Colloquium: Comparison of Astrophysical and Terrestrial Frequency Standards

We have re-analyzed the stability of pulse arrival times from pulsars and white dwarfs using several analysis tools for measuring the noise characteristics of sampled time and frequency data. We show that the best terrestrial artificial clocks substantially exceed the performance of astronomical sources as time-keepers in terms of accuracy (as defined by cesium primary frequency standards) and stability. This superiority in stability can be directly demonstrated over time periods up to two years, where there is high quality data for both. Beyond 2 years there is a deficiency of data for clock/clock comparisons and both terrestrial and astronomical clocks show equal performance being equally limited by the quality of the reference timescales used to make the comparisons. Nonetheless, we show that detailed accuracy evaluations of modern terrestrial clocks imply that these new clocks are likely to have a stability better than any astronomical source up to comparison times of at least hundreds of years. This article is intended to provide a correct appreciation of the relative merits of natural and artificial clocks. The use of natural clocks as tests of physics under the most extreme conditions is entirely appropriate; however, the contention that these natural clocks, particularly white dwarfs, can compete as timekeepers against devices constructed by mankind is shown to be doubtful.Comment: 9 pages, 2 figures; presented at the International Frequency Control Symposium, Newport Beach, Calif., June, 2010; presented at Pulsar Conference 2010, October 12th, Sardinia; accepted 13th September 2010 for publication in Reviews of Modern Physic

An Optical Lattice Clock with Spin-polarized 87Sr Atoms

We present a new evaluation of an 87Sr optical lattice clock using spin polarized atoms. The frequency of the 1S0-3P0 clock transition is found to be 429 228 004 229 873.6 Hz with a fractional accuracy of 2.6 10^{-15}, a value that is comparable to the frequency difference between the various primary standards throughout the world. This measurement is in excellent agreement with a previous one of similar accuracy

Evaluating and Minimizing Distributed Cavity Phase Errors in Atomic Clocks

We perform 3D finite element calculations of the fields in microwave cavities and analyze the distributed cavity phase errors of atomic clocks that they produce. The fields of cylindrical cavities are treated as an azimuthal Fourier series. Each of the lowest components produces clock errors with unique characteristics that must be assessed to establish a clock's accuracy. We describe the errors and how to evaluate them. We prove that sharp structures in the cavity do not produce large frequency errors, even at moderately high powers, provided the atomic density varies slowly. We model the amplitude and phase imbalances of the feeds. For larger couplings, these can lead to increased phase errors. We show that phase imbalances produce a novel distributed cavity phase error that depends on the cavity detuning. We also design improved cavities by optimizing the geometry and tuning the mode spectrum so that there are negligible phase variations, allowing this source of systematic error to be dramatically reduced.Comment: To appear in Metrologi

Influence de paramètres physico-chimiques sur la cristallisation d'oxalates de lanthanides et d'actinides, précurseurs d'oxydes : orientation des microstructures

International audienceLa précipitation d'actinides en solution au sein de précurseurs a base de ligands oxalate, dont le traitement en température permet l'obtention d'oxydes, est l'une des voies envisagées pour le recyclage des actinides dans le cadre des systèmes nucléaires du futur. Une compréhension approfondie de l'influence des paramètres physico-chimiques de la précipitation, tels que la température ou la présence d'additifs dans le milieu, est essentielle. Elle permet en effet de contrôler puis d'orienter les caractéristiques morphologiques et structurales du précurseur oxalate et ainsi de moduler les caractéristiques de l'oxyde final produit après traitement thermique. L’étude présentée se focalise uniquement sur l'influence d'un polyol, le glycérol (1,2,3-propanetriol), et d'un phosphate, le NTMP (acide nitrilotrimethylphosphonate), sur la précipitation de l'oxalate de plutonium (III). Le système est simule, dans un premier temps, par l'oxalate de néodyme, ce lanthanide présentant des propriétés chimiques voisines de celles du plutonium (III)

Density of plantation, soil humidity and soil depth influence catabolic structure of microbial communities in maize crop system.

International audienc

Oxydes d'actinides à façon pour la fabrication de combustibles nucléaires de Génération IV

International audienceCette étude s’inscrit dans le cadre des recherches menées sur le traitement-recyclage descombustibles nucléaires du futur et, plus précisément, sur la fabrication de nouveaux combustiblessous forme de pastilles à partir de poudres d’oxydes à base d’actinides de différentesmorphologies. Différents procédés de conversion sont envisagés pour la synthèse d’oxydesd’actinides à partir de l’uranium et du plutonium issus du traitement des combustibles usés.Plusieurs études menées au CEA sur le procédé de conversion par voie oxalique ont permisd’aboutir à des oxalates et à des oxydes d’actinides de morphologies très variées en fonction desparamètres de précipitation

Caractérisation de la recharge de la nappe a l'echelle parcellaire par l'irrigation par submersion : une approche couplée Hydrologie-géophysique, Milieux poreux et transferts hydriques.

National audienc

STUDIES OF MULTIOBJECTIVE OPTIMIZATION OF BATCH FREE-RADICAL POLYMERIZATION PROCESS

multiobjective optimization, batch free-radical polymerization, nonlinear programming The precise control of the polymer properties during its manufacture process is one of the most important issues for the development of chemical process control techniques. The free-radical polymerization process is a typical example of a multivariable open-loop control design problem. In this work, the batch free-radical polymerization of styrene process is analyzed by the multiobjective optimization technique, which shows in a more realistic way the multivariable aspects of the polymerization manufacture. The process objectives include monomer conversion, polymer molecular weight, polydispersity index, and initiator residue. The simulated results have confirmed that multiobjective optimization is a powerful and helpful technique, offering a consistent strategy for the solution of polymerization reactor optimization problems