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 nearl y 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 an d of impurities, a density of (2.32908350 +/- 0.00000036) g/cm(3) has been assigned to a near-perfect silicon crystal. In comparison to the measurements based on a previous sphere, the relative combined uncerta inty has been reduced from 4.1 x 10(-7) to 1.5 x 10(-7).