Current State of Avogadro ${}^{28}$ Si sphere S8

In 2011, the first determination of the Avogadro constant with ²⁸Si material was published with an overall relative uncertainty of 3 × 10^-8. The results were based on measurements of a very pure single crystal of highly enriched silicon-28. Where the result of crystal properties, molar mass, and lattice constant could be performed with satisfying uncertainty, the measurements on the spheres made of the same crystal were overshadowed by an unexpected effect; the surface of both silicon spheres were contaminated by small amounts of copper (Cu) and nickel (Ni). Although the contaminations were quantitatively measured and their impact into the different quantities was taken into account, the Avogadro group was worried about its negative influence. With the knowledge of various etching procedures and after due consideration, a special chemomechanical technique was applied to remove the metallic contaminations. Herein, we describe the decontamination process, the subsequent measurements, and their impact on the overall uncertainty of the Avogadro constant.