1,721,032 research outputs found
Effects of galactic cosmic rays on the surface of icy bodies from the outer solar system : clues for the formation of organic matter found in ultracarbonaceous antarctica micrometeorites
No full textLes météorites et particules de poussière interplanétaire apportent des contraintes sur la formation et l’évolution de la matière dans le système solaire. Les micrométéorites, dont certaines proviennent des régions externes du système solaire, représentent la source dominante de matière extraterrestre arrivant sur Terre. Les micrométéorites collectées dans les neiges antarctiques sont dans un excellent état de conservation du fait de conditions géographiques et météorologiques favorables à leur préservation. La collection CONCORDIA/CSNSM de micrométéorites contient en particulier des micrométéorites peu altérées thermiquement lors de leur entrée atmosphérique. Certaines sont caractérisées par une très haute teneur en matière organique, dépassant 50% en volume, très largement au dessus des valeurs habituelles trouvées dans les météorites. Cette matière organique présente de plus la spécificité d’être fortement enrichie en deutérium et contient jusqu’à cinq fois plus d’azote celle extraite des météorites.Les différents scénarios proposés pour expliquer la formation de cette matière et satisfaisant à l’ensemble des caractéristiques de ces micrométéorites impliquent des corps parents orbitant au-delà de Neptune, dans la ceinture de Kuiper ou dans le nuage de Oort. La température y est suffisamment basse pour condenser à leur surface les molécules volatiles comme l’azote et le méthane tandis qu’ils sont exposés à l’action radiochimique du rayonnement cosmique galactique. Afin de contraindre ces scénarios, des expériences ont été conduites en exposant différentes glaces N2-CH4 aux faisceaux d’ions du GANIL simulant ce rayonnement. L’évolution chimique des glaces au cours de l’irradiation et pendant le recuit des échantillons a été suivie par spectroscopie infrarouge au moyen de deux dispositifs disponibles au CIMAP : la chambre d’analyse CASIMIR et le nouvel appareil IGLIAS. Des analyses complémentaires ex situ ont été menées par spectrométrie de masse. Les résultats apportant des éléments de réponse à l’origine de la matière organique des micrométéorites ultracarbonées ainsi que sur l’origine de leur enrichissement isotopique seront présentés et discutés.Extraterrestrial materials, such as meteorites and interplanetary dust particles, provide constraints on the formation and evolution of organic matter in the young solar system. Micrometeorites represent the dominant source of extraterrestrial matter at the Earth’s surface, some of them originating from large heliocentric distances.Micrometeorites recovered from Antarctica snows provide a unique source of pristine interplanetary dust particles, which underwent a minimal weathering at atmospheric entry. A few percent are characterized by very large carbon content with at least 50% in volume, much higher than the value found in meteorites. This organic matter exhibits extreme deuterium excesses and is unusually nitrogen-rich.Several formation scenarios have been proposed for the formation of the N-rich organic matter observed in UCAMMs, suggesting that these particles come from a parent body orbiting beyond the nitrogen snow line, in the outer Solar System where they are exposed to ions from the galactic cosmic rays. We experimentally evaluate the scenario involving high energy irradiation of icy bodies subsurface orbiting at large heliocentric distances by irradiating N2-CH4 ices with swift heavy ions provided by the GANIL facility. Chemical evolution was monitored by Fourier transform infrared spectroscopy with two experimental set-up : CASIMIR and IGLIAS. Ex situ mass spectroscopy measurement where also conducted. Results concerning the origin of the organic matter found in ultracarbonaceous micrometeorites and the origin of its deuterium enrichment will be presented and discussed
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
Complex organic molecules under irradiation : matrix and temperature effects
No full textLa détection de nombreuses molécules organiques complexes (MOCs) dans l’espace notamment dans les comètes mais aussi sur terre à l’intérieur des micrométéorites a permis de stimuler et d’élargir considérablement le champ de recherche sur ces molécules. Les travaux effectués au cours de cette thèse ont porté sur l’irradiation des MOCs en phase solide. Différentes installations (GANIL, GSI et ATOMKI) ont été utilisées pour simuler les effets des rayonnements cosmiques sur la pyridine et deux nucléobases: adénine et cytosine. La radiorésistance des glaces de pyridine pure et des mélanges de pyridine et d’eau à différentes concentrations à 12 K a été étudiée. Le dispositif CASIMIR a été utilisé pour préparer les glaces