Gamma-radiation decontamination of paper based cultural heritage - should the recommendations be supplemented

Great amounts of books and different documents are important cultural heritage artifacts, and thus preserving paper based objects is an important mission in the community dealing with cultural heritage. Paper is a versatile material formed from wood pulp or plant fiber and its use goes back to the 2nd century BC. It is a cellulose based material often contaminated by fungi, especially if stored in inadequate conditions, i.e. in moist atmospheres. The use of gamma irradiation for decontamination of cultural heritage is increasing in the past years because of the many advantages of this method compared to the classical treatments for decontamination. Its use on paper based materials is also increasing. It is known that cellulose is degraded by ionizing irradiation, but at which doses and in which conditions is precisely distinguished. In this work we investigated the changes induced by gamma irradiation on paper, and the influence of different irradiation conditions. We investigated different doses commonly used for decontamination of cultural heritage, but also different dose rates and conditions like irradiation of paper in air, pre-autoclaved and pre-vacuumed paper, to determine the influence of such conditions on the irradiation process. The visual changes were investigated by colorimetry, while the changes in the chemical composition were investigated by Fourier-transform infrared spectroscopy. The obtained results have shown that the dose rate under which irradiation is performed makes an important impact on the preservation of paper when it is irradiated in air, i.e. higher dose rates should be applied to preserve the paper. Vacuuming of the paper did not show a difference compared to paper irradiated in air, but decontaminating (autoclaving) did. Namely, the paper that is microbiologically contaminated shows a visual change at lower doses than the non-contaminated paper. These findings are new in literature since most investigations and thus the recommendations for the use of different doses are determined on non-contaminated paper, while our results have shown that these observations are not the same as in real conditions.