The kilogram (kg), the only remaining SI unit based on a physical object, is set to relinquish its unique status in 2018. The new kg definition will rely on mass measurements based on Planck’s constant and Avogadro’s constant.

A 2015 study achieved the required precision (2.0 · 10^–8 relative standard uncertainty) using X-ray crystal density to count the silicon atoms in a one-kilogram ²⁸Si-enriched single crystal sphere [1]. The isotopic enrichment reduced background noise produced by ³⁰Si and ³¹Si present in natural silicon.

Verifying these results required an extremely precise evaluation of the ultratrace impurities present in the silicon sphere. Giancarlo D’Agostino, Istituto Nazionale di Ricerca Metrologica, Torino, Italy, and colleagues examined two 5.2-gram samples from the sphere using instrumental neutron activation analysis, collecting their data at research reactor neutron sources in Italy and Australia.

The team quantified the concentrations of 12 elements and determined the detection limits of another 54 elements. They determined that the difference between the mass of an idealized one-kilogram silicon sphere and the actual sphere is 1.154 μg.

• Purity of ²⁸Si-Enriched Silicon Material Used for the Determination of the Avogadro Constant,
  Giancarlo D’Agostino, Marco Di Luzio, Giovanni Mana, Massimo Oddone, John W. Bennett, Attila Stopic,
  Anal. Chem. 2016.
  DOI: 10.1021/acs.analchem.6b01537

References

• [1] Improved measurement results for the Avogadro constant using a28Si-enriched crystal,
  Y. Azuma, P. Barat, G. Bartl, H. Bettin, M. Borys, I. Busch, L. Cibik, G. D’Agostino, K. Fujii, H. Fujimoto, A. Hioki, M. Krumrey, U. Kuetgens, N. Kuramoto, G. Mana, E. Massa, R. Meeß, S. Mizushima, T. Narukawa, A. Nicolaus, A. Pramann, S. A. Rabb, O. Rienitz, C. Sasso, M. Stock, R. D. Vocke, A. Waseda, S. Wundrack, S. Zakel,
  Metrologia 2015, 52, 360–375.
  DOI: 10.1088/0026-1394/52/2/360