30Si Mole Fraction of a Silicon Material Highly Enriched in 28Si Determined by Instrumental Neutron Activation Analysis

The latest determination of the Avogadro constant, carried out by counting the atoms in a pure silicon crystal highly enriched in 28Si, reached the target 2x10-8 relative uncertainty required for the redefinition of the kilogram based on the Planck constant. The knowledge of the isotopic composition of the enriched silicon material is central; it is measured by isotope dilution mass spectrometry. In this work, an independent estimate of the 30Si mole fraction was obtained by applying a relative measurement protocol based on Instrumental Neutron Activation Analysis. The amount of 30Si isotope was determined by counting the 1266.1 keV gamma-photons emitted during the radioactive decay of the radioisotope 31Si produced via the neutron capture reaction 30Si(n,gamma)31Si. The x(30Si) = 1.043(19)x10-6 mol mol-1 is consistent with the value currently adopted by the International Avogadro Coordination.