The 2019 SI kilogram redefinition was underpinned by counting atoms in near-perfect enriched silicon-28 spheres to determine the Avogadro constant

When the International System of Units redefined the kilogram in 2019 — moving it off the physical platinum-iridium artifact ("Le Grand K") and onto a fixed value of the Planck constant — the new definition had to be realized experimentally by independent routes. One of the two was the silicon-sphere method: an international collaboration built spheres of highly enriched silicon-28 — "99.9995 percent silicon-28," each "about a 94-mm diameter sphere" with "a mass of 1 kg" — so that researchers could effectively count the atoms in a known mass of a pure, regular crystal and from that determine the Avogadro constant. Fixing the Avogadro constant is tied through the molar Planck constant to fixing the Planck constant, which is what anchors the redefined kilogram.

The work was coordinated by the International Avogadro Coordination, a consortium of national metrology institutes. The Tier-1 NIST page itself names the collaborators: "the work was done by researchers at PTB in Germany, NMIJ in Japan, NIM in China, METAS in Switzerland, NIST in the U.S., INRiM in Italy, BIPM in France, and IRMM in Belgium." Australia's NMIA/CSIRO — where the spheres were polished — is not on that NIST roster and rests on Tier-3 reporting (see the Powerhouse record cited at the Leistner note below). The vivid physical consequence of the manufacturing precision this required is recorded separately at claim-avogadro-silicon-spheres-among-roundest-objects-ever-made, and the craft dimension — that the final polishing depended on one optician's fingertips — at claim-leistner-atomic-feeling-hand-polished-avogadro-spheres.