1,720,959 research outputs found
Les clathrate hydrates deutérés comme nouveau matériau modérateur d'une source de neutrons très froids
No full textVery Cold Neutrons (VCN) cover a wide spectral range within the long-wavelength tail of typical sources for cold neutrons, from energies below 1 meV down to the domain of ultracold neutrons (UCN) at a few hundreds of neV. Dedicated high-intensity VCN sources have the potential to enhance existing neutron scattering techniques, such as small angle neutron scattering (SANS), as well as to improve energy resolution in time-of-flight or neutron spin-echo spectrometers. In particle physics, higher VCN intensities could increase the sensitivity of experiments employing beams of slow neutrons, such as the search for neutron anti-neutron oscillations and in-beam searches for a static neutron electric dipole moment. The object of this thesis is the first-time investigation of the possible employment of deuterated clathrate hydrates as a novel moderator material for VCN.The efficacy of clathrate hydrates in neutron moderation is linked to the localized low-energy, so-called Einstein modes, of entrapped guest molecules. These facilitate down-scattering of neutrons to very low temperatures. Besides nuclear scattering of confined molecules, dioxygen (O2) provides an additional path for neutron slowdown by exploiting the zero-field splitting of the magnetic triplet ground state of molecular oxygen. The absence of a dispersion relation of this excitation constitutes a “cooling cascade” - a process where neutrons are progressively slowed down by increments of the field-splitting energy of 0.4 meV.To quantify the neutronic properties of these compounds, a comprehensive neutron scattering campaign was conducted in the framework of this thesis. This investigation examined the hydrate's structure through neutron diffraction, its dynamics through inelastic neutron scattering, and its total neutron cross section with transmission experiments on fundamental physics beamlines. This effort was complemented by the development of thermal scattering libraries (TSL) for clathrate hydrates based on ab-initio molecular dynamics simulations and density functional theory, as well as a magnetic scattering model for dioxygen. This work was part of the HighNESS project, with the goal of expanding the scientific potential of the European Spallation Source (ESS), through innovative moderator designs.The experimental study presented here focuses on two particular hydrates hosting deuterated tetrahydrofuran (THF-d) and dioxygen (O2). The developed method for producing binary hydrates hosting both THF-d and O2 turned out to yield a high purity of the hydrate structure up to 98.44 ± 1.56%, with dioxygen filling up to 80.1 ± 1.1%. The thermal expansion of these materials was determined from neutron diffraction data and reflects the complex guest-host interactions of these compounds.Experiments on neutron spectrometers measured the dynamic structure factor S(q, ω) over a large phase space in absolute units. The down-scattering of confined dioxygen at 0.4 meV was measured for the first time in a binary clathrate hydrate. Additionally, it was found that the THF-d molecule exhibits excitations at 2.9 and 4.7 meV, which allow bypassing of the “cooling cascade,” leading to faster thermalization of neutrons in a moderator.Transmission experiments to measure the total cross-section were performed at various fundamental physics beamlines. These experiments confirmed the expected Bragg scattering behaviour up to a Bragg-cutoff of about 20 Å. The data also show a significant increase of the cross-section for higher wavelengths. This increase is modelled as a SANS contribution, which could be relevant for future TSL of clathrate hydrates.The established manufacturing technique, experimental data and theoretical models provide a foundation for quantitatively assessing the potential of clathrate hydrates as moderator materials in various moderator reflector configurations at the ESS and other emerging neutron sources worldwide.Les Neutrons Très Froids (VCN) couvrent une gamme spectrale allant des neutrons froids (<1 meV) aux Neutrons Ultra Froids (UCN, quelques centaines de neV). Les sources VCN à haute intensité peuvent améliorer les techniques de diffusion de neutrons, telles que SANS, ainsi que la résolution énergétique des spectromètres à temps de vol ou à spin-echo. En physique des particules, des faisceaux VCN plus intenses