345 research outputs found
Multipole (E1, M1, E2, M2) transition wavelengths and rates between states with n<= 6 in heliumlike carbon, nitrogen, oxygen, neon, silicon, and argon
Transition wavelengths and rates are given for E1, E2, M1, and M2 transitions
between singlet and triplet S, P, D, and F states in heliumlike ions of
astrophysical interest: carbon, nitrogen, oxygen, neon, silicon, and argon. All
possible transitions between states with n <= 6 are considered. Wave functions
and energies are calculated using the relativistic configuration-interaction
(CI) method including both Coulomb and Breit interactions. For transitions to
the ground state, the present theoretical wavelengths agree to five digits with
precise measurements.Comment: 8 pages of text 97 pages of tables submitted to Atomic & Data Nuclear
Datable
Les cherts Archéens de la ceinture de roches vertes de Barberton (3.5-3.2Ga), Afrique du Sud. Processus de formation et utilisation comme proxys paleo-environnementaux
Les cherts archéens permettent de contraindre les environnements primitifs qui ont vu l apparition de la vie sur Terre. Ces roches siliceuses se forment selon trois processus : les C-cherts (cherts primaires) se forment par précipitation chimique de silice océanique sur le plancher, sous la forme d une boue siliceuse ou en tant que ciment dans les sédiments de surface; les F-cherts (cherts de fracture) précipitent dans les fractures de la crôute depuis les fluides circulant; les S-cherts (cherts secondaires) sont issus de la silicification de roches préexistantes lors de la percolation de fluides enrichis en silice. Ces processus sont largement acceptés mais des questions majeures subsistent : comment reconnaître ces différents types de chert ? Quelle est l origine de la silice et sous quelle forme a-t-elle précipité ? Quel signal chimique est porté par les cherts et comment s en servir pour les reconstructions paléo-environnementales ? Ces questions sont abordées à travers trois sites de la ceinture de roches vertes de Barberton, en Afrique du Sud. L approche adoptée combine l analyse des structures sédimentaires et de déformation, de la pétrologie et de la composition chimique et isotopique de ces unités. Dans ces sites, la formation des cherts est étroitement liée à l environnement de mise en place. La sédimentation clastique (turbidites) est à l origine des C-cherts de Komati River, déposés sous la forme d une boue siliceuse par adsorption de silice sur les particules argileuses en suspension. En absence de contribution continentale, les alternances de cherts noirs et blancs de Buck Reef sont interprétées comme issues de variations climatiques à l échelle saisonnières (chert noir), voire glaciaires/inter-glaciaires (chert blanc). Les cherts de fracture de Barite Valley sont liés à la précipitation de silice depuis une suspension colloïdale thixotrope remontant à travers la croûte. La composition chimique des cherts est contrôlée par leur environnement de mise en place, et représente un mélange entre une phase siliceuse et une phase contaminante, indépendamment des processus qui ont précipité la silice. Les cherts de Komati River et de Barite Valley sont enrichis en Al, K, Ti, HFSE et en REE, ce qui est attribué à la contamination de la matrice siliceuse par la présence de phyllosilicate. Une telle contribution clastique peut expliquer les larges gammes de 30Si dans les cherts de Komati River (-0.69 à +3.89 ), et la majorité des valeurs positives est probablement liée à la contribution de l eau de mer. Dans les dykes de Barite Valley, les 30Si très négatifs (-4.5 à +0.22 ) sont cohérents avec l origine hydrothermale basse température des fluides initiaux. A Buck Reef, l absence de contribution continentale s exprime dans les cherts blancs par une minéralogie exclusivement microquartzitique et par des concentrations extrêmement faibles en éléments traces (i.e. HFSE et REE<1ppm). 2% de carbonates et 3-4% de matériel continental (e.g. argiles) suffisent à masquer le signal siliceux dans ces cherts purs. Nous ne pouvons conclure sur la présence d un signal océanique dans ces cherts par manque de fiabilité des proxys océaniques modernes (appauvrissement en LREE, enrichissement en La et Y). Reconnus à la fois dans des quartz océaniques, hydrothermaux, magmatiques et pegmatitiques, ils ne permettent pas d identifier un signal d eau de mer dans les cherts archéens. Les 18O de ces cherts indiquent la présence de circulations fluides secondaires à moins de 100C, et leurs 30Si négatifs ou positifs (-2.23 et +1.13 en moyenne) montrent la contribution de fluides différents au moment de leur formation. Le couplage des observations pétrologiques et de terrain est la seule approche fiable pour reconnaître le mode de mise en place des cherts. Leur composition chimique dépend plus des conditions environnementales que des caractéristiques du fluide initial.Archean cherts potentially constrain the primitive environment in which life emerged and evolved. These siliceous rocks formed by three processes : C-cherts (primary cherts) formed by the chemical precipitation of oceanic silica, either as a siliceous ooze (or silica gel) on the seabed, or as cement within still soft sediments at the surface ; F-cherts (fracturefilling cherts) precipitated from circulating fluids in concordant or crosscutting veins in the shallow crust ; S-cherts (secondary cherts) are the result of the metasomatism (silicification) of preexisting rocks during the percolation of silica-rich fluids. These processes are generally accepted but major questions remain unsolved : how to recognize various chert types ? Where does the silica come from and how did it precipitate ? What chemical signal is hosted in cherts and how can it be used for paleo environmental reconstructions ? These questions are addressed here using three sites in the Barberton Greenstone