Australian scientists redefining the kilogram

Australian scientists will tomorrow take delivery of the silicon they will use to redefine the kilogram.

Scientists from the National Measurement Institute (NMI) and CSIRO's Australian Centre for Precision Optics (ACPO) will use the silicon to create a new international standard for the kilogram unit.

The kilogram is a base unit in the International System (SI), important in science, commerce and everyday life.

Of all the seven SI base units, however, it is the only one still defined by a physical object: a lump of metal, known as the International Prototype, sitting in a vault in France.

All other units have evolved with time and are now defined in terms of a fundamental constant of nature, reproducible by anyone, anywhere, at any time.

The dilemma faced by the scientific community is that if the International Prototype changes in mass for some reason, like material loss when getting cleaned, then the mass values of all subjects measured in kilograms around the world will also change.

A decision was made, under the guidance of the International Bureau of Weights and Measures (BIPM) near Paris, which houses the prototype, for the international scientific community to focus on redefining the kilogram based on one of two unchanging natural phenomena: a quantity of light or the mass of a fixed number of atoms.

The Australian scientists, working on what is called the Avogadro project, will make a perfect sphere from the single crystal of pure silicon to use as the standard, then calculate the number of atoms in that sphere.

While a physical object will still be necessary for calibrating scales and balances, the silicon atoms in the sphere will always remain the same. Once the number of atoms in the sphere is determined, the definition of the kilogram can be based on an atomic weight from then on.

"The only people who can make what is likely to be the roundest object in the world are our colleagues at CSIRO's ACPO," said Dr Barry Inglis, chief executive of NMI.

The best sphere the ACPO team has made had a total out-of-roundness of 35 nanometres. That is, the diameter varies by an average of only 35 millionths of a millimetre, making it probably the roundest object in the world.

"My laboratory has maintained the International Prototype kilogram since 1889," said Alain Picard of the BIPM, who is bringing the silicon to Australia. "We are really pleased that this international collaboration will finally let us improve the definition of the unit of mass by basing it on a physical constant."

It has taken three years to produce the 20 cm long cylinder of silicon. The special silicon, known as monoisotopic silicon, was made in Russia and grown into a near perfect crystal in Germany. It will take around 12 weeks to make one sphere 93 mm in diameter (the team will make two).