Primary mass standards based on atomic mass

Project Description

The CIPM recommended last year (CI-1993, recommendation no. 4) that laboratories should pursue their work in developing independent methods of monitoring the stability of mass standards. Ultimately the aim would be to monitor the stability of the international kilogram prototype and in due course open the way to a new definition of the unit of mass based upon fundamental or atomic constants:

1 kg = 10^3 * {NA} * µ,

with NA, the Avogadro constant, and µ, the atomic mass unit. For the determination of NA, the density, molar mass and atomic volume of a highly pure silicon crystal will be measured by IRMM, IMGC and PTB. The atomic mass unit will be measured by GSI and MSL.

The present project concerns a wide European effort on that part of this research aimed at new and improved primary mass standards based specifically on atomic mass.

The work, whlch has been in progress many years, covers a broad scope, ranging from relative atomic mass determination (in ion spectrometry, to studies of macroscopic samples of silicon, including crystallography, density determination and mass stability.

The intention is to help coordinate the research and the seeking of funds from appropriate European Community programmes.

Final Report 2003-03-05

The project has summarised the activities to replace the kilogram artefact by the mass of a certain number of silicon atoms, which were planned and performed also within the scope of a worldwide collaboration coordinated by of the Working Group on the Avogadro Constant (WGAC) of the CIPM Consultative Committee for Mass and Related Quantities (CCM).
Defining the unit of mass by such an atomic procedure requires a very accurate determination of the Avogadro constant, NA, with a relative uncertainty close to 1x10-8. At present the most important limiting factor is the measurement of the molar mass of silicon, e.g. the determination of the natural isotopic composition. Recently, several improvements in crystal characterisation and density determination have been achieved and a relative uncertainty of about 10-7 could be reached. Using single crystals of natural silicon, the uncertainty attained for that value, especially for the molar mass, has now reached practical limits.

The available data are all contained in a database [i]. The combination of this value with the 1998 CODATA recommended value of the Si lattice parameter [ii] leads to an Avogadro constant NA = 6. 022 133 9(27) x 1023 mol-1, a candidate for consideration in future adjustments of the values of the constants by CODATA. This value is in disagreement with the CODATA 1998 recommended value for NA based on Planck´sconstant as determined mainly by watt balance experiments by more than 1 x 10-6, relatively.
Further reduction of the uncertainty using a 99.99% 28Si crystal is planned and subject for a new international cooperation.