Abstract
The new definition of the International System of Units (SI) of the kilogram based on a fixed numerical value of the Planck constant, one of the fundamental physical constants, came into force on May 20, 2019. AIST has measured the Planck constant with world’s highest level of precision and contributed substantially to the new definition of the kilogram by X-ray crystal density (XRCD) method with 28Si-enriched spheres. In an accurate determination of the Planck constant by the XRCD method, the surface characterization of the Si sphere is indispensable. We measured the composition and the thicknesses of the oxide and carbonaceous layer existing on the Si sphere surface by X-ray photoelectron spectroscopy (XPS) and succeeded in reducing the uncertainty caused by the surface layer in the determination of the Planck constant. This paper describes an accurate thickness measurement of the surface layer on a 28Si-enriched sphere using XPS.
