Abstract
An improved attempt of several national metrology institutes toreplace the present definition of the kilogram with the mass of acertain number of 12C atoms is described. This requires thedetermination of the Avogadro constant, NA, via the silicon routewith a relative uncertainty better than 2.10-8.Previously, the limiting factor is the measurement of the averagemolar mass. Consequently, a world-wide collaboration has been setup, to produce, approximately, 5 kg of $^{28}{\rm Si}$ single-crystal withan enrichment factor greater than 99.985% to be used for animproved determination of NA. The first successful tests of alltechnological steps (enrichment of SiF4, purification and synthesisof silane, deposition of polycrystalline $^{28}{\rm Si}$ , single crystalgrowth) for the production of high-purity $^{28}{\rm Si}$ are described.
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