Texture analysis of ancient coins with TOF neutron diffraction

Two ancient Greek coins were studied by time-of-flight (TOF) neutron diffraction to investigate their authenticity. Diffraction spectra were analyzed with the Rietveld programs GSAS and MAUD for phase proportions and texture. Both samples revealed very similar textures, consistent with pressing between dies. Also, both samples had similar phase proportions around 85% of silver and 15% of copper. The coin analysis revealed limitations of currently used processing methods for TOF neutron diffraction data. Despite the advantages of neutron diffraction for non-destructive bulk analyses, great care must be paid to the procedures to avoid artifacts. The conventional irregular and sparse coverage used at HIPD-LANSCE is problematic if absorption effects exist and the harmonic texture expansion is extended beyond lmax = 6. Direct texture methods, such as EWIMV, are less affected by coverage limitations and a new method to deal with absorption is introduced.