Volume determination of two spheres of the new28Si crystal of PTB

In the scope of the redetermination of Avogadro's constant N A , a new isotopically enriched silicon crystal has been produced, from which two spheres were manufactured. After the crystal properties, the lattice parameter and molar mass, as well as the masses of the two spheres have been determined, the volume of the spheres was also measured. For this, the sphere interferometer of PTB was used. The methods of the interferometric measurements have been improved and the major contributions to the uncertainty have been investigated thoroughly. As a result, the total uncertainty could be reduced significantly, yielding a substantial impact on the determination of Avogadro's constant. The mean diameter of each sphere was measured twice with a repeatability of ±2 × 10 −10 , and the relative uncertainty of the 'apparent' volume, which disregards the comparatively small influence of the optical effects of surface layers, was reduced to 7 × 10 −9 . The final results of the volumes and comments on their uncertainties are given.