CSIRO’s Australian Centre for Precision Optics is the only place in the world capable of fabricating round objects with the accuracy required for the Avogadro Project.
Two very special spheres have been made by CSIRO as part of an international project to redefine the kilogram.
The kilogram, the unit of mass in the International System (SI), is one of seven base units used in science, commerce and everyday life. It is the only one still defined by a physical object.
A kilogram is currently defined as: ‘that mass which is equal to the International Prototype’. The International Prototype being a cylinder of platinum and iridium kept in a vault at the International Bureau of Weights and Measures (BIPM) near Paris, France.
All the other SI units are defined in terms of a fundamental constant of nature so that anyone, anywhere can reproduce them and they don't change over time.
Around the world, scientists are working on new ways to define the kilogram.
Background to the Avogadro project
The international scientific community, under the auspices of the BIPM, decided to focus on two methods, and CSIRO was involved in one of them.
Known as the Avogadro Project, the plan is to bring together enough atoms of one substance – silicon – to make a kilo.
Attention has focused on silicon because:
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its characteristics are very well understood
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a single crystal of the right size can be grown
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its atomic structure is extremely uniform
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its widespread use in the computer industry means it can be obtained with relative ease at high purity and resonable cost.
A spherical shape was chosen because a sphere has no edges that might get damaged and only one dimension has to be measured in order to calculate its volume.
This work was done with the close cooperation of Australia's National Measurement Institute (NMI) and CSIRO's Australian Centre for Precision Optics (ACPO).
ACPO's work in creating precision optical components for applications such as astronomy and photonics, has meant that CSIRO is able to contribute the necessary cutting, grinding, polishing and measurement skills to make the spheres perfect.
CSIRO's involvement
ACPO's optical engineers have made 14 spheres for this project. The last two are very special as they were made from an almost perfect crystal grown from an almost perfectly pure type of silicon.
The old spheres are a mixture of the three isotopes of silicon (isotopes having the same number of protons but different numbers of neutrons). The new spheres were made from just one isotope: silicon 28. The monoisotopic silicon, as it is known, was made in Russia while the near perfect crystal was grown in Germany.
ACPO's expert optical engineers made two new spheres from this material, each 93 mm in diameter.
The best sphere the ACPO team has ever 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.
These two spheres are not as round as that: their out-of-roundness is 'only' about 63 nm and 70 nm. What's more important for this project is that the surface be perfectly smooth and free of defects: which it is.
No-one else on Earth can achieve this level of accuracy.
Outcomes
On Friday 4 April 2008, these two spheres were presented to representatives of the Avogadro project.
Now it's up to these countries, and others in this major international effort such as Italy, Belgium, Japan and the US, to determine what is effectively the number of atoms in a sphere. Once the number of atoms is known, the definition of the kilogram can be based on it from then on.
While a physical object will still be necessary for calibrating scales and balances, the silicon atoms in an Avogadro’s sphere will always remain the same.
Watch CSIRO's The story of the spheres (Video).
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