International Avogadro Project
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| International Avogadro Project | |
|---|---|
| Name | International Avogadro Project |
| Formation | 1990s |
| Purpose | determination of the Avogadro constant and realization of the kilogram |
| Headquarters | Various laboratories |
| Region served | International |
| Key people | Paul J. B. Hodgkinson; Richard S. Davis; Peter Becker |
International Avogadro Project The International Avogadro Project was a multinational metrology effort to determine the Avogadro constant with extreme accuracy using silicon-sphere methods, intended to underpin the 2019 redefinition of the kilogram and to interlink atomic-scale constants with macroscopic mass standards. The effort connected national metrology institutes such as Physikalisch-Technische Bundesanstalt, National Physical Laboratory (United Kingdom), National Institute of Standards and Technology, and International Bureau of Weights and Measures laboratories, and drew on expertise from crystallography, surface science, and materials engineering. The project produced a chain of measurements that complemented efforts using the Watt balance (also known as the Kibble balance) and influenced decisions at meetings of the International Committee for Weights and Measures.
Background and Objectives
The project originated from initiatives at Bureau International des Poids et Mesures-affiliated laboratories and proposals discussed at the General Conference on Weights and Measures and in workshops involving International Organization for Standardization observers. Its primary objective was a high-precision determination of the Avogadro constant (NA) by counting atoms in nearly perfect single-crystal silicon spheres, linking atomic masses to the macroscopic kilogram prototype held at the International Prototype of the Kilogram. Secondary objectives included development of ultra-pure silicon growth techniques pioneered at facilities associated with Siemens and Torch glassmaking research, refinement of x-ray crystal density methods used at European Space Agency-linked labs, and standardization of surface layer characterization methods used at Centre National de la Recherche Scientifique laboratories.
Methodology and Experimental Techniques
The methodology combined precision sphere fabrication, x-ray crystallography, optical interferometry, and chemical and isotopic analysis. Single-crystal silicon spheres were produced from Float-zone silicon and Czochralski process boules with isotopic enrichment in 28Si through centrifuge and laser-based separation developed in collaboration with research teams at Max Planck Society institutes and companies like Mitsubishi Electric. Lattice parameter measurements used combined x-ray and optical interferometry techniques refined at Physikalisch-Technische Bundesanstalt and National Metrology Institute of Japan (NMIJ). Mass and volume determinations employed optical interferometers related to designs from National Institute of Standards and Technology and mirror-polished spheres were characterized by atomic force microscopy and ellipsometry methods developed in Lawrence Berkeley National Laboratory-collaborative studies. Isotopic composition assays relied on multi-collector inductively coupled plasma mass spectrometry as practiced at International Atomic Energy Agency-linked labs and on thermal ionization mass spectrometry protocols from Oak Ridge National Laboratory.
Results and Impact on the Kilogram Redefinition
The project produced determinations of NA with uncertainties competitive with Kibble-balance determinations of the Planck constant, presenting values that were considered by delegations from France, United Kingdom, United States, Germany, Japan, and Italy at CGPM sessions. Combined analyses influenced the consensus that led to the 2018 resolution redefining the kilogram based on a fixed value of the Planck constant, ratified at the General Conference on Weights and Measures in 2018 and implemented in 2019. The silicon-sphere results were cross-compared with Kibble-balance results from National Research Council (Canada), Laboratoire National de métrologie et d'Essais, and Physikalisch-Technische Bundesanstalt to validate the new definition, and contributed to improved realization strategies for national mass standards at institutes such as VNIIM and KRISS.
International Collaboration and Organization
The project operated through formal collaborations among national metrology institutes, coordinated working groups under the auspices of the International Bureau of Weights and Measures and the Consultative Committee for Mass and Related Quantities, and through technical committees including members of Comité International des Poids et Mesures subgroups. Funding and technical contributions came from agencies like European Commission research programs, national research councils such as the Engineering and Physical Sciences Research Council and Japan Society for the Promotion of Science, and from interlaboratory exchanges modeled after CIPM MRA comparisons. Regular meetings were held at sites including PTB, NPL, and BIPM headquarters, and results were published in collaboration with journals associated with Royal Society publishing and the Institute of Physics.
Criticisms, Limitations, and Controversies
Criticisms addressed the difficulty of achieving consistent sphere surface characterization across institutes and discrepancies between silicon-sphere and Kibble-balance results that prompted scrutiny at International Committee for Weights and Measures meetings. Limitations included dependence on isotopic enrichment facilities regulated by export controls involving Wassenaar Arrangement considerations, supply-chain constraints tied to semiconductor industry partners like Intel and TSMC, and debates over uncertainty budgets raised by delegations from Russia and China. Controversies also emerged concerning proprietary fabrication techniques owned by commercial partners, and about the relative cost-effectiveness of maintaining parallel realization routes compared to consolidating around the Kibble balance model advocated by some working groups.
Legacy and Subsequent Developments
The project left a legacy of improved crystallographic methods, standardized surface metrology protocols, and new isotope-ratio measurement capabilities that have been adopted by institutes including PTB, NPL, and NIST. It fostered sustained collaboration between metrology, semiconductor, and surface-science communities represented by institutions such as Max Planck Society, Lawrence Livermore National Laboratory, and CERN. Subsequent developments include applications of silicon-sphere techniques in nanoparticle metrology used by ISO committees and ongoing comparisons between atomic-count and Kibble-balance realizations carried out under the oversight of BIPM and discussed at CGPM sessions. The methodological advances have also influenced precision measurements in fields associated with CODATA adjustments and high-accuracy determinations of fundamental constants.