  The determination of the Avogadro constant plays a key role in the
redefinition of the kilogram in terms of a fundamental constant. The present
experiment makes use of a silicon single-crystal highly enriched in 28Si that
must have a total impurity mass fraction smaller than a few parts in 109. To
verify this requirement, we previously developed a relative analytical method
based on neutron activation for the elemental characterization of a sample of
the precursor natural silicon crystal WASO 04. The method is now extended to
fifty-nine elements and applied to a monoisotopic 28Si single-crystal that was
grown to test the achievable enrichment. Since this crystal was likely
contaminated, this measurement tested also the detection capabilities of the
analysis. The results quantified contaminations by Ge, Ga, As, Tm, Lu, Ta, W
and Ir and, for a number of the detectable elements, demonstrated that we can
already reach the targeted 1 ng/g detection limit.
