New sphere interferometer for a possible new definition of the kilogram

The kilogram is the last SI-unit still defined in relation to an artefact: one kilogram is the mass of the international kg-prototype. Therefore a new definition of the unit of mass on base of a natural constant has been taken into considerations. One way could be a precise determination of Avogadro’s constant, for which the volume of a single crystal silicon sphere has to be determined with a relative uncertainty of 10-8. For feasibility studies a sphere interferometer has been developed in PTB based on commercially available optics and has been modified several times to achieve smaller uncertainties. Promising results of the other contributing quantities - mass, lattice spacing and isotopic composition - led to the decision to set up a new interferometer for spheres. In collaboration with Zeiss and Jenoptik the best realisation for the optical parts, Fizeau lenses and collimators, were found to ensure best possible optical properties. The demands were: operation in vacuum, reference face < λ/30 and a wave front quality of the whole optical system better λ/10. The sphere interferometer will be introduced in detail and first measurements of the new 28Si sphere will be presented.