Density of Silicon Crystals

Measurements of silicon crystal density for the determination of the Avogadro constant are reviewed. Volume, mass and hydrostatic-weighing measurements are considered from the technical standpoints of error sources and uncertainty of measurement. The outlook on the possibility of achieving experimental uncertainties of less than 10 -7 is described. Attention is paid to the necessity of reducing the sources of uncertainty in the density arising from the surface, such as silica layers and damaged crystalline layers.