THE SILICON PATH TO THE KILOGRAM

A more accurate value of the Avogadro constant N-A will be a key input parameter to tabulated values of the fundamental constants and a new definition of the unit of mass. The contribution summarizes the activi ties of the Physikalisch-Technische Bundesanstalt (PTB) in replacing t he kilogram artefact by the mass of a certain number of silicon atoms. Defining the unit of mass by such an atomic procedure requires a dete rmination of the Avogadro constant with a relative measuring uncertain ty of better than 5 . 10(-8). At present, the most limiting factors se em to be the -up to now- unknown density of self-point defects and the precise determination of the isotope composition of the silicon mater ial. In collaboration with the Wacker-Siltronic company in Germany and several other european research institutes, PTB has initiated a scien tific program for the improvement of the purity of silicon crystals, e specially by a significant reduction of the carbon and oxygen content and by homogeneity investigations of density and lattice parameter wit h respect to vacancies and Si self-interstitials. As a second independ ent method for the study of the isotope composition -besides mass spec troscopy- the interaction of the crystal with neutrons producing promp t gamma-rays was proved to be sensitive enough. This method is now und er development at PTB.