Elemental Analysis of Achaemenid and Parthian Silver Coins Using PIXE Technique and Comparing their Silver Content Using Statistical Analysis

Researchers and archaeologists have always been interested in the elemental analysis of ancient Iranian coins. However, a systematic analysis of the Achaemenid coins is still lacking. The Iranians minted coins since the time of the Medes. Darius I (552-486 B.C) is the first Achaemenid king have used the coinage system to regulate trade across his empire. The Achaemenid gold coins were called Daric, while their silver coins were called Sigloi. Daric was a thick gold coin, which had a standard weight of 8.4 grams. The gold used in the coins was of very high quality with a purity of 95.83%. The Sigloi weight about 5.6 grams and its purity was more than 96%. The face of the coins carried an image of a Persian bowman, thought it might be the King himself. The flip side of the coins bear irregular indentations. The Achaemenid coins bear no inscription, hence it is not possible to find the date of mintage with any accuracy. The Achaemenid coins were mainly minted in Sardis, Lyida, Lybia, Pamphilia, and Sidon. During the Parthian period (247 B.C – 224 A.D), commerce was carried out with silver, copper, and bronze coins. The face of the coins bore a relatively accurate image or profile of the King, and the flip side typically (for about 5 centuries), bore the image of of Arsaces sitting on a throne and holding a bow . This indicates the respect the Parthian kings held for the founder of their dynasty. The Parthian coins have markings that are abbreviations for city where the coins were minted. From these markings it is clear that coins were minted in Nisa, Dara, Hekatompylos (near present Damghan), Thymbrax (Sari), Syrinc (near Sari), Ecbatana (near modern Hamedan), Susa, Seleucia, Raga (Ray), Marv, Heart, Kangavar and Nahavand.  In this work, 27 Achaemenid silver coins and 62 Parthian silver coins were analyzed. The Achaemenid coins and 20 of Parthian coins are belonging to the National Museum of Iran and the rest of Parthian coins are kept in Tamashagah-e Pool Museum. The concentrations of the chemical elements presenting in the coins have been measured using PIXE analysis. This analysis is carried out by bombarding the samples with protons accelerated through a 2MV electric potential to an energy of 2 MeV. The proton beam current that was used in this analysis was between 2 and 3 Nano-amperes. The beam was supplied by the 3MV Van de Graff accelerator belonging to the Physics and Accelerators School of the Nuclear Science and Technology Research Center. As a result of the proton bombardment, characteristic X-rays are emitted. The energy of these X-rays is specific to each element and can be used to identify the elements present in the sample. The number of X-ray photons of a specific energy can be used to determine the concentration of these same elements within the sample. With this technique, one can measure the concentration of Ag, Au, Pb, Zn, Al, S, Cu, Cl, Ca, Ti, Mn, and Fe in the coins with an accuracy of parts per million (ppm). Since the metallic composition of the coins is of most importance in deciding their value, we measured the concentration of metals in these coins. The average concentration of silver in the Achaemenid and Parthian coins, were 91% and 86% respectively. The Parthian coins also contained three times as much copper impurity as the Achaemenid coins and 5 times as much gold impurity. However, the concentration of lead and iron impurities in the Parthian coins were less than those of the Achaemenid coins. The statistical analysis of these coins indicated that the Achaemenid coins can be separated from the Parthian coins through elemental analysis. This analysis also showed that most of the Achaemenid coins in our sample were minted in the same place.