Comparison of EDXRF and PIXE in the analysis of ancient gold coins

The metal contents of several ancient uncleaned gold coins, with compositions ranging from about 35% to 99% Au, were determined and compared by using two techniques: EDXRF and PIXE. These techniques have proved to be a powerful tool in numismatic investigations, due to their ability of rapid, simultaneous and nondestructive multielement analysis. The results obtained with the two techniques for coins where gold is present in concentrations above 94% showed to be in good agreement. However, they appeared to diverge for coins with a lower standard of fineness. In this case, the Au contents determined by PIXE are higher than those obtained by EDXRF. This may be due to the well known surface enrichment effect and to the circumstance that the protons used as incident beam in PIXE analysis have a shorter penetration depth than the primary X-rays generated by the XRF tube. (orig.)