English: Silicon sphere used in the Avogadro project being inspected by Achim Leistner, master optician at the Australian Centre for Precision Optics (part of the CSIRO). The sphere, which has a diameter of about 93.6 mm, was cut from a single crystal of 99.9995% pure silicon-28 and has a mass of 1 kg. The number of atoms in the sphere has been calculated using X-ray crystallography with an uncertainty of less than 2×10-8. It is round to within one part in two million.
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