May 25 2007
Scientists at the Institute for Crystal Growth in Berlin have produced the crucial silicon crystal that could lead to a redefinition of the international mass unit kilogram. Up to now, the value of the kilogram is defined as being equal to the mass of the international prototype of the kilogram which is stored in Paris, France.
Many countries have their own copies of the prototype. The German copy is stored in Braunschweig at the Physikalisch-technische Bundesanstalt PTB. The problem is: Handling of the prototypes, above all cleansing, leads to a loss of atoms, i.e. a loss of mass. Experts assume that meanwhile deviations up to 70 micrograms (0.000 000 07 kg) have arisen. Since in today's chemistry and physics experiments and measurements are conducted with far smaller masses, exact and internationally standardized definitions of mass are of great importance.
To obtain such a definition, scientists worldwide explore different possibilities. A promising approach is to define the unit kilogram by the exact number of atoms of a specific element it contains. Scientists of the international Avogadro Project have chosen silicon (Si) because it’s properties are very well known. Now the scientists at the Institute for Crystal Growth in Berlin have produced a unique silicon crystal of highest perfection and purity. From this piece of crystal two spheres of one kilogram each will be cut for further studies.
First, the balls must be compared with the old kilogramm prototype, in order to determine exactly their actual mass. Then the spherical geometry has to be measured precisely. Finally, the scientists of the Avogadro Project will determine the atomic lattice. All these measurements must be extremely precise with an error margin of less than one in a hundred million. If these conditions are fulfilled, one can compute the number of the silicon atoms in the crystal ball and thus specify a mass unit that does not change over time.
The raw material was very precious already: Several weeks ago, Russian scientists delivered 6 kilograms of highly purified silicon 28. The rod costs approximately 1,2 million Euro. After further purification processes and after growing a single crystal out of it, the remaining 5 kilograms of crystalline silicon 28 cost approximately 2 million Euro, more than 20 times the price of gold.