Abstract: The Avogadro constant is an important input parameter for the international effort to redefine the unit of mass, the kilogram on the basis of a fundamental physical constant. The so called explicit-constant definition nowadays favours a relationship of the mass unit to 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. A measurement uncertainty below 2 x 10 -8 is therefore required. This paper reports the progress and the achievements of an international research project aimed at determining the Avogadro constant - which can be recalculated in h without losing accuracy - by counting the atoms in enriched 28 Si spheres. The counting procedure assumes a perfect crystal where the molar volume and the atomic volume have to be measured. The progress was possible by applying isotope dilution mass spectrometry as a new and extremely accurate method for the determination of the molar mass of enriched silicon. Because of the unexpected metallic contamination of the surface oxide layer, the present measurement uncertainty has reached 3 x 10 -8 , which is by a factor 1.5 close to the target. The value of the Avogadro constant, NA = 6. 022 140 92 (18) x 10 23 mol -1 is up to now the most accurate input for the new kilogram approach and differs by 1.5 x 10 -7 from the CODATA 2006 adjustment. The value is, therefore, in disagreement to the value published by NIST in 2007, but in agreement with the NPL watt balances result updated in 2010.
Publication Year: 2010
Publication Date: 2010-01-01
Language: en
Type: article
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