behaviors, morphologies and ionic conductivities of the composite electrolytes were investigated. The obtained results show that the composite electrolyte with a composition of 60SDC-40BZCY532 has the highest conductivity. In contrast, the composite electrolyte with a composition of 40SDC-60BZCY532 shows the lowest conductivity. In summary, the results show that BaZrO3-BaCeO3 based proton-conducting ceramic materials represent very promising materials for future ITSOFCs electrolyte applications. / <p>QC 20150423</p>
224

Methods for ℓp/TVp Regularized Optimization and Their Applications in Sparse Signal Processing

Yan, Jie 14 November 2014 (has links)
Exploiting signal sparsity has recently received considerable attention in a variety of areas including signal and image processing, compressive sensing, machine learning and so on. Many of these applications involve optimization models that are regularized by certain sparsity-promoting metrics. Two most popular regularizers are based on the l1 norm that approximates sparsity of vectorized signals and the total variation (TV) norm that serves as a measure of gradient sparsity of an image. Nevertheless, the l1 and TV terms are merely two representative measures of sparsity. To explore the matter of sparsity further, in this thesis we investigate relaxations of the regularizers to nonconvex terms such as lp and TVp "norms" with 0 <= p < 1. The contributions of the thesis are two-fold. First, several methods to approach globally optimal solutions of related nonconvex problems for improved signal/image reconstruction quality have been proposed. Most algorithms studied in the thesis fall into the category of iterative reweighting schemes for which nonconvex problems are reduced to a series of convex sub-problems. In this regard, the second main contribution of this thesis has to do with complexity improvement of the l1/TV-regularized methodology for which accelerated algorithms are developed. Along with these investigations, new techniques are proposed to address practical implementation issues. These include the development of an lp-related solver that is easily parallelizable, and a matrix-based analysis that facilitates implementation for TV-related optimizations. Computer simulations are presented to demonstrate merits of the proposed models and algorithms as well as their applications for solving general linear inverse problems in the area of signal and image denoising, signal sparse representation, compressive sensing, and compressive imaging. / Graduate
225

Les oncogènes NUP98-PHF23 et NUP98-HOXD13 confèrent un potentiel aberrant d’auto-renouvellement aux progéniteurs thymiques

Tardif, Magalie 09 1900 (has links)
No description available.
226

Desenvolvimento de estratégias analíticas visando a determinação de macroelementos, microelementos e elementos traço em rações de cães e gatos empregando técnicas de espectrometria atômica / Development of analytical strategies aiming the determination of macroelementos, microelementos and trace elements in feeds for dogs and cats employing atomic espectrometry techniques

