Realisation of the kilogram with the XRCD method – improved XRF evaluation and its impact

The realisation of the kilogram at PTB is based on the established X-ray crystal density method (XRCD). This method is based on a high-precision characterisation of spheres of isotope-enriched 28 Si. The crystal and material parameters are considered constant and do not need to be re-determined when the realisation of the kilogram is updated. Therefore, the re-determination is primarily based on new measurements of the volume and the mass of the surface layers of the 28 Si spheres. This paper presents new insights in the XRF analysis method used in the XRCD method by PTB and its effects on the volume and mass of 28 Si spheres used as primary mass standards, which provides a basis for participation in the key comparison CCM.M-K8.2024. In the key comparisons CCM.M-K8 from 2019, 2021 and 2024, the definition of the kilogram was realised using the AVO28-S8c and Si28kg01a spheres, as outlined in this document. When applying the new model for the spectral components of the XRF spectrum, realisation results of both spheres show an average mass increase of 13.5 µg.