Short- and long-term effect of radiation damage in cotton paper The analysis of cultural heritage artefacts To recreate conditions as close as possible to a real synchrotron with ionizing radiation has rapidly increased experiment, in this study we used the PUMA beamline (Figure 1). The paper samples were irradiated with different X-ray doses in popularity over the last decades - in using the macro beam (1 x 2 cm²) and with a photon energy particular at synchrotron sources - leading to of 7.22keV (wavelength 1.7 Å). The effect of paper moisture many fascinating new discoveries. However, content was investigated using different silica gels to maintain three different relative humidity levels during irradiation (Figure 1a). the damage inflicted to the materials under study during the irradiation is still largely FIGURE 1 unknown. Organic samples, such as paper, are especially vulnerable to degradation by X-rays. In this work we quantify the radiation damage on a molecular level and show that the chemical reactions triggered promote cellulose degradation and continue to alter the material for several months after the irradiation took place. Paper artefacts are an important part of our cultural heritage. MONITORING THE RADIATION DAMAGE Aside from drawings and other paper-based artworks, archival ON THE MOLECULAR SCALE and library material such as private letters and public documents also give us a unique insight into the written heritage of past When evaluating radiation damage, it is important to look generations. A large part of this immense cultural heritage beyond macro-scale observations, such as a color change for treasure is still hidden from the public at large, but is of enormous instance, and focus on the changes incurred in the molecular importance to historians and codicologists. Special care is needed structure of the material, due to the interaction with photons. To for their conservation as they can easily be destroyed by fire, characterize the depolymerization, indicator of hydrolytic scissions water damage or attacks from microorganisms and insects. of cellulose, and hence molecular length shortening and fiber While the composition of paper is complex and varied considerably damage, size-exclusion chromatography (SEC-MALS-DRI) was over the course of history, its basic structural level consists of used. High-performance liquid chromatography (HPLC-FLD- a network of plant fibers made of cellulose. Cellulose is the DAD) allowed the quantification of hydroxyl free radicals, which most abundant naturally occurring macromolecule on earth. are indicators of the cellulose auto-oxidation phenomena. This It is a polysaccharide with the chemical formula (CHO). To approach was complemented by macro-scale observations 6 10 5n such as UV luminescence, spectrometry and photography. understand the fundamental radiochemistry of paper, in this research we used a simple paper (Whatman no. 1), made of cotton linters, which is almost exclusively cellulose (99%). SHORT AND LONG TIME SCALES Our analysis showed that chemical damage happened on the IRRADIATION molecular scale during the irradiation, starting from a relatively The use of ionizing radiation, and especially synchrotron X-ray, low dose of 21 Gy. It was observed that the higher the dose for the analysis of paper artefacts has increased rapidly over the of SR-X radiation absorbed by the paper, the greater the last decades [1]. For example, the use of X-ray fluorescence depolymerization and the higher the concentration of hydroxyl imaging allows scientists to reveal blacked-out ink writings, thus free radicals. This behavior can be seen in Figure 2, which unearthing information that was beyond reach for centuries [2] shows glycosidic scissions as a function of the dose for paper However, when the X-ray photons interact with cellulose, they samples irradiated at different relative humidity conditions. The can break chemical bonds and depolymerize the molecules. impact of humidity is unmistakable; there is less damage in This initiates further chemical degradation that can continue on humid paper than in dry paper. larger timescales than the irradiation duration. 74