Isotopically enriched silicon: the atomic path to the kilogram?

Progress in the experiments for the redefinition of the unit of mass by the X-ray crystal density method is hampered mainly by the insufficient accuracy of silicon molar mass. This paper reports on considerations to calibrate prompt (n,/spl gamma/) spectrometry against data obtained by mass spectrometry. When natural silicon, where df/spl ap/10/sup -6/ is known from mass spectroscopy, is compared with isotopically enriched silicon, two difficulties are overcome: (a) the cross sections etc. cancel out, and (b) the gain in isotopic enrichment is roughly valid for df, too. So, from df=10/sup -6/ we end up with df=6.3/spl times/10/sup -8/ for f/spl ges/0.9995. This material then is used for the fabrication of the 1 kg sphere on which the density and the lattice constant are determined.