A research team headed by Dr. Ken-ichi Fujii of the National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has succeeded in upgrading the accuracy of the Avogadro constant, which is crucially important for establishing a atomic mass standard, based on the number of atoms, in collaboration with the Institute for Reference Materials and Measurements (IRMM) of the European Union. The new value was recognized by the Committee on Data for Science and Technology (CODATA). A set of fundamental physical constants totally revised on the basis of the AIST-IRMM measurement was published on December 9, 2003. A fundamental physical constant reported from Japan was officially approved by the CODATA.

The Avogadro constant refers to the number of atoms or molecules contained in a mole of matter. Some of important quantum mechanical constants related to the microscopic world involving atoms and subatomic particles, such as the Planck’s constant can be derived from the Avogadro constant. As these constants are of particular importance among many constants used in physics and chemistry, they are designated as fundamental physical constants. They have crucial significance for science and technology, constitute the underlying bases for many other physical constants and exert spin-off effects to a broad spectrum of science.

The technique used for determining the Avogadro constant at NMIJ/AIST and IRMM is termed the X-ray crystal density method. MIJ/AIST was responsible for measuring the density of silicon crystal and lattice constant (i.e. interatomic distance), while IRMM took care of determining molar mass (average of atomic masses). NMIJ/AIST developed a laser interferometer (Photo 1) capable of assessing geometrical shape of a 1-kg silicon sphere of tens of nanometer sphericity fabricated through an ultra-precision polishing process with an accuracy of a few nanometers.

Furthermore, a breakthrough metrological method to identify a density difference of silicon crystal with an extremely high accuracy was developed (Photo 2). Consequently, the crystal density could be determined with an accuracy as fine as eighth order of magnitude to succeed in improving the accuracy of the Avogadro constant. The result was published jointly by MIJ/AIST and IRMM on a US science journal, the IEEE Transactions on Instrumentation and Measurement 52(2): 646-651, 2003. The study upgraded the accuracy of the Avogadro constant to 2x107, the best value ever achieved through a direct measuring technique without depending upon other fundamental physical constants. The data was appreciated by CODATA, and overall renovation of around 200 physical constants followed.

Photo 1. A laser interferometer to measure diameter of a silicon sphere.

Photo 2. A setup for ultra-precision comparison of solid density.

The effective figure of the Avogadro constant is currently at a level of 107. If the value could be upgraded by another digit, it would become possible to redefine the mass unit based on the international prototype of the kilogram, replacing with an atomic mass standard based on the number of atoms. The accomplishment of MIJ/AIST and IRMM will serve not only for consolidating fundamental physical constants, basic data of science and technology to be shared by human beings, but also opening the way to the implementation of atomic mass standard.

The Comité International des Poids et Mesures (CIPM), composed of delegates from member countries of the Convention of Meter, is going to launch an international project for further upgrading the accuracy of the Avogadro constant, and MIJ/AIST is expected to join the project. Besides, the method of ultra-precision comparison of solid density makes it possible to quantify microscopic defects within a silicon crystal, as never before, and is expected to provide an innovative means of crystal characterization to the semiconductor industry.