The kilogram is the only SI base unit that is still defined by a material object - a piece of platinum-iridium alloy kept in a vault in Sèvres, near Paris. All the other units are related to atomic or fundamental constants, such as the speed of light or the charge on an electron, and metrologists now agree that they must redefine the kilogram.

Researchers have proposed two ways to redefine the unit of mass: one is based on the Planck constant, and the other relies on measurements of the Avogadro constant - the number of atoms or molecules in one mole of a substance. One problem with defining the kilogram in terms of the Avogadro constant is that it is necessary to count the number of atoms in a sample with more accuracy than is practically impossible.

To overcome this, Becker and co-workers determined the lattice parameter, density and molar mass of an almost perfect single-crystal of silicon using an X-ray crystal density molar mass (XRCDMM) method. Then they calculated the Avogadro constant by dividing the molar volume - the ratio of the mean molar mass of silicon to the density of the crystal - by the atomic volume. The atomic volume is the volume occupied by a single silicon atom, which the researchers calculated by measuring the lattice spacing of the crystal.

Although the PTB value of 6.022 135 3 x 1023 mol-1 agrees well with previous experiments, it still has a measurement uncertainty of 3.4 x 10-7. The group now hopes to reduce this figure to the order of 10-8 with the help of the international Avogadro Constant Working Group. Moreover, the new value disagrees with the latest recommended CODATA value by about 1 part in 106 for reasons that are not currently understood.