Investigations on Diameter Measurements of the Avogadro Silicon Sphere Dependent on the Position in the Field of View of the Interferometer Optics

The goal of the International Avogadro Project is the redetermination of Avogadro´s constant with an overall uncertainty of 2 times10^-8 at the most. The project partners, therefore, perform measurements of the molar mass and the lattice parameter of a silicon single crystal and of the mass and the volume of a macroscopic high-precision silicon sphere that is made of the same crystal. The results are combined to calculate Avogadro´s constant. The determination of the volume of the Avogadro sphere requires unequalled precision in absolute interferometry. For this purpose, a special Fizeau interferometer with spherical reference faces has been developed at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany. In particular, systematical influences on an experimental setup can only be detected if relevant influencing parameters are varied. The sphere interferometer of the PTB offers a unique way of investigating the influence of its optical components. The measurement strategy and results are presented in this paper.