Belt, South Africa, which contain a variety of cherts deposited in very different environments. The approach combines field description of sedimentary and deformation structures, the characterization of various chert petrologies, and the study of their chemical and isotopic composition. In these three sites, chert formation strongly depends on the environmental setting. Clastic sedimentation is directly linked to C-chert formation at Komati River, where the silica was deposited as a viscous, siliceous ooze by sorption process onto suspended clay particles. A continental contribution is absent at Buck Reef, and the black and white banded cherts (C-cherts) are interpreted to have formed by chemical precipitation of oceanic silica during seasonal (black chert) and maybe glacial/inter-glacial (white chert) climatic variations. The fracture-filling cherts from Barite Valley precipitated from a thixotropic colloidal suspension that migrated upward through the crust. The chemical compositions of cherts from these three sites are essentially controlled by the environment of deposition, and represent mixtures of a siliceous and contaminant phases, independent from the silica precipitation mode. Komati River C-cherts and Barite Valley F-cherts are both enriched in Al, K, Ti, HFSE and REE, which represents the contamination by phyllosilicates of the microquartzitic fabrics. Such a clastic contribution may account for the wide range of 30Si in Komati River cherts (-0.69 to +3.89 ) although the majority of positive values is attributed to seawater involvement. In the dykes, 30Si is strongly negative (-4.5 to +0.22 ) and is consistent with the low-temperature hydrothermal nature of these fluids. At Buck Reef, the lack of continental contribution is expressed in the white cherts, by a mineralogy exclusively composed of microquartz, and by extremely low trace element contents, i.e. HFSE and REE below 1ppm. We calculate that 2% of carbonates and 3-4% of clastic particles (i.e. clay, feldspar) would be enough to mask the silica composition in these high purity cherts. A marine signature was not recognized in their geochemistry because of the unreliability of commonly used modern proxys (i.e. LREE depletion, La and Y enrichment). These features were identified in oceanic, hydrothermal, magmatic and pegmatitic quartz and thus do not reliably identify an oceanic signal in Archean cherts. Because the 18O values in these white cherts indicates secondary fluid circulations at <100C, their negative or positive 30Si values (-2.23 and +1.13 in average) most probably represent different fluid contributions at the time they formed. The combination of field and petrological observations appears to be the most reliable approach to classify cherts and to deduce their origin, and we show here that their chemical composition depends more on the environmental conditions than on the primary fluid characteristics.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF
Semi-quantitative analysis of bulk chondritic material using X-Ray fluorescence spectroscopy
73rd Annual Meeting of the Meteoritical-Society, New York, NY, July 26-30, 2010International audienceSynchrotron radiation X-ray fluorescence (SR-XRF) is a method of choice to analyze fragile, unique meteoritic samples, requiring no sample preparation. It is a nondestructive, multielemental, quantitative method, easily coupled to diffraction and speciation for a detailed sample characterization. The composition of samples thicker than a few microns is however difficult to obtain due to the high attenuation of the characteristic X-rays resulting in non-detection of low-Z elements (Z≤ 14)
Holographic Methods as Local Probes of the Atomic Order in Solids
In the last fifteen years several techniques based on the holographic
principle have been developed for the study of the 3D local order in solids.
These methods use various particles: electrons, hard x-ray photons, gamma
photons, or neutrons to image the atoms. Although the practical realisation of
the various imaging experiments is very different, there is a common thread;
the use of inside reference points for holographic imaging. In this paper we
outline the basics of atomic resolution holography using inside reference
points, especially concentrating to the hard x-ray case. Further, we outline
the experimental requirements and what has been practically realized in the
last decade. At last we give examples of applications and future perspectives.Comment: 14 pages, 6 figure
Связь физико-химических превращений в минеральной части Назаровского угля с процессами образования связанных отложений
Predictive Modeling in a VoIP System, Journal of Telecommunications and Information Technology, 2013, nr 4
An important problem one needs to deal with in a Voice over IP system is server overload. One way for preventing such problems is to rely on prediction techniques for the incoming traffic, namely as to proactively scale the available resources. Anticipating the computational load induced on processors by incoming requests can be used to optimize load distribution and resource allocation. In this study, the authors look at how the user profiles, peak hours or call patterns are shaped for a real system and, in a second step, at constructing a model that is capable of predicting trends
QED theory of transition probabilities and line profiles in highly-charged ions
A rigorous QED theory of the spectral line profiles is applied to transition
probabilities in few-electron highly charged ions. Interelectron interaction
corrections are included as well as radiative corrections. Parity nonconserving
(PNC) amplitudes with effective weak interactions between the electrons and
nucleus are also considered. QED and interelectron interaction corrections to
the PNC amplitudes are derived
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