Measurement of the Si Mole Fraction in the New Avogadro Silicon Material by Neutron Activation and High-Resolution γ-Spectrometry.

The use of new silicon single crystals highly enriched in Si recently produced for the upcoming redetermination of the Avogadro constant requires knowledge of their molar masses. The isotopic composition data are collected independently in different laboratories but all using the virtual element technique with multicollector inductively coupled plasma mass spectrometers. In this framework, the comparison of the results with an independent measurement of the amount of at least one of the depleted isotopes is useful to limit hidden systematic errors. To this aim, the Si mole fraction of a sample of the new material was measured using a relative measurement protocol based on instrumental neutron activation analysis. The protocol is similar to that previously applied with the AVO28 silicon material used for the last determination of the Avogadro constant value with the exception that unknown and standard samples are not coirradiated. The x(Si) = 5.701 × 10 mol mol estimate is close to the expected one and is given with a standard uncertainty of 8.8 × 10 mol mol. This value, if adopted, gives a contribution to the relative standard uncertainty of the Avogadro constant of 6.3 × 10.