Measurement of the Avogadro Constant and Mise EN Pratique of an Atomic Definition of the Kilogram by A 28Si Single-Crystal

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 times 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 99.985% and of sufficient chemical purity to be used for the determination of N
_A
with a target relative measurement uncertainty better than 2 times 10
^-8
. The present article describes the cooperation work-plan, the specifications applied, and quantifies the expected improvement