engleski

Electron spin resonance and thermoluminescence for assessment of color changes in irradiated nacres

Radiation induced color change of nacre that were detected by measurements in CIELAB color space are likely to be caused both by color centers arising of electronic excited states and/or unpaired electrons in free radicals produced by high-energy irradiation. Nacre or mother-of- pearl, a common ornament material in cultural heritage (CH) objects is a natural biocomposite based on crystalline calcium carbonate, predominately in aragonite form. Two different nacre types, one white and the other yellowish, obtained from Museum of Arts and Crafts in Zagreb were irradiated to various doses in the range commonly applied in CH treatment. The goal of this research was to identify the nature of color centers formed upon irradiation of nacre. Electron spin resonance (ESR) was used to detect and identify species containing unpaired electrons Thermoluminscence (TL) provided information on trapped electrons and holes since on heating trapped states rapidly decay into lower-energy states with emission of photons. Even for unirradiated nacres both TL and ESR results were significantly different. Strong signals of Mn in various ESR-active states were observed in white nacre but not in brownish one. In irradiated brownish nacre TL signal intensities and maximum temperatures changed in an unexpected way. The TL response of white nacre was more regular with some changes in the shape of spectra at the doses over 6 kGy. ESR signals of carbonate and radicals were detected in both nacres, the signal increased with dose but there were some variations revealing inhomogeneity of nacre.

cultural heritage ; radiation treatment ; nacre ; color ; termoluminescence ; EPR

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