Towards an atomic realization of the kilogram: The measurements of N_A and N_Ah

Abstract
An improved attempt of several national metrology institutes to replace the present definition of the kilogram with the mass of a certain number of ¹²C atoms is described. This requires the determination of the Avogadro constant, N_A, via the silicon route with a relative uncertainty better than . Previously, the limiting factor is the measurement of the average molar mass. Consequently, a world-wide collaboration has been set up, to produce, approximately, 5 kg of ²⁸Si single-crystal with an enrichment factor greater than 99.985% to be used for an improved determination of N_A. The first successful tests of all technological steps (enrichment of SiF₄, purification and synthesis of silane, deposition of polycrystalline ²⁸Si, single crystal growth) for the production of high-purity ²⁸Si are described.