GERMANY: A solution to the kilogram headache

The kilogram is a massive headache for scientists, writes Geoff Brumfiel (pictured) in a news item in the latest issue of Nature.com. It is officially defined as the mass of a 122-year-old cylinder of platinum and iridium, kept at the International Bureau of Weights and Measures in Paris. But the cylinder's mass seems to be changing as it ages, prompting several groups of scientists to seek a replacement.

They hope to define the kilogram by referring to a physical constant rather than an antique object.

Brumfiel says the latest result from a team led by Peter Becker of the Federal Institute of Physical and Technical Affairs in Braunschweig, Germany, published on arXiv (P. Andreas et al.), comes closer than ever to ending the cylinder's reign.

The team measured the number of atoms in a sphere of silicon-28 to calculate Avogadro's constant to nine significant figures. The constant refers to the number of atoms in a sample whose bulk mass in grams equals the relative atomic mass of the element. This general relationship makes Avogadro's constant a fixed point from which to define mass.

The big challenge was making the silicon sphere. In an ordinary sample of silicon, 92% of the atoms are silicon-28; the remainder are a mix of silicon-29 and silicon-30. A gas centrifuges purified silicon-28 to 99.99%, which Becker's team used to grow a 5kg crystal that could be fashioned into two near-perfect spheres.

Using laser interferometery, the team mapped each sphere's surface to measure its volume and used X-ray diffraction to image its crystal structure. Calculating the volume taken up by each atom of silicon allowed them to work out how many atoms were in the whole sphere, and derive Avogadro's constant.

The main rival to the silicon sphere method relies on a watt balance which measures the mass of a test cylinder by suspending it using a combination of electrical currents and magnetic fields.

But the two methods produce slightly different values for the kilogram. Brumfiel quotes Becker as saying that he hopes refining the current measurements of the spheres can reduce the uncertainty. "We need a couple of years, but we can see the end of the tunnel," Becker says.