Determination of the Void Volume in Silicon Single Crystals for Use in the Precision Measurement of Avogadro's Constant

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This project is supported by NSF as a Cooperative Activity in Materials Sciences between the National Science Foundation and the European Commission. The kilogram is the only remaining fundamental unit that is still defined by a specific artifact: the standard kilogram, kept at the International Bureau of Weights and Measures in Paris. A more fundamental standard is needed and could be defined if Avogadro's number were known more accurately. An international consortium has been working for the last decade to determine Avogadro's number more accurately by measuring the physical properties of highly perfect silicon crystals. These studies, however, revealed a surprising occasional variation in the density of nominally equal quality float-zone silicon caused by the presence of small voids. This project aims to develop a method for determining the volume of these voids accurately through high-precision measurements of copper dissolved in silicon. The project is being carried out in collaboration with the Institute for Reference Materials of the European Commission, the National Institute of Standards and Technology, and the German Physikalisch-Technische Bundesanstalt.