The Avogadro Project A 25 Year Quest

The Avogadro constant is an important input parameter for the international effort to redefine the kilogram on the basis of a fundamental physical constant, probably the Planck constant h. The measured uncertainty of this quantity will be the associated uncertainty of the kilogram which is up to now, per definition, zero. The talk will cover the achievements of an international research project aimed at determining the Avogadro constant by counting the atoms in an isotopically enriched silicon crystal. Progress was helped considerably by applying isotope dilution mass spectrometry as a new and extremely accurate method for the determination of the molar mass of enriched silicon. The value of the Avogadro constant is now the most accurate input for the new kilogram approach and differs relatively by only 16 × 10^-8 from the CODATA 2006 adjustment value.

Graphic provided by the speaker.

Immediately following the talk, a poster session featuring NIST metrology programs will be held in the Hall of Flags to mark the occasion of World Metrology Day 2011 and the International Year of Chemistry.

Anyone outside NIST wishing to attend must be sponsored by a NIST employee and receive a visitor badge.

For more information, contact Kum Ham at 301-975-4203.

Colloquia are videotaped and available in the NIST Research Library.

Peter Becker

Department of Quantum Optics and Length PTB (Braunschweig)