Featured Research

from universities, journals, and other organizations

The 'new' kilogram is approaching

Date:
February 10, 2011
Source:
Physikalisch-Technische Bundesanstalt (PTB)
Summary:
With the aid of a single crystal of highly enriched 28Si, the Avogadro constant has now been measured as exactly as never before. Within the scope of the redefinition of the kilogram, the value permits the currently most exact realization of this unit.

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.

Related Articles


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 silicon 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.


Story Source:

The above story is based on materials provided by Physikalisch-Technische Bundesanstalt (PTB). Note: Materials may be edited for content and length.


Journal Reference:

  1. B. Andreas, Y. Azuma, G. Bartl, P. Becker, H. Bettin, M. Borys, I. Busch, M. Gray, P. Fuchs, K. Fujii, H. Fujimoto, E. Kessler, M. Krumrey, U. Kuetgens, N. Kuramoto, G. Mana, P. Manson, E. Massa, S. Mizushima, A. Nicolaus, A. Picard, A. Pramann, O. Rienitz, D. Schiel, S. Valkiers, A. Waseda. Determination of the Avogadro Constant by Counting the Atoms in a ^{28}Si Crystal. Physical Review Letters, 2011; 106 (3) DOI: 10.1103/PhysRevLett.106.030801

Cite This Page:

Physikalisch-Technische Bundesanstalt (PTB). "The 'new' kilogram is approaching." ScienceDaily. ScienceDaily, 10 February 2011. <www.sciencedaily.com/releases/2011/02/110209112706.htm>.
Physikalisch-Technische Bundesanstalt (PTB). (2011, February 10). The 'new' kilogram is approaching. ScienceDaily. Retrieved October 24, 2014 from www.sciencedaily.com/releases/2011/02/110209112706.htm
Physikalisch-Technische Bundesanstalt (PTB). "The 'new' kilogram is approaching." ScienceDaily. www.sciencedaily.com/releases/2011/02/110209112706.htm (accessed October 24, 2014).

Share This



More Matter & Energy News

Friday, October 24, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

IKEA Desk Converts From Standing to Sitting With One Button

IKEA Desk Converts From Standing to Sitting With One Button

Buzz60 (Oct. 24, 2014) — IKEA is out with a new convertible desk that can convert from a sitting desk to a standing one with just the push of a button. Jen Markham explains. Video provided by Buzz60
Powered by NewsLook.com
Ebola Protective Suits Being Made in China

Ebola Protective Suits Being Made in China

AFP (Oct. 24, 2014) — A factory in China is busy making Ebola protective suits for healthcare workers and others fighting the spread of the virus. Duration: 00:38 Video provided by AFP
Powered by NewsLook.com
Real-Life Transformer Robot Walks, Then Folds Into a Car

Real-Life Transformer Robot Walks, Then Folds Into a Car

Buzz60 (Oct. 24, 2014) — Brave Robotics and Asratec teamed with original Transformers toy company Tomy to create a functional 5-foot-tall humanoid robot that can march and fold itself into a 3-foot-long sports car. Jen Markham has the story. Video provided by Buzz60
Powered by NewsLook.com
Police Testing New Gunfire Tracking Technology

Police Testing New Gunfire Tracking Technology

AP (Oct. 24, 2014) — A California-based startup has designed new law enforcement technology that aims to automatically alert dispatch when an officer's gun is unholstered and fired. Two law enforcement agencies are currently testing the technology. (Oct. 24) Video provided by AP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:

Strange & Offbeat Stories

 

Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:  

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins