Density determination of a small isotopically enriched silicon single crystal

For a new determination of the Avogadro constant using small crystals of isotopically enriched silicon, the density of a sample was determined at Physikalisch-Technische Bundesanstalt. The sample has a mass of 58 g and a Si-28 enrichment of about 99.98%. Its density was compared by the pressure-of-flotation method to the density of a large hollow transfer standard that was manufactured from natural silicon, to have the same density. The flotation measurement yielded a relative density difference of 0.64(10) (.) 10(-6). The density of the transfer standard was then measured by hydrostatic weighing, which is traceable to a primary density standard. Thus, the density of the small Si-28 sample was determined to be 2320.08031(40) kg/m(3), i.e., with a relative standard uncertainty of 0.17 (.) 10(-6).