Avogadro Project: First isotope-pure silicon single-crystal

One goal of the Avogadro Project, is to determine the quantity of the Avogadro constant with unprecedented accuracy. This would also give rise to a definition of the kilogramme, which is still embodied by a prototype, referenced to a fundamental constant. Measurements performed on silicon spheres with a natural isotopic composition did not allow relative uncertainties of less than 3 · 10^–7 to be achieved for the Avogadro constant. Therefore, it was decided to repeat the measurements with isotopically pure silicon. For this purpose, PTB coordinated a cooperation agreement between eight national metrology institutes, international research laboratories and Russian research institutions and industrial firms. The first result of this cooperation is the single-crystal now available. The crystal was a joint-production of the Central Office for Centrifuge Development of the Russian Nuclear Ministry in St. Petersburg (production and enrichment of SiF₄ gas), the Institute for Ultrapure Materials in Nishni-Novgorod (converting SiF₄ gas into SiH₄ gas, chemical purification and separation of a poly-crystal) and the Institute for Crystal Growth in Berlin (growing the Si single-crystal by means of zone purification procedures). The chemical purity required for further use of the material in producing the sphere was achieved after several float zone melting processes. At PTB, the impurity concentrations were determined by infrared spectrometry: carbon 3 · 10¹⁵ cm^–3, oxygen 4 · 10¹⁵ cm^–3 and boron 3 · 10¹³ cm^–3. The content of more than 99.99% ²⁸Si in the crystal, determined at the Institute for Reference Materials and Measurements of the EU in Geel, was maintained through out the entire production process. This is an important prerequisite for successful measurements on a ²⁸Si sphere. The material required, a 5 kg crystal, will be delivered within schedule in 2006.