The 'new' kilogram is approaching: Avogadro constant determined with enriched silicon-28

Feb 09, 2011
The high-purity silicon sphere of the Avogadro experiment reflects a copy of the international kilogram prototype - the last embodiment of a unit via a physical body. The sphere, in contrast, stands for the definition on the basis of atomic properties or fundamental constants. Credit: PTB

A milestone in the international Avogadro project coordinated by the Physikalisch-Technische Bundesanstalt (PTB) has been reached: With the aid of a single crystal of highly enriched 28Si, the Avogadro constant has now been measured as exactly as never before with a relative overall uncertainty of 3 • 10-8. Within the scope of the redefinition of the kilogram, the value NA = 6.02214078(18) • 1023 mol-1 permits the currently most exact realization of this unit. The results have been published in the most recent edition of the journal Physical Review Letters.

The crucial phase of the long-term Avogadro project - which is coordinated by PTB - started in 2003: In that year, several national metrology institutes launched - together with the Bureau International des Poids et Mesures (BIPM) and in cooperation with Russian research institutes - the ambitious project of having approximately 5 kg of highly enriched 28Si (99.99 %) be manufactured as a single crystal, of measuring the Avogadro constant with it and of achieving - by the year 2010 - a measurement uncertainty of approx. 2 • 10-8. Meanwhile, the first measurements have been completed on the two 1 kg spheres of 28Si - which had been polished in Australia - and their density, lattice parameter and surface quality have been determined.

The single steps: After an extensive check of the crystal perfection, the influence of the crystal lattice defects was assessed. Then, the lattice parameter was determined at the Italian metrology institute (INRIM) by means of an X-ray interferometer, and confirmed by comparison measurements with a natural Si crystal at the American NIST. At BIPM, NMIJ (Japan) and PTB, the masses of the two silicon spheres were linked up in vacuum to the international mass standards. In the respective Working Groups of NMIJ, NMI-A (Australia) and PTB, the sphere volume was measured optically - with excellent agreement - by means of interferometers with different beam geometries. The surface layer (basically composed of silicon dioxide) was spectroscopied with electron radiation, X-ray radiation and synchrotron radiation in accordance with different procedures, analyzed and taken into account for the determination of the density. The unexpectedly high metallic contamination of the sphere surfaces with copper and nickel silicides which occurred during the polishing process was measured, and its influence on the results of the sphere volume and of the sphere mass was assessed. This resulted in a higher measurement uncertainty.

What was decisive for the success achieved - i.e. a relative overall measurement uncertainty of 3 • 10-8 - was the development of a new mass-spectrometric method for the determination of the molar mass at PTB.

The result is a milestone on the way towards a successful realization of the new kilogram definition on the basis of fundamental constants whose values have been fixed. At present, the agreement of this value with other realizations of the kilogram is not good enough to change the existing definition of the mass unit. The present state of the Avogadro project is, however, so promising that - on the basis of new measurements with improved sphere interferometers - the measurement uncertainty of 2 • 10-8 demanded by the Consultative Committee for the Mass (CCM) will in the near future be achieved on contamination-free spheres and will probably even be undercut.

Explore further: Superstring theorist at University of Florida wins 2015 Heineman Prize

More information: Andreas, B.; Azuma, Y.; Bartl, G.; Becker, P.; Bettin, H.; Borys, M.; Busch, I.; Gray, M. et al. (2011), "An accurate determination of the Avogadro constant by counting the atoms in a 28Si crystal", Phys. Rev. Lett. 106 (3): 030801 (4 pages), doi:10.1103/PhysRevLett.106.030801

Provided by Physikalisch-Technische Bundesanstalt

4.2 /5 (11 votes)
add to favorites email to friend print save as pdf

Related Stories

Accurate Avogadro constant may help redefine the kilogram

Oct 20, 2010

( -- A new accurate determination of the Avogadro constant has used the method of "counting" the atoms in a 1 kg sample of an almost perfect silicon sphere highly enriched with the isotope 28Si. They used ...

Measuring the density of ultra-pure water

Dec 18, 2007

For oceanography – and there in particular for the description of ocean currents – accurate measurements of the density of sea water are of great importance. For this purpose, measuring instruments are ...

Nano Measurement in the 3rd Dimension

Jul 06, 2009

From the motion sensor to the computer chip - in many products of daily life components are used whose functioning is based on smallest structures of the size of thousandths - or even millionths - of millimetres. ...

New standard mass made with ISTC help

Mar 03, 2005

This program began three years ago and it involves scientists from eight countries. The task is immense. A new standard mass will be created with maximum possible precision on a modern level of development for world science ...

Recommended for you

Cooling with molecules

Oct 22, 2014

An international team of scientists have become the first ever researchers to successfully reach temperatures below minus 272.15 degrees Celsius – only just above absolute zero – using magnetic molecules. ...

User comments : 4

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Feb 09, 2011
How is the sphere held during the experiments?, surely at this level of precision, even touching something else is enough to deform the crystal and knock atoms off?
5 / 5 (1) Feb 10, 2011
NA = 6.02214078(18) • 1023 mol^-1

should be

NA = 6.02214078(18) • 10^23 mol^-1
not rated yet Feb 11, 2011
A more reliable and compact notation would be to simply skip the ten and Karat, and use "E" notation:

NA = 6.02214078(18)E23 /mol

This string uses 7 fewer characters by the time you count spaces, and will work regardless of the formating limitations of the website or other media.
not rated yet Feb 11, 2011
shut your mouth Quantum one wants to hear your shit today, down south d-dbag