Status of the kilogram realisation using the XRCD method at PTB

Since the establishment of the revised SI the unit of mass, the kilogram, can be realised by two primary methods based on Planck's constant - the “Kibble balances” and the “XRCD” method (X-ray crystal density). The XRCD method is based on a high-quality characterisation of spheres made of isotopically enriched 28 Si silicon, which can be divided into two main activities, the determination of crystal/material parameters and the determination of the macroscopic volume of 28 Si spheres and their surface layers. This subdivision is of particular interest since the material parameters are considered constant and are not re-determined when the realisation is “reactivated”. Before and after the redefinition of the kilogram, comparisons of the most accurate realisation experiments were carried out. This publication discusses the parameters, methodology and instrumentation used by PTB for the participation in CCM.R-kg-P1 (Pilot Study) before redefinition in comparison to the results of the experiments for the participation in CCM.M-K8.2019 and CCM.M-K8.2021 after redefinition.