ABSOLUTE MEASUREMENT OF THE DENSITY OF SILICON-CRYSTALS IN VACUO FOR A DETERMINATION OF THE AVOGADRO CONSTANT

An absolute measurement of the density of silicon crystals used for a determination of Avogadro constant is described. Two 1-kg spheres of n early perfect geometries were fabricated from a single-crystal silicon ingot both for measuring their densities and for determining the dens ity distribution in the ingot. A scanning type optical interferometer was used to measure their diameters in vacuo. Their volumes were obtai ned by fitting the diameters to a series of spherical harmonics. Thick ness of oxide layers on their surfaces was measured by using an ellips ometer to evaluate its effect on their volumes. The masses of the sphe res were measured by using a precision balance for the prototype kilog ram. The relative total uncertainty of the densities, determined by di rect measurements of their masses and volumes in vacuo, is estimated t o 1.1 x 10(-7).