Differential PIXE analysis of Mesoamerican jewelry items

Abstract

Gold jewelry items of Mesoamerican origin (from Peru, Colombia, Mexico, etc,…) are usually cast in Tumbaga: a man-made gold-copper-silver alloy containing a large proportion of copper. In order to give the objects a colour close to that of pure gold, ancient Mesoamerican goldsmiths experimented with a procedure to eliminate less noble metals (like copper and silver) from the surface. RBS may be used to identify a possible enrichment in gold in the most external layer of the items but due to the low capability of this technique to separate scattered particles on gold and silver and due to the low Rutherford cross section for α-particles on copper by comparison with those on gold, the determination of the exact depth depletion of copper cannot be easily reached. Differential PIXE is an appropriate method to achieve this goal. It takes the relative X-ray intensities of Cu and Au lines into account. By varying the incident proton energy, this ratio is modified in a completely different way if the sample is homogeneous or exhibits a layered or depth profile structure.