The Avogadro constant links the atomic and the macroscopic properties of matter. Since the molar Planck constant is well known via the measurement of the Rydberg constant, it is also closely related to the Planck constant. In addition, its accurate determination is of paramount importance for a definition of the kilogram in terms of a fundamental constant. We describe a new approach for its determination by counting the atoms in 1 kg single-crystal spheres, which are highly enriched with the ${}^{28}\mathrm{Si}$ isotope. It enabled isotope dilution mass spectroscopy to determine the molar mass of the silicon crystal with unprecedented accuracy. The value obtained, $N_A=6.02214078(18)\times {10}^{23}{\mathrm{mol}}^{-1}$, is the most accurate input datum for a new definition of the kilogram.

New results are reported from an ongoing international research effort to accurately determine the Avogadro constant by counting the atoms in an isotopically enriched silicon crystal. The surfaces of two 28 Si-enriched spheres were decontaminated and reworked in order to produce an outer surface without metal contamination and improved sphericity. New measurements were then made on these two reconditioned spheres using improved methods and apparatuses. When combined with other recently refined parameter measurements, the Avogadro constant derived from these new results has a value of N A = 6.022 140 76(12) × 10 23 mol-1. The x-ray crystal density method has thus achieved the target relative standard uncertainty of 2.0 × 10-8 necessary for the realization of the definition of the new kilogram.

Improved measurement results for the Avogadro constant using a 28Si-enriched crystal Y Azuma, P Barat, G Bartl et al.-Counting the atoms in a 28Si crystal for a new kilogram definition B Andreas, Y Azuma, G Bartl et al.