Impurities in a ²⁸Si-Enriched Single Crystal Produced for the Realization of the Redefined Kilogram

The practical realization of the unit of mass is possible by manufacturing a perfect one-kilogram sphere from a ²⁸Si-enriched single crystal. The mass of the sphere can be determined in terms of a fixed value of the Planck constant by counting the number of silicon atoms in the core of the single crystal. To reach the target 2.0 × 10^-8 relative standard uncertainty, the mass of the surface layer and the mass deficit due to point defects such as impurities and vacancies must be investigated and corrected for. A sample of a ²⁸Si-enriched single crystal produced to test the possibility of obtaining material at a scale useful to the dissemination of mass standards was measured by instrumental neutron activation analysis to check the purity with respect to a large number of possible contaminant elements. The results collected in a neutron activation experiment performed with the high thermal neutron flux available at the 20 MW OPAL research reactor are described. The data collected in this study showed that the produced material has a purity level never achieved with silicon used to manufacture previous one-kilogram spheres.