Search

The isotopic compositions (molar mass, M) of the first and new silicon crystals highly enriched in $^{28}\text{Si}(> 99.99\ \%)$ used for the determination of the Avogadro constant were measured using a high resolution multicollector ICP mass spectrometer and applying a modified isotope dilution mass spectrometry technique. Relative uncertainties $u_{\text{rel}}(M) were obtained: a reduction by almost three orders of magnitude in ten years. This study compares the homogeneities in M(Si) of the first “AVO28” crystal and the first crystal (Si28-23Pr11) of the “Kilogram-2” project.

An overview of the present results and status of the molar mass of a silicon crystal highly enriched in ²⁸Si used for a new determination of the Avogadro constant is given with special emphasis on the cooperation between NIST, NRC, and PTB currently working on this topic. Using an experimental combination of multicollector ICP and isotope dilution mass spectrometry, a new approach developed by PTB for the measurement of the isotopic composition has been applied. It focuses on the isotope ratio treatment of ³°Si/²⁹Si mainly, avoiding the more uncertain measurement of ratios related to the main isotope ²⁸Si., 2012 Conference on Precision Electromagnetic Measurements (CPEM 2012), July 1-6, 2012, Washington, DC, USA, Series: CPEM Digest