pourraient augmenter la sensibilité des recherches sur les oscillations neutron-antineutron et le moment dipolaire électrique statique du neutron. Cette thèse explore l'utilisation des hydrates de clathrate deutérés comme nouveaux modérateurs pour les VCN.L'efficacité des hydrates de clathrate est liée aux modes localisés de basse énergie, dits modes d'Einstein, des molécules invitées confinées, facilitant le ralentissement des neutrons. Le dioxygène (O2) offre une voie supplémentaire pour ralentir les neutrons via le dédoublement en champ nul de l'état fondamental de triplet magnétique de l'oxygène, créant une « cascade de refroidissement » avec des incréments d'énergie de 0,4 meV.Pour quantifier les propriétés neutroniques de ces composés, une campagne de diffusion de neutrons a été menée. Cette étude a examiné la structure de l'hydrate par diffraction des neutrons, sa dynamique par diffusion inélastique des neutrons, et sa section efficace totale des neutrons par des expériences de transmission. Cet effort a été complété par le développement de bibliothèques de diffusion thermique (TSL) pour les hydrates de clathrate basées sur des simulations de dynamique moléculaire ab-initio et la théorie de la fonctionnelle de la densité, ainsi qu'un modèle de diffusion magnétique pour le dioxygène. Ce travail s'inscrit dans le cadre du projet HighNESS, visant à étendre le potentiel scientifique de la Source Européenne de Spallation (ESS) avec des modérateurs innovants.L'étude expérimentale se concentre sur des hydrates contenant le tétrahydrofurane deutéré (THF-d) et le dioxygène (O2). La méthode développée pour produire des hydrates binaires hébergeant le THF-d et l'O2 a permis d'obtenir une structure d'hydrate de haute pureté (98,44 ± 1,56%) avec un remplissage de dioxygène allant jusqu'à 80,1 ± 1,1%. L'expansion thermique de ces matériaux, déterminée à partir des données de diffraction des neutrons, reflète les interactions complexes entre invités et hôtes.Des expériences sur des spectromètres à neutrons ont mesuré le facteur de structure dynamique S(q, ω) sur un large espace de phase. La diffusion vers le bas du dioxygène confiné à 0,4 meV a été mesurée pour la première fois dans un clathrate hydrate binaire. De plus, la molécule de THF-d présente des excitations à 2,9 et 4,7 meV, contournant la « cascade de refroidissement » et conduisant à une thermalisation plus rapide des neutrons.Des expériences de transmission pour mesurer la section efficace totale ont confirmé le comportement attendu pour la diffusion de Bragg jusqu'à une coupure de Bragg d'environ 20 Å. Les données montrent également une augmentation significative de la section efficace pour des longueurs d'onde plus élevées, modélisée comme une contribution SANS dans la section efficace totale, utile pour les futures TSL des hydrates de clathrate.Une base pour une évaluation quantitative du potentiel des hydrates de clathrate en tant que modérateurs a été fournie grâce à la mise au point d'une technique de fabrication, des données expérimentales et des modèles
Les clathrate hydrates deutérés comme nouveau matériau modérateur d'une source de neutrons très froids
No full textVery Cold Neutrons (VCN) cover a wide spectral range within the long-wavelength tail of typical sources for cold neutrons, from energies below 1 meV down to the domain of ultracold neutrons (UCN) at a few hundreds of neV. Dedicated high-intensity VCN sources have the potential to enhance existing neutron scattering techniques, such as small angle neutron scattering (SANS), as well as to improve energy resolution in time-of-flight or neutron spin-echo spectrometers. In particle physics, higher VCN intensities could increase the sensitivity of experiments employing beams of slow neutrons, such as the search for neutron anti-neutron oscillations and in-beam searches for a static neutron electric dipole moment. The object of this thesis is the first-time investigation of the possible employment of deuterated clathrate hydrates as a novel moderator material for VCN.The efficacy of clathrate hydrates in neutron moderation is linked to the localized low-energy, so-called Einstein modes, of entrapped guest molecules. These facilitate down-scattering of neutrons to very low temperatures. Besides nuclear scattering of confined molecules, dioxygen (O2) provides an additional path for neutron slowdown by exploiting the zero-field splitting of the magnetic triplet ground state of molecular oxygen. The absence of a