International Avogadro constant project - measuring isotope ratios of silicon

NOTE! This page is no longer updated and is about to be decommissioned.

The kilogram is the only remaining base SI unit defined by a man-made artefact. For many years now research groups have been trying to find a way to define it in a more independent and easily reproducible manner, like the other units. An international cooperation is focussing on a solution based on determining the Avogadro constant.

The Avogadro constant can be determined via the mass of a specified number of atoms. If the uncertainty on the Avogadro constant is sufficiently small, this definition can be inverted to define the mass - the unit kg - via a known number of particles.

The approach chosen involves measurements of the dimensions, lattice parameters, mass and isotopic composition of single crystals of silicon, one of the purest materials available.

IRMM can determine the isotopic composition of silicon with the accuracy and precision required, and is at the moment the only laboratory in the world able to conduct such measurements.

A silicon single crystal.