Volume measurements of 28Si spheres by an improved optical interferometer and ellipsometer to determine the Avogadro constant

In the determination of the Avogadro constant using the x-ray crystal density method with the
²⁸
Si-enriched crystal prepared by the International Avogadro Coordination, the volume measurement and surface analysis of the
²⁸
Si spheres play a crucial role. For the accurate volume determination, an optical interferometer with a flat etalon has been improved taking account of the geometrical shape of the optical components. Furthermore, a new spectroscopic ellipsometer with a sphere rotation system has been installed for the accurate surface analysis. Details of the optical interferometer and ellipsometer are described.