Measurement of the {2 2 0} lattice-plane spacing of a 28Si x-ray interferometer

The spacing of the {2?2?0} lattice planes of a 28Si crystal, used to determine the Avogadro constant by counting silicon atoms, was measured by combined x-ray and optical interferometry to a relative accuracy of 3.5 ? 10?9. The result is d2?2?0 = (192?014?712.67 ? 0.67)?am, at 20.0??C and 0?Pa. This value is greater by (1.9464 ? 0.0067) ? 10?6d2?2?0 than the spacing in natural Si, a difference which confirms quantum-mechanics calculations. This result is a key step towards a realization of the mass unit based on a conventional value of the Planck or the Avogadro constant.

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