The IMGC volume-density standards for the Avogadro constant

Volume-density standards in the form of single-crystal silicon spheres have been introduced and developed at the IMGC. A new sphere of nearly perfect form has been characterized on the basis of a comprehensive set of interferometric, roundness, mass and hydrostatic measurements. From its mass and volume and from the evaluation of the oxide layer and of impurities, a density of (2.32908350/spl plusmn/0.00000036) g/cm/sup 3/ has been assigned to a near-perfect silicon crystal. In comparison to the measurements based on a previous sphere, the relative combined uncertainty has been reduced from 4.1/spl times/10/sup -7/ to 1.5/spl times/10/sup -7/.<
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