PROJECT TITLE :
Improvements to the Volume Measurement of 28Si Spheres to Determine the Avogadro Constant
Within the determination of the Avogadro constant using the X-ray crystal density method with a 28Si-enriched crystal prepared by the International Avogadro Coordination (IAC) project, the quantity measurement and surface analysis of 28Si spheres play an important role. For correct volume determination, an optical interferometer has been improved taking account of the geometrical form of the optical parts. Furthermore, a replacement spectroscopic ellipsometer with a sphere rotation system has been developed for accurate surface analysis. The optical interferometer and also the ellipsometer have been used for the degree determination and surface analysis of the 28Si spheres repolished by the IAC, respectively. On the premise of the preliminary uncertainty estimation, the relative standard uncertainty of the apparent volume measurement has been reduced from four.4 to two.seven by the development of the interferometer. The quantity of ellipsometric measurement points on the sphere surface has been increased from 20 to 7782 by the installation of the new ellipsometer, yielding additional reliable data on the sphere surface. Details of the enhancements and therefore the preliminary results of the measurements of the 28Si spheres are given.
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