Determining the crystallographic orientation of Avogadro silicon spheres

Abstract

Highly polished spheres, manufactured from silicon single-crystal material, are used in the X-ray crystal density method (XCDM) to determine the Avogadro constant. If the measurement uncertainty associated with this method can be reduced to 0.01 p.p.m., it would be possible to redefine the SI unit of mass, the kilogram, in terms of a fixed number of atoms of a definite species. The spheres are manufactured with a nominal mass of 1 kg and nominal diameter of 90 mm and a surface roughness of 0.5 nm (peak to valley). A goniometer has been constructed to enable the crystallographic orientation of these spheres to be determined using the back-reflection Laue technique. Two spheres have been successfully orientated in this manner by identifying two orthogonal 〈100〉 directions.