Ávila, Dayara Virgínia Lino 29 February 2016 (has links)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work, four methods were developed for analysis of feed for dogs and cats using atomic spectrometry techniques. Initially, the multivariate optimization of sample digestion procedure was employed in wet feed for dogs and cats and then the simultaneous determination of macroelements (Ca, K, Mg and P), microelements (Cu, Fe, Mn, and Zn) and trace elements (Al, B, Ba and Sr) was carried out by optical emission spectrometry with inductively coupled plasma (ICP OES). The results of the elements concentrations in the samples were in agreement with the values provided and regulated by Ministry of Agriculture, Livestock and Supply (Ministério da Agricultura, Pecuária e Abastecimento - MAPA) and American Association of Official Food Control Officials (AAFCO). Limits of detections (LoD) ranged between 0.1 μg g-1 (Ba, Mg and Sr) and 15.0 μg g-1 (P) and limits of quantification (LoQ) between 0.3 μg g-1 (Ba, Mg and Sr) and 49.9 μg g-1 (P). In the second part of the work two analytical methods were optimized for Co and Cr determination in wet food (sachets and cans) for dogs and cats using direct sample analysis (SS) and detection by high resolution continuum source graphite furnace atomic absorption spectrometry (HR-CS GF AAS). Pyrolysis temperatures were 1500 ° C and 1400 ° C for Cr and Co determination, respectively. Atomization temperatures of 2500 ° C for Cr, and 2400 ° C for Co with chemical modifiers were adopted. The optimized method for Co and Cr determination in wet feed was fast, precise and accurate and contributes to the concept of “Green Chemistry”. The results obtained for concentrations of Cr and Co were not compared to any official rule, since there is no regulation for these elements. The LoD's obtained for Cr and Co were 0.01 and 0.05 ug g-1, respectively, and LoQ's obtained were 0.03 to 0.17 ug g-1 for Cr and Co, respectively. Finally, in the third part of the work, an analytical method for determination of Zn in dry feed for dogs and cats using energy dispersive Xray fluorescence spectrometry (EDXRF) was developed. Sample were prepared as pressed pellets. The concentration of Zn in seven dry feed samples for dogs and cats were determinate. The results were statistically compared to the values found by the analytical method by ICP OES. Additionally, paired t-test between concentrations of Zn by EDXRF and ICP OES was applied. The LoD e LoQ obtained for Zn determination were 0.1 e 0.4 mg kg-1, respectively. The independent methods were not significantly different at a 95% confidence level. In our work, the analytical methods proposed were efficient, fast and reliable for the determination of macroelements (Ca, Mg, K and P), microelements (Cu, Fe, Mn and Zn) and trace elements (Al, B, Ba, Co, Cr and Sr) in feeds for dogs and cats using atomic spectrometry techniques. This work was financially supported through the project Universal notice nº 014/2013 approved by CNPq, registered process nº 482416 / 2013-0. / Neste trabalho, foram desenvolvidos quatro métodos para análise de rações de cães e gatos, usando técnicas de espectrometria atômica. Primeiramente, foi empregada a otimização multivariada do procedimento de digestão da amostra para determinação simultânea de macroelementos (Ca, K, Mg e P), microelementos (Cu, Fe, Mn e Zn) e elementos traço (Al, B, Ba e Sr) em rações úmidas de cães e gatos, utilizando a espectrometria de emissão óptica com plasma acoplado indutivamente (ICP OES). Os resultados das concentrações dos elementos nas rações dos animais mostraram-se de acordo com os valores fornecidos e regulamentados pelo Ministério da Agricultura, Pecuária e Abastecimento (MAPA) e pela Associação Americana de Controle Oficial dos Alimentos (AAFCO). Os LoD’s variaram entre 0,1 μg g-1 (Ba, Mg e Sr) e 15,0 μg g-1 (P) e os LoQ’s variaram entre 0,3 μg g-1 (Ba, Mg e Sr) e 49,9 μg g-1 (P). Na segunda parte do trabalho, foram otimizados dois métodos analíticos para determinação de Co e Cr em rações úmidas (sachês e enlatadas) para cães e gatos utilizando análise direta de sólidos (SS) e detecção por espectrometria de absorção atômica de alta resolução com fonte contínua e atomização em forno de grafite (HR-CS GF AAS). Foram estabelecidas temperaturas de pirólise de 1500°C e de 1400°C para a determinação de Cr e Co respectivamente, e temperaturas de atomização de 2500°C para Cr e de 2400°C para Co, sem o uso de modificadores químicos. Os métodos otimizados foram rápidos, precisos e exatos, além de contribuir com a concepção da Química Verde. Os resultados obtidos para as concentrações de Cr e Co não foram comparados com nenhuma norma, uma vez que estes elementos não são regulamentados. Os LoD’s obtidos para Cr e Co foram de 0,01 e 0,05 μg g-1 respectivamente, já os LoQ’s obtidos para Cr e Co foram de 0,03 a 0,17 μg g-1 respectivamente. Por fim, na terceira parte do trabalho, foi desenvolvido um método analítico para determinação de Zn em rações secas destinadas à alimentação de cães e gatos empregando a espectrometria de fluorescência de raios-X com energia dispersiva (EDXRF). As amostras foram preparadas na forma de pastilhas prensadas e a concentração de Zn, determinada em sete rações secas de cães e gatos. Os resultados obtidos foram estatisticamente comparados com os valores encontrados através de um método analítico estabelecido por ICP OES, em que foi aplicado o teste t pareado entre as concentrações de Zn por EDXRF e ICP OES. Os LoD e LoQ obtidos na determinação de Zn através do método foram de 0,1 e 0,4 mg kg-1 respectivamente. Os métodos independentes não apresentaram diferença significativa para um nível de confiança de 95%. Em nosso trabalho, os métodos analíticos propostos foram eficientes, rápidos, e confiáveis para a determinação de macroelementos (Ca, Mg, K e P), microelementos (Cu, Fe, Mn e Zn) e elementos traço (Al, B, Ba, Co, Cr e Sr) em rações de cães e gatos, usando técnicas espectroanalíticas. Este trabalho teve o auxílio financeiro do projeto Universal Edital 014/2013 aprovado pelo CNPq, registrado sob processo nº 482416/2013-0.
227

