Sphere of influence: CSIRO redefines the kilo

120 year old cylinder that defines the world standard for one kilogram to be replaced

The 120 year old International Prototype that defines the world's standard for the mass of one kilogram, held at the International Bureau of Weights and Measurement in Paris (BIPM), is set to be replaced by a perfect silicon crystal sphere developed by scientists at Australia's CSIRO.

According to the CSIRO, the international scientific community, under the auspices of the BIPM, decided to focus on several methods for redefining the kilogram. One of those was the Avogadro Project that sought to bring together enough atoms of one substance to make the perfect kilogram. The sphere was made from one isotope: silicon 28.

Two years of development by the National Measurement Institute and the CSIRO's Australian Centre for Precision Optics (ACPO) have now resulted in the production of the perfect silicon sphere, set to redefine the International System (SI) definition of one kilogram. The CSIRO said the ACPO is the only place in the world capable of fabricating round objects with the accuracy required for the Avogadro project.

The shape of a sphere was chosen to replace the previous cylindrical kilogram definition as it has no sharp edges that could be damaged, and only one dimension has to be measured in order to calculate its volume.

"You can never produce a sharp enough edge to achieve the values of density and mass that you do with a sphere, because the sphere has a very smooth surface, a surface that is on an atomic scale if you like," said CSIRO's master optician Achim Leistner, in a podcast describing the sphere.

"The sphere is actually only a measurement artifact, and in a nutshell by measuring the volume precisely you can determine how many atoms are in the sphere," he said.

The CSIRO said that while a physical object is still necessary for calibrating scales and balances, the silicon atoms in an Avogadro sphere will always remain the same.

Silicon was chosen for several reasons: its characteristics are well understood; a single crystal of the right size can be grown; its atomic structure is extremely uniform; and its widespread use in the IT industry means it can be obtained easily and in good quality.

ACPO engineers produced 14 spheres for the project, with the last two being made from "an almost perfect crystal grown from an almost perfectly pure type of silicon".

The two sphere's out-of-roundness is about 63nm and 70nm, or 63 and 70 millionths of a millimeter respectively. A diamond saw was used in the first stage of shaping, and their surfaces are perfectly smooth and free of defects.

"The silicon, or the gas, was made in Russia. They converted this uranium enriching plant into one that would enrich silicon. Nobody has ever done that before, nobody has ever produced more than a few grams of this single isotope silicon material because its such an expensive exercise. The total cost to produce the material was in the order of about 2.5 million euros," Leistner said.

Leistner said the process of polishing the sphere was done in a manner very much the same as used by Sir Isaac Newtown and others when they polished the first mirrors.

"The kilogram is the only primary standard of the seven other standards that is not based on a natural constant in nature -- in other words it is purely by definition. So when this is now being defined in the silicon crystal this will mean that every other of the SI units will almost automatically get an upgrade because we will know more precisely what the kilogram is," Leistner said.

"The opportunity to work on such a prestigious but important project that will affect not just the kilogram but every other science and every other measurement is something even I can't get my head around properly."

Two weeks ago, the spheres were presented to representatives of the Avogadro project, and are set to replace the metal cylinder in Paris that has defined the kilo for over a century.

Over the next several years a consortium of countries including the US, Japan, Belgium and Italy will work on determining the number of atoms in the sphere in order to confirm the new definition of a kilogram.

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