This paper investigates the use of a method based on Cu decoration and neutron activation to determine the total volume of voids in a silicon single crystal. A measurement protocol was developed and tested in an experiment carried out with a 5 cm3 volume and 10 g mass high-purity natural silicon sample. The few percent uncertainty reached in the determination of the Cu concentration, at a 1014 cm-3 level, makes this method a candidate to set an upper limit to the concentration of the vacancies contributing to the void volume in the enriched silicon material used to determine the Avogadro constant.