Predictors of performance

Danay, Erik 04 April 2011 (has links)
Präsentiert werden drei Studien zum Thema Prädiktion von Leistung. In Studie 1 wurde die Prädiktion von Studienerfolg nicht nur mit Persönlichkeitsmaßen auf Facettenebene sowohl von Fremd- als auch Selbst-Ratings untersucht, sondern auch der Einfluss von faking auf die Kriteriumsvalidität. Ergebnisse zeigten, dass Fremd- über Selbst-Ratings und Intelligenz hinaus Studienerfolg prädizieren. Auch wurde gezeigt, dass Faking die Kriteriumsvaliditäten auf Facettenebene in unterschiedlicher Weise beeinflusst. Studie 2 untersuchte den Einfluss der unterschiedlichen Abstraktionsebene von Prädiktor und Kriterium auf die Kriteriumsvalidität. Dazu wurden Leistungsmotivationsskalen sowohl in einer Mathe-spezifischen Formulierung als auch in einer globalen Formulierung Schülern vorgegeben. Diese Skalen dienten dann als Prädiktoren für Noten in Mathe, Physik und Deutsch. Ergebnisse einer Varianzzerlegung mit MTMM zeigten, dass die Mathe-spezifischen Skalen durchgehend ein Plus an Varianz enthalten, welches unabhängig ist von der Varianz aufgrund der einzelnen Motivationskonstrukte. Folglich messen domänen-spezifische Skalen entweder ein engeres Konstrukt von Leistungsmotivation oder, eher, ein zusätzliches Konstrukt. Das Korrelationsmuster der domänen-spezifischen Varianz mit den drei untersuchten Noten legt nahe, dass es sich bei diesem zusätzlichen Konstrukt um Selbstkonzept handelt. Studie 3 untersuchte die Konstruktvalidität der Big 5 und möglicher higher-order factors nach Kontrolle von möglichen Biases innerhalb des CTCM-1 Ansatzes mit Selbst- und Fremdratings. Ergebnisse zeigten, dass bias-bereinigte Big 5 Maße die Annahme eines higher-order factors wenig plausibel machen. Darüber hinaus konnte ein solcher potentieller Faktor nicht theoriekonform die positive Eigenschaft Intelligenz prädizieren. Insgesamt verdeutlicht dies die Problematik des Einflusses von unterschiedlichen Quellen und Verzerrungen auf die Kriteriumsvalidität von häufig eingesetzten Persönlichkeitsmaßen. / Presented are 3 studies about the prediction of performance. Study 1 analyzes the prediction of academic performance by use of self-ratings, other-ratings and faked-ratings of personality measures not only on domain level but also on facet level. Result showed that other-ratings yield incremental validity above and beyond self-ratings and intelligence. Moreover, against prior findings for domain-level, faking does influence criterion validity on facet-level, with the influence not being uniform in direction. Study 2 analyzed the influence of different levels of abstraction of predictor and criterion in the realm of achievement motivation. For that, various achievement motivation scales were administered both in a global and a math-specific wording. These scales later on served as predictor for grades in math, physics and German. By modeling this data in a MTMM model different sources of variance could be disentangled. Results showed that math-specific scales are the better predictors. More so, these domain-specific scales have uniformly an increase in variance regardless of the positive or negative valence of the various achievement motivation scales. This leads to the conclusion that math-domain-specific scales either measure a narrower construct or, more probable, they tap an additional construct. This is backed by the uniform positive additional variance. Moreover, test-criterion correlation-pattern between the math-domain-specific variance and the three different grades makes it plausible that the additional construct tapped in these scales is self-concept. Study 3 analyzed the construct-validity of personality’s Big 5 and their possible higher order factor after controlling for singular rater biases using a newly developed CTCM-1 approach. Results showed that these bias free Big 5 make the assumption of one higher order factor implausible. Moreover, such a factor would not uniformly predict intelligence as is claimed by advocates of this factor.
228

Contextualizing Customer Feedback: A Research-through-Design Approach - Alternative Approaches and Dialogical Engagement in Survey Design

Svensson, Rasmus January 2023 (has links)
Providing context behind customer feedback remains a challenge for company’s who rely on approaching Customer Experience (CX) through standardized Customer Satisfaction (CS) metrics like Net Promoter Score (NPS), Customer Satisfaction Score (CSAT), and Customer Effort Score (CES). Practical guidelines for monitoring CS throughout the customer journey are limited, creating a gap in academic research. This study addresses this gap by offering practical guidelines for CS, actionable insights, and alternative survey design strategies within the context of invoicing. Utilizing a Research-through-Design (RtD) approach guided by the Double Diamond design model, the study consists of four phases: Discover, Define, Develop, and Deliver. From a service design perspective using qualitative methods, the study acquires and analyzes both organizational and customer insights. Synthesized empirical findings emphasize the need for a more comprehensive approach that targets specific phases of the customer journey utilizing a more customer- centric approach, paving the way for alternative methods that reaches beyond just simply measuring CS. Introducing the concept of a personal companion, the study presents a dialogical approach where surveys are experienced as ongoing interactions rather mere tasks. By highlighting the importance of contextualization, alternative survey approaches, and a dialogical approach, this research aims to guide company’s in managing customer feedback strategies.
229

A Radio Frequency Quadrupole Instrument for use with Accelerator Mass Spectrometry: Application to Low Kinetic Energy Reactive Isobar Suppression and Gas–phase Anion Reaction Studies

