INTERNATIONAL INTERCOMPARISON OF SILICON DENSITY STANDARDS

In order to obtain a more accurate value for the Avogadro constant Na the reliability of density values obtained by different measuring tech niques for silicon single crystals was checked by density determinatio ns with four silicon spheres at PTB, IMGC and NRLM. The diameters of t he spheres (two of PTB and two of NRLM) ranged from 90 mm to 94 mm. Al l the mass measurements (at PTB, IMGC, and NRLM) were carried out in a ir, At IMGC and NRLM the volume was determined by interferometric meas urements, whereas at PTB the volume was obtained from hydrostatic weig hings using two Zerodur volume standards, In addition? at PTB the dens ities of the four spheres were cross-checked by flotation measurements , The maximum relative density difference was found to be 1x10(-6), wh ereas the typical values were 2x10(-7).