Composition and structure of surfaces of silicon spheres used in determination of the Avogadro constant

The Avogadro constant is required to be determined with an uncertainty of less than 1/spl times/10/sup -8/ in order to consider an atomic definition of the kilogram. A single crystal silicon sphere 93.6 mm diameter is used for this determination. Surface contaminants such as oxide and water must be taken into account. The nature of this contamination has been investigated using optical ellipsometry and ion beam analysis.