dispersion relation of this excitation constitutes a “cooling cascade” - a process where neutrons are progressively slowed down by increments of the field-splitting energy of 0.4 meV.To quantify the neutronic properties of these compounds, a comprehensive neutron scattering campaign was conducted in the framework of this thesis. This investigation examined the hydrate's structure through neutron diffraction, its dynamics through inelastic neutron scattering, and its total neutron cross section with transmission experiments on fundamental physics beamlines. This effort was complemented by the development of thermal scattering libraries (TSL) for clathrate hydrates based on ab-initio molecular dynamics simulations and density functional theory, as well as a magnetic scattering model for dioxygen. This work was part of the HighNESS project, with the goal of expanding the scientific potential of the European Spallation Source (ESS), through innovative moderator designs.The experimental study presented here focuses on two particular hydrates hosting deuterated tetrahydrofuran (THF-d) and dioxygen (O2). The developed method for producing binary hydrates hosting both THF-d and O2 turned out to yield a high purity of the hydrate structure up to 98.44 ± 1.56%, with dioxygen filling up to 80.1 ± 1.1%. The thermal expansion of these materials was determined from neutron diffraction data and reflects the complex guest-host interactions of these compounds.Experiments on neutron spectrometers measured the dynamic structure factor S(q, ω) over a large phase space in absolute units. The down-scattering of confined dioxygen at 0.4 meV was measured for the first time in a binary clathrate hydrate. Additionally, it was found that the THF-d molecule exhibits excitations at 2.9 and 4.7 meV, which allow bypassing of the “cooling cascade,” leading to faster thermalization of neutrons in a moderator.Transmission experiments to measure the total cross-section were performed at various fundamental physics beamlines. These experiments confirmed the expected Bragg scattering behaviour up to a Bragg-cutoff of about 20 Å. The data also show a significant increase of the cross-section for higher wavelengths. This increase is modelled as a SANS contribution, which could be relevant for future TSL of clathrate hydrates.The established manufacturing technique, experimental data and theoretical models provide a foundation for quantitatively assessing the potential of clathrate hydrates as moderator materials in various moderator reflector configurations at the ESS and other emerging neutron sources worldwide.Les Neutrons Très Froids (VCN) couvrent une gamme spectrale allant des neutrons froids (<1 meV) aux Neutrons Ultra Froids (UCN, quelques centaines de neV). Les sources VCN à haute intensité peuvent améliorer les techniques de diffusion de neutrons, telles que SANS, ainsi que la résolution énergétique des spectromètres à temps de vol ou à spin-echo. En physique des particules, des faisceaux VCN plus intenses pourraient augmenter la sensibilité des recherches sur les oscillations neutron-antineutron et le moment dipolaire électrique statique du neutron. Cette thèse explore l'utilisation des hydrates de clathrate deutérés comme nouveaux modérateurs pour les VCN.L'efficacité des hydrates de clathrate est liée aux modes localisés de basse énergie, dits modes d'Einstein, des molécules invitées confinées, facilitant le ralentissement des neutrons. Le dioxygène (O2) offre une voie supplémentaire pour ralentir les neutrons via le dédoublement en champ nul de l'état fondamental de triplet magnétique de l'oxygène, créant une « cascade de refroidissement » avec des incréments d'énergie de 0,4 meV.Pour quantifier les propriétés neutroniques de ces composés, une campagne de diffusion de neutrons a été menée. Cette étude a examiné la structure de l'hydrate par diffraction des neutrons, sa dynamique par diffusion inélastique des neutrons, et sa section efficace totale des neutrons par des expériences de transmission. Cet effort a été complété par le développement de bibliothèques de diffusion thermique (TSL) pour les hydrates de clathrate basées sur des simulations de dynamique moléculaire ab-initio et la théorie de la fonctionnelle de la densité, ainsi qu'un modèle de diffusion magnétique pour le dioxygène. Ce travail s'inscrit dans le cadre du projet HighNESS, visant à étendre le potentiel scientifique de la Source Européenne de Spallation (ESS) avec des modérateurs