20,187 research outputs found
Atomic fountains and optical clocks at SYRTE: status and perspectives
In this article, we report on the work done with the LNE-SYRTE atomic clock
ensemble during the last 10 years. We cover progress made in atomic fountains
and in their application to timekeeping. We also cover the development of
optical lattice clocks based on strontium and on mercury. We report on tests of
fundamental physical laws made with these highly accurate atomic clocks. We
also report on work relevant to a future possible redefinition of the SI
second
A Preposterous Universe
Recent announcements that the Cosmic Microwave Background (CMB) - the faint
remnant of the Big Bang - is polarized have caused a stir among cosmologists.
Such polarization has long been predicted but could not be detected, until the
recent DASI and WMAP detections. The polarization is a signature of the motion
of the primordial seeds that led to the galaxies and large-scale structure of
the Universe observed today. Earlier measurements of tiny temperature
fluctuations in the CMB have been used to infer the sizes of the primordial
seeds. The polarization signature should help to determine the fundamental
parameters of cosmology and to explain how large-scale structure arose
[Highwire abstract].Comment: Perspective: Astronomy. See summary at
http://www.sciencemag.org/cgi/content/summary/299/5611/1333 (need sign up) or
full text (free) at http://www.iafe.uba.ar/relatividad/gangui
Back-action cancellation in interferometers by quantum locking
We show that back-action noise in interferometric measurements such as
gravitational-waves detectors can be completely suppressed by a local control
of mirrors motion. An optomechanical sensor with an optimized measurement
strategy is used to monitor mirror displacements. A feedback loop then
eliminates radiation-pressure effects without adding noise. This very efficient
technique leads to an increased sensitivity for the interferometric
measurement, which becomes only limited by phase noise. Back-action
cancellation is furthermore insensitive to losses in the interferometer.Comment: 4 pages, 3 figures, RevTe
Utilization of nuclear methods for materials analysis and the determination of concentration gradients
The various types of nuclear chemical analysis methods are discussed. The possibilities of analysis through activation and direct observation of nuclear reactions are described. Such methods make it possible to analyze trace elements and impurities with selectivity, accuracy, and a high degree of sensitivity. Such methods are used in measuring major elements present in materials which are available for analysis only in small quantities. These methods are well suited to superficial analyses and to determination of concentration gradients; provided the nature and energy of the incident particles are chosen judiciously. Typical examples of steels, pure iron and refractory metals are illustrated
XANES in (TMTSeF)2Re04: Polarization Dependence of the Se K-Edge
We have measured XANES (X-ray Absorption Near Edge Spectra) near the Re Li (i=l,II,III) edges in (TMTTF)2Re04 and (TMTSeF) 2Re04 and the Se K edge in the latter compound. An important dependence of the XANES at the Se K edge on the polarisation of the incident beam with respect to the crystal axes was observed, whose interpretation can give information on the symmetry of the unoccupied conduction band states. The positions of the Re L edges were compared with those in several inorganic compounds containing Re in various oxidation states, and were found to be, within experimental error, the same as those in KRe04, suggesting the molecular environment in the organic materials affects little the chemical state of the perrhenate anion
Effect of row orientation on soil water content and vine water status on a Cabernet franc vineyard in Madrid, spain
The aim of this study was to evaluate the effects of row orienÂŹtation on vine and soil water status in an irrigated vineyard. The trial was developed during 2006, 2007 and 2008, in the South East region of Madrid (Spain) on 5-year old Cabernet franc grapevines (Vitis vinifera L.) grafted onto 140Ru. Plant spacing was 2.5 m x 1.5 m and vines were trained to a VSP. Four orientations were stuÂŹdied: North-South (N-S), East-West (E-W), Northeast-Southwest (N+45) and North-South +20o (N+20). Irrigation (0.4âąET0) started when shoot growth stopped. Soil water availability was measured using a TDR technique with forty buried probes. Row orientation did not have any effect on water consumption in the vineyard. At maturity, leaf water potential was measured at predawn, early morÂŹning, midday and 14:00 solar time, on both canopy sides - sun and shade â ; the early morning measurement was the one that better differentiated treatments. Leaf water potential was a good indicaÂŹtor of plant water status. Differences between (N-S and E-W) and (N+20 and N+45) treatments were obtained both on sun and shade canopy sides, N+20 and N+45 having lower leaf water potentials then drier leaves. The water stress integral shows that N-S and E-W reach the end of maturation with a greater level of hydration than N+45 and N+20. As a whole, N+45 and N+20 orientations, without affecting too much the soil available water content, induce regularly more water stress to the vine at some periods, probably due to an higher sunlight interception in early morning which makes water limitation for the vine more early and thus more severe during the day
High Spectral Purity, Narrow Linewidth Laser Transmitter for Dial Measurements
A laser transmission with very high degree of spectral purity was described. Amplified Spontaneous Emission (ASE) dependence on the oscillator energy and the detuning away from the gain curve center are demonstrated. The effects of both finite laser linewidth and spectral purity on humidity measurements are experimentally demonstrated
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