TOWARD THE REDEFINITION OF THE KILOGRAM FROM THE 28 Si INTERNATIONAL RESEARCH PROJECT

In the international system of units, the kilogram in the only SI base unit still defined by a material artefact. In order to redefine this unit with a fundamental physical constant, an international research project was launched in 2004 for determining the Avogadro constant, N A , by counting the atoms in an isotopically enriched 28 Si crystal. The counting procedure relies on the measurements of the molar and atomic volumes of 1 kg spheres made of the 28 Si crystal. In 2011, the project succeeded in measuring the Avogadro constant with a smallest standard uncertainty, 3.0 × 10 −8 N A . Because of an unexpected metallic contamination at the surface of the spheres, the measurement uncertainty was larger than the target of the project by a factor of 1.5. In order to further reduce the uncertainty, a new international research project was launched in 2012. Outline of the new project and the improvements of the measurements will be introduced at the forum. Note from Publisher: This article contains the abstract only.