Realization of the New Kilogram Using 28 Si-Enriched Spheres and Dissemination of Mass Standards at NMIJ
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ORIGINAL PAPER
Realization of the New Kilogram Using 28Si-Enriched Spheres and Dissemination of Mass Standards at NMIJ N. Kuramoto* , S. Mizushima , L. Zhang , K. Fujita , Y. Ota , S. Okubo H. Inaba
and
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan Received: 27 June 2020 / Accepted: 13 August 2020 The Author(s) 2020
Abstract: The new definition of the kilogram was implemented on May 20, 2019. The kilogram is presently defined by a fixed value of the Planck constant. On the basis of the new definition, the kilogram will be realized at the National Metrology Institute of Japan by the X-ray crystal density method using 28Si-enriched spheres. For the realization, the volume of 28Si-enriched spheres is measured by optical interferometry. The sphere surface characterization by X-ray photoelectron spectroscopy and ellipsometry is also performed. The relative standard uncertainty of the realization is estimated to be 2.4 9 10-8. Details of the realization and future dissemination of mass standards in Japan based on the 28 Si-enriched spheres are described. Keywords: Kilogram; Planck constant; New SI; Mass standards 1. Introduction On May 20, 2019, the International System of Units (SI) was essentially revised. Presently, all SI units are defined in terms of the seven defining constants [1]. In the new SI, the kilogram, the unit of mass, is defined by fixing the value of the Planck constant. Under the new definition, it is in principle possible for each national metrology institute to realize the kilogram independently. To ensure the consistency among the realizations by national metrology institutes, an international comparison of the realizations based on the new definition, CCM.M-K8.2019, is presently organized by the Consultative Committee for Mass and Related Quantities. This comparison is an essential step to the dissemination of the kilogram based on the individual realization by each national metrology institute [2]. There are currently two independent methods that are capable of realizing the definition of the kilogram with relative uncertainties within a few parts in 108 [3]. The first of these relies on determining the unknown mass using an electromechanical balance specially designed for the purpose. The second method compares the unknown mass to the mass of a single atom of a specified isotope by counting
the number of atoms in a crystal, where the mass of the atom is well-known in terms of the Planck constant. The second method is referred to as the X-ray crystal density (XRCD) method. After the confirmation of the international consistency of the realizations, the National Metrology Institute of Japan (NMIJ) will realize the kilogram on the basis of the new definition by the XRCD method. This method realizes the kilogram at a nominal mass of 1 kg using 28Si-enriched spheres [4, 5]. Details of the realization at NMIJ and future dissemination of mass standards in Japan based on the 28Si-enriched sph
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