The new kilogram definition based on counting the atoms in a 28Si crystal

Abstract

The kilogram is the only unit of measure still defined by a physical object. Now, a marathon effort to tie the kilogram to a constant of nature is nearing the finish line. This paper concerns an international research project aimed at determining the Avogadro constant by counting the atoms in an isotopically enriched silicon crystal. The counting procedure was based on the measurement of the molar volume and the volume of an atom in two 1 kg crystal spheres. The novelty was the use of isotope dilution mass spectrometry as a new and very accurate method for the determination of the molar mass of enriched silicon. Because of an unexpected metallic contamination of the sphere surfaces, the relative measurement uncertainty, , results were larger by a factor 1.5 than that targeted. The measured value of the Avogadro constant, mol–1 is the most accurate input datum for the kilogram redefinition and differs only by from the CODATA 2010 adjusted value. This value is midway between the watt-balance values.