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

Abstract

The practical realization of the unit of mass is possible by manufacturing a perfect one-kilogram sphere from a Si-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 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 Si-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.

Avogadro constant measurements using enriched28Si monocrystals

“…Subsequently, these molar masses were independently and consistently re-measured by the National Institute of Standards and Technology (NIST), NMIJ and PTB [5] by using a water solution of tetramethylammonium hydroxide (TMAH) as a basic Si solvent in place of a solution of sodium hydroxide (NaOH). INRIM confirmed the 30 Si fraction and purity by nuclear activation analysis [22,23]. The result,…”

A new28Si single crystal: counting the atoms for the new kilogram definition

“…The analysis quantified nine elements: the Cu concentration was 1.52(27) × 10 13 cm −3 while the concentrations of Cr, Co, Ga, As, Br, La, W and Au were between 5.22(32) × 10 10 cm −3 (W value) and 2.14(27) × 10 8 cm −3 (Au value). The corresponding overall concentration is 1.53(27) × 10 13 cm −3 and the total overall mass fraction is 0.70 (12) ng g −1 . Detection limits of an additional 52 elements detectable by INAA were established within the range 1.3 × 10 15 cm −3 (Ti value) and 3.3 × 10 6 cm −3 (Ir value); Sb is included, with a detection limit of 7.2 × 10 8 cm −3 [12].…”

“…The corresponding overall concentration is 1.53(27) × 10 13 cm −3 and the total overall mass fraction is 0.70 (12) ng g −1 . Detection limits of an additional 52 elements detectable by INAA were established within the range 1.3 × 10 15 cm −3 (Ti value) and 3.3 × 10 6 cm −3 (Ir value); Sb is included, with a detection limit of 7.2 × 10 8 cm −3 [12].…”

A new generation of 99.999% enriched28Si single crystals for the determination of Avogadro’s constant

“…The oxygen and carbon concentration in the 28 Si single crystal was measured by IR spectroscopy (IChHPS RAS and PTB, see section 4.4). Additionally, the instrumental neutron activation analysis was used to check the purity with respect to many other chemical elements [25].…”