Colour changes in proxy rock art paints induced by proton beam irradiation

Particle Induced X-ray Emission (PIXE) is widely used for the elemental analysis of cultural heritage materials; however, irradiation by MeV protons can induce physical and chemical modifications that may compromise the integrity of sensitive samples. In this work, we investigate radiation-induced colour changes in proxy rock art paints subjected to proton irradiation with fluences in the range relevant to PIXE analysis, and higher. Samples consisting of natural colouring materials combined with representative binders and substrates were irradiated using two different accelerator systems: ELISA (at CERN), a compact radio-frequency quadrupole (RFQ) linac delivering a pulsed proton beam; and a conventional tandem accelerator (Tandetron, at the Surrey Ion Beam Centre, SIBC) providing a continuous (DC) beam. Colour changes were quantified using image-based analysis of high-resolution scans acquired before and after irradiation. The dependence of colour change on fluence was characterised; for both of the accelerator systems used, the degree of colour change increased with fluence, saturating at some level. However, the samples tested showed a greater sensitivity to irradiation at ELISA than at SIBC. Although the facilities differ in several respects, one of the main differences is in the beam time structure (DC at SIBC, pulsed at ELISA), raising the possibility that dose rate or temporal structure may play a role in colour modification processes. These findings have implications for the optimisation of PIXE measurements for studies of cultural heritage objects, including in the development of portable systems operating with pulsed beams. Further work is required to establish quantitative damage thresholds and to disentangle the contributions of dose rate, beam profile and material composition.