Eliades, John Alexander 21 August 2012 (has links)
A radio frequency (rf) quadrupole instrument, currently known as an Isobar Separator for Anions (ISA), has been integrated into an Accelerator Mass Spectrometry (AMS) system to facilitate anion–gas reactions before the tandem accelerator. An AMS Cs+ sputter source provided > 15 keV ions that were decelerated in the prototype ISA to < 20 eV for reaction in a single collision cell and re-accelerated for AMS analysis. Reaction based isobar suppression capabilities were assessed for smaller AMS systems and a new technique for gas–phase reaction studies was developed. Isobar suppression of 36S– and 12C3– for 36Cl analysis, and YF3– and ZrF3– for 90Sr analysis were studied in NO2 with deceleration to < 12 eV. Observed attenuation cross sections, σ [x 10^–15 cm^2], were σ(S– + NO2) = 6.6, σ(C3– + NO2) = 4.2, σ(YF3– + NO2) = 7.6, σ(ZrF3– + NO2) = 19. With 8 mTorr NO2, relative attenuations of S–/Cl– ~ 10^–6, C3–/Cl– ~ 10^–7, YF3–/SrF3– ~ 5 x 10^–5 and ZrF3–/SrF3– ~ 4 x 10^–6 were observed with Cl– ~ 30% and SrF3– > 90% transmission. Current isobar attenuation limits with < 1.75 MV accelerator terminal voltage and ppm impurity levels were calculated to be 36S–/Cl– ~ 4 x 10^–16, 12C3–/Cl– ~ 1.2 x 10^–16, 90YF3–/SrF3– ~ 10^–15 and 90ZrF3–/SrF3– ~ 10^–16. Using 1.75 MV, four 36Cl reference standards in the range 4 x 10^–13 < 36Cl/Cl < 4 x 10^–11 were analyzed with 8 mTorr NO2. The measured 36Cl/Cl ratios plotted very well against the accepted values. A sample impurity content S/Cl < 6 x 10^–5 was measured and a background level of 36S–/Cl < 9 x 10^–15 was determined. Useful currents of a wide variety of anions are produced in AMS sputter sources and molecules can be identified relatively unambiguously by stripping fragments from tandem accelerators. Reactions involving YF3–, ZrF3–, S– and SO– + NO2 in the ISA analyzed by AMS are described, and some interesting reactants are identified.
230

A Radio Frequency Quadrupole Instrument for use with Accelerator Mass Spectrometry: Application to Low Kinetic Energy Reactive Isobar Suppression and Gas–phase Anion Reaction Studies

Eliades, John Alexander 21 August 2012 (has links)
A radio frequency (rf) quadrupole instrument, currently known as an Isobar Separator for Anions (ISA), has been integrated into an Accelerator Mass Spectrometry (AMS) system to facilitate anion–gas reactions before the tandem accelerator. An AMS Cs+ sputter source provided > 15 keV ions that were decelerated in the prototype ISA to < 20 eV for reaction in a single collision cell and re-accelerated for AMS analysis. Reaction based isobar suppression capabilities were assessed for smaller AMS systems and a new technique for gas–phase reaction studies was developed. Isobar suppression of 36S– and 12C3– for 36Cl analysis, and YF3– and ZrF3– for 90Sr analysis were studied in NO2 with deceleration to < 12 eV. Observed attenuation cross sections, σ [x 10^–15 cm^2], were σ(S– + NO2) = 6.6, σ(C3– + NO2) = 4.2, σ(YF3– + NO2) = 7.6, σ(ZrF3– + NO2) = 19. With 8 mTorr NO2, relative attenuations of S–/Cl– ~ 10^–6, C3–/Cl– ~ 10^–7, YF3–/SrF3– ~ 5 x 10^–5 and ZrF3–/SrF3– ~ 4 x 10^–6 were observed with Cl– ~ 30% and SrF3– > 90% transmission. Current isobar attenuation limits with < 1.75 MV accelerator terminal voltage and ppm impurity levels were calculated to be 36S–/Cl– ~ 4 x 10^–16, 12C3–/Cl– ~ 1.2 x 10^–16, 90YF3–/SrF3– ~ 10^–15 and 90ZrF3–/SrF3– ~ 10^–16. Using 1.75 MV, four 36Cl reference standards in the range 4 x 10^–13 < 36Cl/Cl < 4 x 10^–11 were analyzed with 8 mTorr NO2. The measured 36Cl/Cl ratios plotted very well against the accepted values. A sample impurity content S/Cl < 6 x 10^–5 was measured and a background level of 36S–/Cl < 9 x 10^–15 was determined. Useful currents of a wide variety of anions are produced in AMS sputter sources and molecules can be identified relatively unambiguously by stripping fragments from tandem accelerators. Reactions involving YF3–, ZrF3–, S– and SO– + NO2 in the ISA analyzed by AMS are described, and some interesting reactants are identified.

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