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

Area of Science:

• Metrology
• Solid State Chemistry
• Nuclear Physics

Background:

• The international kilogram unit is defined by a physical artifact, the International Prototype Kilogram.
• Advancements in materials science and metrology aim to realize the kilogram based on fundamental physical constants.
• Silicon-28 (28Si) enriched single crystals offer a pathway to a new definition of the kilogram due to their isotopic purity and crystalline perfection.

Purpose of the Study:

• To assess the purity of a 28Si-enriched single crystal for manufacturing a one-kilogram mass standard.
• To investigate the feasibility of achieving the required 2.0 × 10-8 relative standard uncertainty for mass dissemination.
• To evaluate the material's suitability for realizing the unit of mass based on the Planck constant.

Main Methods:

• Instrumental neutron activation analysis (INAA) was employed to determine elemental impurities.
• High thermal neutron flux from the OPAL research reactor was utilized for sensitive detection of contaminants.
• The study focused on quantifying surface layer mass and mass deficit from point defects.

Main Results:

• The 28Si-enriched single crystal exhibited an exceptionally high purity level, surpassing that of silicon used in previous kilogram spheres.
• INAA identified and quantified a wide range of potential contaminant elements.
• The material's purity is a critical step towards achieving the target uncertainty for mass standards.

Conclusions:

• The produced 28Si-enriched silicon material demonstrates a purity level suitable for the next generation of mass standards.
• Further characterization and correction for surface and defect-related mass are necessary for precise realization of the kilogram.
• This research advances the practical realization of the unit of mass using silicon spheres.