Density measurements of silicon crystals by hydrostatic weighing

Through research for a determination of the Avogadro constant by the X-ray crystal density method, a primary density standard based on silicon crystal s was established at the National Research Laboratory Metrology (NRLM) with a relative combined standard uncertainty of 1.0 x 10(-7). To investigate t he cause of the e;existing discrepancy in the molar volume of silicon in si ngle crystals, a new hydrostatic weighing system has been developed for eva luating the consistency in the absolute density values of the silicon cryst als used for the determination of the Avogadro constant. This system measur es the density of a solid material under study with respect to that of a pr imary standard with a relative combined standard uncertainty of 1.3 x 10(-7 ). Some measurement results on silicon spheres fabricated from different si licon ingots are described.