This paper describes an attempt to replace the present definition of the kilogram with the mass of a certain number of carbon atoms. This requires determination of the Avogadro constant, N_(A), with a relative uncertainty of 1 X 10~(-8). Silicon crystals are used in this determination. At present, a factor limiting factor is the measurement of the average molar mass of natural Si. Consequently, a worldwide collaboration has been set up, to produce, approximately, 5 kg of ~(28)Si single-crystal with an enrichment factor greater than 99.985percent and of sufficient chemical purity to be used for the determination of N_(A) with a target relative measurement uncertainty better than 2 X 10~(-8). The present article describes the cooperation work-plan, the specifications applied, and quantifies the expected improvement.
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