MATERIAL RESPONSE AS A CRITERION FOR THE APPROACH TO RADIATION TREATMENT OF CULTURAL HERITAGE OBJECTS

Radiation decontamination offers fast and effective protection to damaged cultural heritage (CH) objects. Due to wider availability of irradiation sources like 60Co gamma radiation the treatment of CH objects became important in 1970's. At Laboratory for Radiation Chemistry and Dosimetry, (LRCD), Ruđer Bošković Institute, Zagreb, Croatia where CH objects are being treated for almost 25 years [1]. Linear accelerator of electrons (LINAC) is another common type of irradiator that is somewhat less suitable for heritage treatment because of lower penetration and homogeneity of electron beam. Radiation treatment has numerous advantages like radiation being a very effective and universal biocide with almost unlimited penetration, temperature independent, leaving no residues and not inducing secondary radioactivity. CH objects may be packed during treatment to protect them and to prevent recontamination. Radiation consolidation is also an option. In disastrous situations decontamination of CH objects may be necessary to ensure safety of restores and other personnel that handles it. Contaminant response to irradiation is based on experiences attained in radiation sterilization and sanitization of medical materials, pharmaceuticals, food and the like. In processing of CH objects doses are selected according to contamination - up to 0.5 kGy for disinsection, 4-10 kGy for fungi control while 5-20 kGy ensures microbiological decontamination. The dose may be fine tuned depending on the materials that constitute the object and its state. The decision is usually made by irradiation plant staff, in cooperation with restorer and other specialists. In radiation treatment of heritage objects special consideration should be given to protection from any unwanted side effects. In disastrous circumstances fungi and bacteria are likely contaminants so higher doses may be required. In order to meet the high standards of conservators and restores the effects of such doses should be assessed for a particular cultural heritage object. Because of that the knowledge of the material response to irradiation is of utmost importance. The most recent knowledge on the subject will be presented. For example, plain wooden objects can withstand any desired radiation dose other materials are less radiation resistant, especially paper and some textiles. Pigments, varnishes, glass, decorative stones and other ornaments made of various materials that have diverse radiation sensitivities are often present on the same object. The extent of inflicted damage to the heritage object has also to be taken into account when deciding on the radiation dose to be applied. At LRCD we are recently begun investigation of radiation effects in selected materials that are common constituents of CH objects like textiles since sensitivity of natural fibers to biological deteriorants is a serious problem. Examples of studies [2, 3] performed on model and authentic CH textiles will be presented. The tests were conducted on model samples of the most common textile materials in CH items. To ensure that the radiation effects will be observed a high dose in contact with air was applied. A part of the model samples were heat- or UV -aged before exposure to γ-irradiation. Changes were determined by a number of analytical techniques The results indicated that even a dose more than 20 times higher than otherwise recommended caused only slight changes comparable to or lesser than those caused artificial aging. That is another confirmation that textile CH items can be safely radiation treated for fungi control and probably microbiological decontaminated what is especially important in catastrophic circumstances. [1] B. Katušin-Ražem, D. Ražem, M. Braun, Radiat. Phys. Chem. 78 (2009), 7/8 ; 729-731. [2] K. Kavkler, A. Demšar, Polym. Degrad. Stab. 97 (2012), 5 ; 786-792. [3] I. Pucić, K. Kavkler, B. Mihaljević, to be published