engleski

IBA and NAA as complementary non-destructive tools for the authentication of silver coinage

The IAEA coordinated research project entitled ‘Enhancing Nuclear Analytical Techniques to Meet the Needs of Forensics Sciences’ (CRP F11021) [1, 2] develops a specific program targeting the authentication of Cultural Heritage goods. One application domain is the analysis of precious metals, in particular of collectible ancient silver coinage potentially exposed to counterfeiting, for which the synergetic analytical merits of IBA and neutron activation analysis (NAA) and their non- destructive character (which allows to preserve evidence) are highly adapted. The contribution reports the results of an inter-laboratory measurement program on reference silver targets and authentic ancient silver coins implying particle accelerators and neutron-based laboratories worldwide. First, the analytical performances of PIXE and of two NAA methods, namely prompt -ray activation analysis (PGAA) and instrumental neutron activation (INAA) were explored in terms of accuracy and sensitivity, and their complementarity for the determination of fineness, i.e. the concentration in Ag, was evaluated. XRF, a routine method for the analysis of alloys, was applied for sake of comparison. The second objective was to test and validate the interoperability of this approach through an international inter- comparison program. The studied materials are four Ag certified reference standards (MBH AGA1, AGA3, AGQ2 and Ag500) and a series of Portuguese Reis silver coins from the 17th to 19th centuries and French 5 Francs from the 1960’s. The results highlight the benefits of combining PIXE and neutron activation methods to determine the alloy composition for a wide range of elements where they outperform XRF in terms of precision and sensitivity. When measuring coins instead of standard reference materials, ascertaining the well-established legal Ag-Cu alloy content may be hindered by the Cu surface loss over time. The Cu concentrations measured using PIXE appear biased by the low probing depth of the method (max 20 µm) while NAA, as a bulk analyzing technique, gives the correct composition. PIXE allows measuring a dozen of impurities of the silver alloy, notably Au, Pb, Bi, Zn, Pd and Sb which are potential tracers of the origin of the metal. In spite of the coins surface composition alteration it is shown that the trace element concentrations determined by PIXE still reflect the silver fingerprint. [1] https://www.iaea.org/newscenter/news/new- research-project-to-focus-on-use-of-nuclear- techniques-in-forensic-science

IBA ; NAA ; authetication ; cultural heritage ; silver coinage

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