This paper describes an attempt to replace the present definition of the kilogram with the mass of a certain number of silicon atoms. At present, a limiting factor is the measurement of the average molar mass of natural Si. Consequently, a world-wide 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 of 2 X 10~(-8).
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