3,328 research outputs found
A study of Kapton degradation under simulated shuttle environment
A system was developed which employs a source of low energy oxygen ion to simulate the shuttle low Earth orbit environment. This source, together with diagnostic tools including surface analysis ans mass spectroscopic capability, was used to measure the dependence of ion energy of the oxygen induced CO signals from pyrolytic graphite and Kapton. For graphite the CO signal was examined at energies ranging form 4.5 to 465 eV and for Kapton from 4.5 to 188 eV. While the relative quantum yields inferred from the data are reasonably precise, there are large uncertainties in the absolute yields because of the assumptions necessary to covert the measured signal strengths to quantum yields. These assumptions are discussed in detail
Oxygen interaction with space-power materials
Data from the space shuttle flights have established that many materials experience relatively rapid degradation when exposed to the low Earth orbit ambient atmosphere, which is predominately atomic oxygen. While much was learned from samples flown on the shuttle, laboratory simulations of the shuttle environment are necessary for a detailed understanding of the various interactions which contribute to the observed degradations. These laboratory experiments are particularly important for predicting the deterioration to be expected for materials aboard orbiting power systems, which will be exposed for long periods of time and could have components operating at very high temperatures. By using a mass spectrometer to synchronously detect molecules emitted from the surface as a result of amplitude modulated oxygen ion bombardment, quantum yields were obtained as a function of ion energy. A technique was developed to obtain preliminary yield data by slowly scanning the mass setting of the mass spectrometer; measurements were extended down to zero modulation frequency; yield data was obtained for the insulating materials (Nomex, Kevlar, and Teflon) used in the construction of electrodynamic tethers; a heated sample holder was constructed to investigate the effect of sample temperature on quantum yields; and the instrumentation was developed to observe the mass spectrometer signal as a function of time during and following bombardment of the sample by a brief (approximately 1 millisecond) pulse of ions
Chemische maatlatten zoute watersystemen: risicobeoordeling Nederlandse zoute watersystemen voor de watersysteemverkenning
Electroweak and QCD corrections to Higgs production via vector-boson fusion at the LHC
The radiative corrections of the strong and electroweak interactions are
calculated at next-to-leading order for Higgs-boson production in the
weak-boson-fusion channel at hadron colliders. Specifically, the calculation
includes all weak-boson fusion and quark--antiquark annihilation diagrams to
Higgs-boson production in association with two hard jets, including all
corresponding interferences. The results on the QCD corrections confirm that
previously made approximations of neglecting s-channel diagrams and
interferences are well suited for predictions of Higgs production with
dedicated vector-boson fusion cuts at the LHC. The electroweak corrections,
which also include real corrections from incoming photons and leading
heavy-Higgs-boson effects at two-loop order, are of the same size as the QCD
corrections, viz. typically at the level of 5-10% for a Higgs-boson mass up to
\sim 700 GeV. In general, both types of corrections do not simply rescale
differential distributions, but induce distortions at the level of 10%. The
discussed corrections have been implemented in a flexible Monte Carlo event
generator.Comment: 33 pages, LaTeX, 24 postscript figure
Classification of aerosol properties derived from AERONET direct sun data
International audienceAerosol spectral measurements by sunphotometers can be characterized by three independent pieces of information: 1) the optical thickness (AOT), a measure of the column aerosol concentration, 2) the optical thickness average spectral dependence, given by the Angstrom exponent (?), and 3) the spectral curvature of ? (??). We propose a simple graphical method to visually convert (?, ??) to the contribution of fine aerosol to the AOT and the size of the fine aerosols. This information can be used to track mixtures of pollution aerosol with dust, to distinguish aerosol growth from cloud contamination and to observe aerosol humidification. The graphical method is applied to the analysis of yearly records at 8 sites in 3 continents, characterized by different levels of pollution, biomass burning and mineral dust concentrations. Results depict the dominance of fine mode aerosols in driving the AOT at polluted sites. In stable meteorological conditions, we see an increase in the size of the fine aerosol as the pollution stagnates and increases in optical thickness. Coexistence of coarse and fine particles is evidenced at the polluted sites downwind of arid regions
The temperature and chronology of heavy-element synthesis in low-mass stars
Roughly half of the heavy elements (atomic mass greater than that of iron)
are believed to be synthesized in the late evolutionary stages of stars with
masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly
iron) capture neutrons and progressively build up (through the
slow-neutron-capture process, or s-process) heavier elements that are
subsequently brought to the stellar surface by convection. Two neutron sources,
activated at distinct temperatures, have been proposed: 13C and 22Ne, each
releasing one neutron per alpha-particle (4He) captured. To explain the
measured stellar abundances, stellar evolution models invoking the 13C neutron
source (which operates at temperatures of about one hundred million kelvin) are
favoured. Isotopic ratios in primitive meteorites, however, reflecting
nucleosynthesis in the previous generations of stars that contributed material
to the Solar System, point to higher temperatures (more than three hundred
million kelvin), requiring at least a late activation of 22Ne. Here we report a
determination of the s-process temperature directly in evolved low-mass giant
stars, using zirconium and niobium abundances, independently of stellar
evolution models. The derived temperature supports 13C as the s-process neutron
source. The radioactive pair 93Zr-93Nb used to estimate the s-process
temperature also provides, together with the pair 99Tc-99Ru, chronometric
information on the time elapsed since the start of the s-process, which we
determine to be one million to three million years.Comment: 30 pages, 10 figure
Classical field theory. Advanced mathematical formulation
In contrast with QFT, classical field theory can be formulated in strict
mathematical terms of fibre bundles, graded manifolds and jet manifolds. Second
Noether theorems provide BRST extension of this classical field theory by means
of ghosts and antifields for the purpose of its quantization.Comment: 30 p
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