et suivre l’évolution de celles-ci sous irradiation par spectroscopie d’absorption infrarouge. Cela permet de quantifier la destruction de la pyridine, de déterminer les sections efficaces de destruction associées et de détecter les molécules produites pendant l’irradiation. Les sections efficaces de destruction de la pyridine diminuent avec l’augmentation de la concentration de la pyridine dans la glace d’eau. L’effet est plus important dans le cas des ions de hautes énergies. La radiorésistance de deux autres COMs aromatiques, les nucléobases cytosine (un cycle) et l’adénine (deux cycles) a été étudiée en phase solide en fonction de la température (17 K à 335 K). Les films de cytosine ont été irradiés avec les ions Calcium (230 MeV) au GSI et les films d’adénine ont été irradiés à ATOMKI avec les protons (0.8 MeV) et les ions Helium (3.2 MeV) et au GSI avec les ions Calcium (230 MeV). À l’aide de la spectroscopie infrarouge, les sections efficaces de destruction de la cytosine et de l’adénine ont été déterminées. L’inventaire des nouvelles molécules produites a été effectué. Les sections efficaces de destruction de la cytosine et de l’adénine sont plus importantes à basse température qu’à température ambiante. L’effet de l’augmentation de la destruction de l’adénine est plus important pour les ions légers. La dépendance entre la section efficace de destruction de l’adénine et le pouvoir d’arrêt électronique suit une loi de puissance. Dans les deux cas (basse température 12-20 K et à température ambiante 300K), n est de l’ordre de 1.5, il n’y a pas un fort effet de la températureThe detection of many complex organic molecules (COMs) in space, particularly in comets but also on earth inside micrometeorites, has made it possible to stimulate and considerably broaden the field of research on these molecules. The work carried out during this thesis focused on the irradiation of COMs in solid phase. Different irradiation facilities (GANIL, GSI and ATOMKI) were used to simulate the effects of cosmic radiation on pyridine and two nucleobases: adenine and cytosine. The radioresistance of pure pyridine ice and mixtures of pyridine and water at different concentrations at 12 K has been studied. The CASIMIR set-up was used to prepare the ices. The processing of the ices under irradiation was followed by infrared absorption spectroscopy. This allowed to quantify the pyridine destruction, to determine the associated destruction cross sections and to detect the molecules produced during irradiation. Pyridine destruction cross sections decrease with increasing concentration of pyridine in water ice. The effect is greater in the case of high energy ions. The radioresistance of two other aromatic COMs, the nucleobases cytosine (one cycle) and adenine (two cycles) was studied in solid phase as a function of temperature (17 K to 335 K). Cytosine films were irradiated with Calcium ions (230 MeV) at GSI and adenine films were irradiated at ATOMKI with proton (0.8 MeV) and Helium (3.2 MeV) ions and at GSI with Calcium ions (230 MeV). Using infrared spectroscopy, cytosine and adenine destruction cross sections were determined. The inventory of produced new molecules has been carried out. The destruction cross sections of cytosine and adenine are greater at low temperature than at room temperature. The effect of adenine destruction is greater for lighter ions. The dependence between the adenine destruction cross section and the electronic stopping power follows a power law. In both cases (low temperature 12-20 K and room temperature 300K), n is of the order of 1.5, there is no strong effect of temperature
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
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
Swift Heavy Ion Induced Electron Emission from Solids
Full text linkQuite a number of experiments have been performed on electron emission from solids induced by slow (projectile velocity Vp \u3c 1 atomic unit) or medium velocity heavy ions (projectile energy Ep \u3c 1 MeV/u). Only a few experiments have been made with fast heavy ions (projectile atomic number Zp \u3e 8, Ep \u3e 2 MeV/u) concerning either electron emission yields -y, or double differential electron energy spectra d2n(0)/dEd0 as a function of the observation angle 0. We present the results obtained so far on electron emission induced by fast (Ep \u3e 2 MeV/u) heavy ions (Zp ~ 6). Topics discussed include experimental results for electron yields, -energy and -angular distributions and channelling phenomena as well as the theoretical approaches. We also present new results from recent studies on the evolution of electron yields and doubly differential electron spectra with target thickness for Ar (13.6 MeV/u) obtained at GANIL (the french heavy ion accelerator Grand Accelerateur National d\u27Ions Lourds )
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