Realization of the Kilogram Based on the Planck Constant at NMIJ

A change in the definition of the kilogram has been proposed. The new definition will be based on the Planck constant. After the change, the National Metrology Institute of Japan (NMIJ) will realize the kilogram using the X-ray crystal density method with a 28 Si-enriched sphere. A pilot study to confirm the international equivalence of the ability to realize the kilogram from the new definition is ongoing, where the volume measurement of a 28 Si-enriched sphere by optical interferometry and the sphere surface characterization by spectroscopic ellipsometry and X-ray photoelectron spectroscopy required for the realization were performed by NMIJ independently. The mass of the sphere was determined with a relative standard uncertainty of $2.4\times 10^{\mathrm {\mathbf {-8}}}$ based on the new definition. This paper provides details of the measurements, the data analysis, and the uncertainty evaluation.