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.

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 July 28, 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 July 28, 2014).

Share This




More Matter & Energy News

Monday, July 28, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Europe's Highest Train Turns 80 in French Pyrenees

Europe's Highest Train Turns 80 in French Pyrenees

AFP (July 25, 2014) — Europe's highest train, the little train of Artouste in the French Pyrenees, celebrates its 80th birthday. Duration: 01:05 Video provided by AFP
Powered by NewsLook.com
TSA Administrator on Politics and Flight Bans

TSA Administrator on Politics and Flight Bans

AP (July 24, 2014) — TSA administrator, John Pistole's took part in the Aspen Security Forum 2014, where he answered questions on lifting of the ban on flights into Israel's Tel Aviv airport and whether politics played a role in lifting the ban. (July 24) Video provided by AP
Powered by NewsLook.com
Creative Makeovers for Ugly Cellphone Towers

Creative Makeovers for Ugly Cellphone Towers

AP (July 24, 2014) — Mobile phone companies and communities across the country are going to new lengths to disguise those unsightly cellphone towers. From a church bell tower to a flagpole, even a pencil, some towers are trying to make a point. (July 24) Video provided by AP
Powered by NewsLook.com
Algonquin Power Goes Activist on Its Target Gas Natural

Algonquin Power Goes Activist on Its Target Gas Natural

TheStreet (July 23, 2014) — When The Deal's Amanda Levin exclusively reported that Gas Natural had been talking to potential suitors, the Ohio company responded with a flat denial, claiming its board had not talked to anyone about a possible sale. Lo and behold, Canadian utility Algonquin Power and Utilities not only had approached the company, but it did it three times. Its last offer was for $13 per share as Gas Natural's was trading at a 60-day moving average of about $12.50 per share. Now Algonquin, which has a 4.9% stake in Gas Natural, has taken its case to shareholders, calling on them to back its proposals or, possibly, a change in the target's board. Video provided by TheStreet
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:
from the past week

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