Status of a silicon lattice measurement and dissemination exercise

Although no longer explicitly directed toward measurement of the Avogadro constant, we continue significant effort on lattice period measurement in monocrystalline silicon.There are currently operating an up-gradad xray/optical interferometer and a recently established lattice comparator.We report the status of measurements with these two instruments. SummarvAlmost twenty years ago, several groups at NBS participated in a measurement of the Avogadro constant [l, 2 1 .As in the case of efforts which are currently underway elsewhere, the NBS work involved optical interferometry of the spatial period of a Si monocrystal (as registered by x-ray interferometry), density determination using a dimensionally standardized artifact, and estimation of mean molar mass from absolute isotopic abundances.The result obtained was limited by imprecision and inaccuracy in density and molar mass to a claimed accuracy near 1 ppm while suffering from a large systematic error (1.8ppm) in the lattice spacing measurement which came to light through work at the PTB reported in and around 1980 [3, 41.Subsequently, the NBS x-ray/optical interferometry experiment, XROI, was re-established with several technical improvements, including on-line trajectory characterization.This latter capability showed, around 1986, that the original systematic error arose from an interplay of trajectory curvature, inadequate characterization of this curvature (by x-ray fringe contrast) and x-ray intensity distribution [5].Through this work it became evident that a result of significantly improved accuracy could, in principle, be