innovants.L'étude expérimentale se concentre sur des hydrates contenant le tétrahydrofurane deutéré (THF-d) et le dioxygène (O2). La méthode développée pour produire des hydrates binaires hébergeant le THF-d et l'O2 a permis d'obtenir une structure d'hydrate de haute pureté (98,44 ± 1,56%) avec un remplissage de dioxygène allant jusqu'à 80,1 ± 1,1%. L'expansion thermique de ces matériaux, déterminée à partir des données de diffraction des neutrons, reflète les interactions complexes entre invités et hôtes.Des expériences sur des spectromètres à neutrons ont mesuré le facteur de structure dynamique S(q, ω) sur un large espace de phase. La diffusion vers le bas du dioxygène confiné à 0,4 meV a été mesurée pour la première fois dans un clathrate hydrate binaire. De plus, la molécule de THF-d présente des excitations à 2,9 et 4,7 meV, contournant la « cascade de refroidissement » et conduisant à une thermalisation plus rapide des neutrons.Des expériences de transmission pour mesurer la section efficace totale ont confirmé le comportement attendu pour la diffusion de Bragg jusqu'à une coupure de Bragg d'environ 20 Å. Les données montrent également une augmentation significative de la section efficace pour des longueurs d'onde plus élevées, modélisée comme une contribution SANS dans la section efficace totale, utile pour les futures TSL des hydrates de clathrate.Une base pour une évaluation quantitative du potentiel des hydrates de clathrate en tant que modérateurs a été fournie grâce à la mise au point d'une technique de fabrication, des données expérimentales et des modèles
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dunkle Materie von der Elektron- bis zur Kernstreuung
Full text linkThe recent development of direct detection experiments can be divided into two main approaches: On the one hand, to give weakly interacting massive particles (WIMPs) as Elizabeth Gibney puts it "a final chance to reveal itself", on the other hand, to reach out for candidates beyond the WIMP-paradigm. In this regard, new possibilities in the search for dark matter arise by simultaneously lowering detection thresholds successively and exploiting new kinds of atomic or nuclear phenomena besides the well known dark matter nucleus scattering, namely dark matter electron scattering, the Migdal effect or Bremsstrahlung. While the interaction of dark matter particles and target materials differs for each effect, the observable quantity for each of these phenomena ultimately is the energy deposited in the detector. For dark matter nucleus scattering, some of the kinetic energy of a dark matter particle is transferred to the nucleus of a Standard Model particle. This energy deposition is then converted to measurable signals i.e. ionization, scintillation light or heat. The Migdal effect on the other hand induces ionization signals in addition to the nuclear recoil (3-body process), when a sub-GeV dark matter particle scatters off the nucleus. Finally, when dark matter couples directly to electrons (i. e. electron scattering), ionization and excitation in the electron system of the target atoms can be detected. This means that, depending on the dark matter mass and its coupling to the Standard Model particles, a variety of detection avenues exist at direct detection experiments. The deposited energy of dark matter particles within the detector can mainly (with the exeption of bubble chambers) be measured via scintillation, ionization or heat. Understanding the limits of each effect therefore becomes a key aspect of exploiting the complete potential of current and future direct detection experiments. This is especially the case for dark matter scenarios that go beyond the WIMP paradigm by involving e.g. multiple components. This thesis addresses a comprehensive analysis of the above described dominating theoretical effects induced by dark matter interaction with the Standard Model constituents, primarily taking liquid Xenon as an example. In order to achieve this, the astrophysical parameters, the possible ways of interaction and the detector response for each interaction need to be computed. This results in anticipated interaction rates for each effect and detector. Comparing these anticipated detection rates allows us to draw conclusions for possible blind spots and challenges in setting direct detection limits for particular dark matter scenarios
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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