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The most expensive spheres in the world

The international Avogadro Project coordinated by the PTB, is entering its last phase with two new, isotopically enriched silicon spheres.

The scientists hope that the new spheres will now bring about the crucial breakthrough in determining the Avogadro constant, i.e. the number of particles in one mole of amount of a substance. With the spheres used till now, made of a silicon isotope mixture, the accuracy which would be necessary for the re-definition of the kilogram could not be reached.

On 10th April 2008 the two 28SI spheres came to the PTB from the last stage of their production process in Australia. After the raw silicon from Russia had been processed into a cylindrical single crystal in the Institute for Crystal Growth (IKZ), Berlin, Australian colleagues at the National Measurement Institute (NMI) in Sydney had the last stage of production in their hands: to polish the silicon body into a perfect sphere. It is now the job of the PTB to determine the topography, the volume and the mass of the spheres with smallest measurement uncertainties. A measurement uncertainty of 0.1 nm (corresponding to the thickness of a monoatomic layer) is demanded for the radius of the sphere which is around 5 cm, and an uncertainty of 5 µg, for the mass of the sphere weighing approximately 1 kilogram.

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