Investigation of a cleaning procedure for silicon spheres used in the realization and dissemination of the redefined kilogram via combined spectroscopic and gravimetric measurements

Edyta Beyer,Michael Borys,Michael Mecke,Michael Kolbe

doi:10.1051/ijmqe/2020016

Abstract

The new definition of the SI kilogram requires new methods of realizing this unit. The X-ray crystal density method is a primary realization method and uses silicon spheres. The spheres get cleaned before each measurement, in order to remove surface contaminations and thus reduce their uncertainty contribution to the realization. Therefore, cleaning is an inherent part of the realization and dissemination of the kilogram. A cleaning method for silicon spheres is investigated, concerning its suitability as a part of the realization of the redefined kilogram. Six silicon spheres were used to determine the repeatability of the established cleaning method. Measurements of the spheres' mass and the quantification of their surface layer mass after cleaning were carried out in several cycles resulting in 29 mass and surface measurements. The repeatability of the cleaning method applied shows a standard deviation in the order of two micrograms for both the mass and the surface layer. The cleaning method therefore sufficiently fulfils these requirements.

Highlights

Since the revision of the SI in May 2019, the kilogram has no longer been defined as the mass of the international prototype of the kilogram
The mass deposition determined via X-ray fluorescence analysis (XRF) and the elemental ratios are combined to determine the mass contributions of each sublayer and the overall surface layer
The mass deposition of oxygen dO combined with the ratio of the X-ray photoelectron spectroscopy (XPS) elemental atomic fractions of SiO2 and the remaining oxygen yields the mass deposition of silicon and oxygen in the whole surface

Summary

Introduction

Since the revision of the SI in May 2019, the kilogram has no longer been defined as the mass of the international prototype of the kilogram. The new definition is based on natural constants whose numerical values have been fixed with a defined uncertainty of zero. These constants are the Planck constant, the unperturbed ground-state hyperfine transition frequency of 133Cs and the speed of light in vacuum [1,2,3]. Primary methods are needed for the practical realization of the kilogram definition [2] whereby the X-ray crystal density (XRCD) method is one method of realizing this unit and uses monocrystalline isotopically enriched 28Si spheres. The 28Si spheres work as primary mass standards. Spheres made of natural silicon can be used as secondary mass standards to disseminate and maintain the kilogram

Methods

Results

Conclusion