Proton beam irradiation induces invisible modifications under the surface of painted parchment

Ion beam analysis plays an important role in cultural heritage (CH) studies as it offers a combination of simultaneous and complementary analytical techniques (PIXE/PIGE/RBS) and spatially resolved mapping functions. Despite being considered non-destructive, the potential risk of beam-induced modifications during analysis is increasingly discussed. This work focuses on the impact of proton beams on parchment, present in our CH in form of unique historical manuscripts. Parchment is one of the organic, protein-based CH materials believed to be the most susceptible to radiation-induced changes. Various modification patterns, observed on parchment cross-sections by optical and electron microscopy are reported: discoloration (yellowing), formation of cavities and denaturation of collagen fibers. Considerable modifications were detected up to 100 µm deep into the sample for beam fluences of 4 µC/cm2 and higher. The presence of ultramarine paint on the parchment surface appears to increase the harmful effects of proton radiation. Based on our results, a maximum radiation dose of 0.5 µC/cm2 can be considered as ‘safe boundary’ for 2.3 MeV PIXE analysis of parchment under the applied conditions.