Volume Measurement of a 28Si-Enriched Sphere to Determine the Avogadro Constant at NMIJ

To determine the Avogadro constant by the International Avogadro Coordination using the x-ray crystal density method, the volume of a28Si-enriched sphere, named Si28kgOla, was measured at NMIJ. The volume was determined from the diameter measurement in 2175 directions by an optical interferometer. Taking into account the results of the surface characterization of the sphere by x-ray photoelectron spectroscopy and spectroscopic ellipsometry, the volume of the Si core excluding the surface layers was determined with a relative uncertainty of $\pmb{2\ \times\ 10^{-8}}$ . Details of the interferometer and the uncertainty analysis of the volume measurement are presented.