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

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.