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Silicon quest for the ultimate kilogram standard

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Oz scientists and engineers are preparing to create the ultimate kilogram standard - a pair of polished balls lovingly crafted from a single crystal of silicon-28.

According to Reuters, the team from Australia's National Measurement Institute will take more than 12 weeks to hone the spheres. They will weigh exactly one kilogram, with imperfections of "less than 35 millionths of a millimeter".

Crucially, the team's balls will be immune to corrosion, contamination and moisture - unlike the existing platinum-iridium alloy standard, housed in Paris. Scientists have long called for a more stable yardstick, and back in 2005 the US's National Institute of Standards and Technology (NIST) argued that "relying on an actual physical artifact leaves the kilogram vulnerable to misinterpretation". Worse still, it means that scientists have to be dragged kicking and screaming to France to "verify their own standard masses".

Peter Mohr from NIST told LiveScience that changes of as much as 50 parts per billion have been seen over a hundred years in the French standard, something which is "not a serious problem for banana salesmen, but could impact subatomic physicists", as we put it the time.

NIST's other objection to the physical standard was that it is vulnerable to accidental destruction. Mohr warned: "A meteor could strike Paris - destroying the prototype."

NIST therefore proposed the kilogram be "redefined in terms of a natural phenomenon, such as the number of atoms in a silicon crystal, or the amount of magnetic force required to levitate an object in a watt balance".

It's NIST's former suggestion that now offers the prospect of safeguarding the kilogram. When finished, the silicon balls will be "measured for volume in Australia, Germany and Japan, then measured for mass". Belgian scientists will then "look at the molar mass of the crystal used to calculate the number of molecules in each sphere".

Project leader Walter Giardini explained to Reuters: "The aim is not to change the value of the kilogram, but to ensure its stability for all future times. It will no longer depend on an actual physical object and this is going to allow us to relate the mass to the individual atoms."

In case you're wondering how exactly Australia came to lead the world in ball-polishing, Reuters notes that boffins Down Under "have the most expertise in grinding near-perfect spheres, having been turning them out for clients including NASA since the early 1990s".

Giardini said: "We have developed technology so that we can see what we are getting, whether they [the balls] are slightly oval or flat. We are trying for an accuracy of two parts in 100 million.

"We are doing everything to really create a perfect object. It's not only near-perfect in roundness, but also the crystal purity, the atomic species and so on. Silicon is a very nice material to use that we understand well, makes good crystals and can be worked."

The crystal balls project is a collaboration between scientists from Australia, Belgium, Germany, Italy, Japan, Russia and the United States, in co-operation with the International Bureau of Weights and Measures, which holds the existing platinum-iridium sphere. ®

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