Diameter comparison of a silicon sphere for the International Avogadro Coordination project

In the International Avogadro Coordination (IAC) Project using the x-ray crystal density method, volume measurements of silicon spheres by optical interferometry play a crucial role for the density measurement of the ²⁸Si enriched crystal. To confirm the reliability of the volume measurement, specified diameters of a silicon sphere were measured independently by optical interferometers at PTB (Germany), NMIA (Australia) and NMIJ (Japan). The sphere was washed before the measurement with the same procedure. The directions of the diameters were identified with reference marks prepared on the surface of the sphere. Details on the comparison results are presented.