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  • 1
    Publication Date: 2012-12-12
    Description: Spacecraft-to-spacecraft tracking observations from the Gravity Recovery and Interior Laboratory (GRAIL) have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Zuber, Maria T -- Smith, David E -- Watkins, Michael M -- Asmar, Sami W -- Konopliv, Alexander S -- Lemoine, Frank G -- Melosh, H Jay -- Neumann, Gregory A -- Phillips, Roger J -- Solomon, Sean C -- Wieczorek, Mark A -- Williams, James G -- Goossens, Sander J -- Kruizinga, Gerhard -- Mazarico, Erwan -- Park, Ryan S -- Yuan, Dah-Ning -- New York, N.Y. -- Science. 2013 Feb 8;339(6120):668-71. doi: 10.1126/science.1231507. Epub 2012 Dec 5.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA. zuber@mit.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23223395" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A Si(220) double-crystal monochromator for the energy range 10–30 keV is presented. It will be used for EXAFS as well as powder diffraction measurements. To determine the requirements for this monochromator we looked, apart from mean considerations, at the requirements dictated by EXAFS in transmission mode. For good data analyses the proper shape, amplitude, and location at the energy axis of each wiggle is required. Moreover it is essential to separate the wiggles from background and noise. For the latter a high flux through the sample is desirable, which can be achieved by horizontal focusing of the beam. For that we have chosen to bend the second crystal sagitally. The sagittal bending radius is adjustable between 50 and 0.8 m, because for different energies different sagittal radii are necessary to focus the beam on the sample. The mean meridional radius of the second crystal is fixed at 130 m, which is an optimization for 20 keV. The meridional radius of the first crystal can be tuned between 100 and 500 m. When this radius is set to 130 m the energy resolution is calculated to be 6, 3, and 35 eV for 10, 20, and 30 keV (for perfectly bent crystals). By changing the meridional radius of the first crystal, future users of this monochromator can make the trade off between resolution and intensity. Movement of the monochromator exit beam, during a scan, will occur due to the monochromator geometry, but is reduced as much as possible by using an asymmetrically cut second crystal, with an asymmetry angle of 2.5°. The average exit beam movement of the monochromator for a 1-keV scan is 20 μm. For 40% of the energy range (10–30 keV) the exit beam position remains within 10 μm. For the second crystal no translation stage is used. When using a translation stage it is difficult to keep the change in flux within acceptable limits, because an alignment error of 0.01 arcsec at 30 keV will result in a change in intensity of 1%.
    Type of Medium: Electronic Resource
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