Elemental characterization of the Avogadro silicon crystal WASO 04 by neutron activation analysis

Impurity measurements of the Si-28 crystal used for the determination of the Avogadro constant are essential to prevent biased results or underestimated uncertainties. A review of the existing data confirmed the high purity of silicon with respect to a large number of elements. In order to obtain direct evidence of purity, we developed a relative analytical method based on neutron activation. As a preliminary test, this method was applied to a sample of the Avogadro natural silicon crystal WASO 04. The investigation concerned 29 elements. The mass fraction of Au was quantified to be (1.03 +/- 0.18) x 10(-12). For the remaining 28 elements, the mass fractions were below the detection limits, which ranged between 1 x 10(-12) and 1 x 10(-5).