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
²⁸
Si 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
²⁸
Si sample was determined to be 2320.08031(40) kg/m
³
, i.e., with a relative standard uncertainty of 0.17 10
^-6