Non-destructive determination of the Silver Content in Roman Coins dated to 308-310 A.D. by combined Use of the PIXE-alpha, XRF and DPAA Techniques

The quantitative determination of the composition of ancient coins can give useful information to archaeologists, numismatists and conservators. The use of non-destructive techniques is preferable especially when a large number of samples have to be investigated. However analyses have to be performed with caution since ancient alloys present often surface effects due to corrosion/oxidation processes or to the manufacturing techniques (the presence of enriched patinas with nobles metals) that produce meaningful variation of the composition respect with the interior of the coins. At the INFN/CNR LANDIS laboratory of Catania, a non-destructive protocol of analysis, based on the combined use of the portable PIXE-alpha [1] and XRF spectrometers and of the new DPAA (Deep Protons Activation Analysis) method [2], was developed for studying the Roman "folles" dated to 300 - 330 A.D. belonging to the Misurata treasure (Lybia), one of the most important finding of coins in the Mediterranean Area. The use of the PIXE-alpha system allowed to determine the composition of the few microns thick silver-enriched patina of the bronze coins while the DPAA techniques, making use of high energy proton beams, was used to determine the silver content in the core of the samples, minimizing the surface effects due to the enriched patina; the XRF techniques was used to explore the composition of the interface between the patina and the substratum of the coins. In the present work the data obtained from the analysis of 15 coins dated back to 308-310 A.D. are presented and discussed; results allowed archaeologists to make some historical hypothesis about this period of the Roman empire.