The Avogadro experiment and the watt balance are the pillars of the redefinition of the kilogram. They represent two independent possibilities of realizing the redefinition. Today, the unit "kilogram" is defined as the mass of the international prototype of the kilogram which is kept in a safe in Sèvres near Paris. International efforts are aimed at relating all base units, such as the second, the meter, the kilogram and the ampere, in future only to fundamental constants in order to create a system of units which is independent of artifacts. For this purpose, it is determined in the Avogadro experiment how many atoms are contained in almost perfect silicon spheres. The next possibility for a redefinition of the kilogram will present itself on the occasion of the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) in 2018. At the same time, the results of the Avogadro experiment must beconsistent with those of the watt balance. It is only then that a safe and exact redefinition of the unit of mass is guaranteed. The realization of the Avogadro experiment is based on the contributions of different working groups of PTB and of national and international partners.
Trailer zum Avogadro-ProjektTrailer zum Avogadro-Projekt
Was muss man tun, um die rundesten Kugeln der Welt zu „bauen“? Wie sehen die Arbeitsschritte dafür aus? Wie das Handling der Kugeln, die Lagerung, der Transport und einige der Messungen? Der Videoclip zeigt, wie die Kugeln zu handhaben sind, und macht die Vorteile dieser möglichen „Massenormale“ deutlich.
A chopper-stabilized amplifier with excellent properties has been developed at PTB and is used in a measurement set-up for the Avogadro Project to measure voltage on thermocouples. This has led to a reduction in noise amplitude by a factor of 4.
The second and the metre have already surged ahead. The kilogram, the ampere and the other physical base units are now trying to catch up. This is one way of describing what is currently happening in the world of metrology, in the world of measuring. The International System of Units is facing a redefinition of its foundations: fundamental constants are to serve as the defining reference values...
Zur gezielten Fertigung spannungsfreier Einkristalloberflächen, die als Laserspiegel in kombinierten optischen und Röntgeninterferometern dienen, wurde ein geschlossener Bearbeitungskreislauf entwickelt. Einmal auf den Polierkopf montiert wird das Werkstück in dieser Einheit in die Schleif-/Poliermaschine, den Ebenheitsprüfer und das Orientierungsdiffraktometer eingebracht. Netzebenenabweichungen...
Die Messung des Gitterparameters von Silizium mit Röntgeninterferometern verlangt die extrem präzise Relativbewegung von zwei Teilen eines Interferometerkristalls mit Winkelfehlern bis zu 0,001 arcsec im Vakuum ruckfrei über einige Millimeter. Als Teil der Kombination einer hochpräzisen Linearführung mit einer geregelten Winkelfeinverstellung wurde ein kinematisch beschränkter...
In Vorbereitung für die Zeit nach einer Neudefinition des Kilogramms werden bereits jetzt Untersuchungen durchgeführt, die für das Monitoring von Masse-Gebrauchsnormalen auf Basis von Siliziumkugeln erforderlich sind. Mit spektralellipsometrischen Messungen werden dazu die Schichtdickentopografien verschiedener in der PTB hergestellten Siliziumkugeln erfasst und miteinander verglichen, um den...
Working Group 3.44 "Avogadro Constant" coordinates the research activities at PTB and the cooperation with institutes all over the world for the precise determination of the Avogadro constant. This is to create a foundation for the redefinition of the unit of mass "kilogram" on the basis of a fundamental constant.
Preparation of the spheres
The material of the spheres was manufactured in Russia by the Central Design Bureau of Machine Building in St. Petersburg and by the Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences in Nishni Novgorod. Then, single crystals without dislocations were drawn in the Leibniz-Institut für Kristallzüchtung (IKZ) in Berlin, and from these, the spheres were manufactured. WG 4.33 "X-ray Optics" prepares the samples of the 28Si crystal, and WG 5.56 "Manufacturing Technology" manufactures the spheres with as little a form deviation as possible.
Determination of the isotopic ratio and of the molar mass
The isotopic ratio and the molar mass are measured at PTB's Working Group 3.11 "Inorganic Analysis" by means of isotope dilution mass spectrometry (IDMS). (The relative atomic masses of the naturally occurring silicon isotopes 28Si, 29Si and 30Si are known with sufficient accuracy from other measurements.)
Determination of the volume in the sphere interferometer
The volume of the silicon spheres is determined by WG 5.41
"Interferometry on Spheres" at 20 °C and in vacuum with the aid of a spherical Fizeau interferometer. In cooperation with WG 4.33 "X-ray Optics", this working group also performs numerical simulations for the interferometer. The temperature measurements of the volume and lattice parameter determinations are compared with one another and with the temperature realization of WG 7.42 "Applied Thermometry" with the aid of an electronic temperature reference point (20 °C) which has been developed by WG 3.43 "Solid State Density ".
On the surface of the spheres, an oxide layer and possibly other surface layers can be found which must be taken into account when determining the sphere volume and the sphere mass. The required knowledge of the thickness and of the structure of the surface layers on the spheres is determined by the following Working Groups with the methods indicated:
Density comparisons of the spheres and of material samples
WG 3.43 "Solid State Density" performs density comparisons of the spheres and of other samples. The density of the thermal oxide on silicon spheres has also been determined in this WG with the aid of density comparison measurements. Working Group 3.43: "Solid State Density"
Measurement of the lattice parameter
WG 4.33 "X-ray Optics" establishes X-ray interferometers for relative and absolute measurements of the lattice parameter. The optical part of the X-ray scanning interferometer is manufactured by WG 5.21 "Length and Angle Graduations".
Investigations of impurities of the sphere material
WG 3.14 "Optical Analysis" determines the contents of the main impurities boron, carbon, oxygen and nitrogen by means of low
temperature infrared spectroscopy. A result of the impurities is that the mass of the real silicon sphere differs slightly from the mass of a sphere which consists only of silicon atoms.
Working Group 3.14: Optical Analysis
Nationale und Internationale Partner
National and international partners
Dresden University: Measurement of the hydrogen concentration in the crystal by Deep Level Transient Spectroscopy (DLTS)
Halle-Wittenberg University: Measurement of the vacancy concentration in the crystal by positron annihilation experiments
Federal Institute of Metrology (METAS, Switzerland): Measurement of the surface layers of the Si spheres with X-ray photoelectron spectroscopy (XPS)
National Institute of Metrology (NIM, China), National Institute of Standards and Technology (NIST, USA) and National Research Council of Canada (NRC): Measurement of the molar mass of isotopically enriched silicon with IDMS
International Bureau of Weights and Measures (BIPM): Mass determinations of the silicon spheres in air and in vacuum and guarantee of the traceability to the international prototype of the kilogram
Istituto nazionale di ricerca metrologica (INRIM, Italy): Lattice parameter measurements at 20 °C and in vacuum with the aid of an X-ray scanning interferometer
NMIA: Diameter measurements of spheres at 20 °C and in vacuum with the aid of a Saunders interferometer
National Metrology Institute of Japan (NMIJ, Japan)
oMeasurement of the mass of the spheres in air and in vacuum
Measurement of the diameters at 20 °C and in vacuum with an interferometer
Investigation of the oxide layer thickness with spectral ellipsometry
Investigation of the homogeneity of lattice distances in the 28Si crystal
National Institute of Metrology (NIM, China): Measurements in the fields of the molar mass, the volume, the surface layers and the mass
