It has now been more than 40 years ago since the first attempts were started to find a way of defining the kilogram based on an atomic or fundamental physical constant. A sufficiently accurate determination of the drift is not yet possible, but it is evident that there is a need for a new definition of the mass unit. The mass of the international prototype of the kilogram (IPK) expressed in terms of the SI unit kilogram is invariable by definition, but since 1889 the mass differences between the IPK and its official and national copies have drifted by about 50 µg, or 5 × 10 −8 in relative terms, on the average. The kilogram is the only base unit of the International System of Units (SI) still defined by a material prototype as stated by the 1st General Conference on Weights and Measures in 1889.

The intention of this project is also to show the availability of 28Si single crystals as a guarantee for the future realisation of the redefined kilogram. Then the paper describes the activities of a follow-up project, conducted by PTB, to produce a new generation of highly enriched silicon in order to demonstrate the quasi-industrial and reliable production of more than 12 kg of the 28Si material with enrichments of five nines. First, this paper reviews previous efforts from the very first beginnings to an international collaboration with the goal of producing a 28Si single crystal with a mass of 5 kg, an enrichment greater than 0.9999 and of sufficient chemical purity. The solution chosen subsequently was the usage of silicon highly enriched in 28Si from Russia. The first attempt, in which silicon of natural isotopic composition was used, failed. The method applied in this case is based on the x-ray crystal density experiment using silicon crystals.

One prerequisite for this is that the Avogadro constant, N A, which defines the number of atoms in a mole, needs to be determined with a relative uncertainty of better than 2 × 10 −8. A metrological challenge is currently underway to replace the present definition of the kilogram.

Avogadro hexagonal lattice