Certifying the Degree of Perfection of Silicon Single Crystals for the Avogadro Project by Measuring the Amount of Copper Silicide Precipitated in the Voids

A procedure has been developed for determining the number of missing atoms in a silicon crystal. Possible voids are filled with Cu ⁵Si by indiffusion of copper. After removal of the interstitially dissolved copper, the samples are digested and the copper content is determined by isotope-dilution mass spectrometry. The detection limit of this highly specific and accurate measurement method was developed sufficiently to certify the degree perfection of the silicon single crystals used to enable a redetermination of the Avogadro constant, N_A, to the required level: lower than 2 times 10 ^-8 N_A combined standard uncertainty