Effect of gamma irradiation on thermally treated freshwater Pearls: Structural and spectroscopic investigations for dosimetric applications

Aragonite, a type of calcium carbonate, and an organic matrix make up the majority of pearls. Gamma irradiation causes important structural and optical changes in pearls, leading to the color changing from the typical pearly white to gray or black hues. This work investigates the potential of natural pearls as efficient gamma dosimeters or an irradiation indicator. Color changes are attributed to the formation of CO 2 − radicals and interactions with trace elements such as Mn 2+ were analyzed across doses ranging from 0 to 50 kGy delivered by a Cobalt-60 source. Three methods were used to assess the effects of thermal treatments carried out at temperatures of 100, 300, and 500 °C: colorimetry (L∗, a∗, b∗, ΔE) to measure color changes; electron paramagnetic resonance (EPR) spectroscopy to investigate dosimetric properties; and X-ray diffraction (XRD) to examine structural changes. Results show that thermal treatment at 300 °C optimized dosimetric properties by removing the organic phase (conchiolin) and inducing a transition from aragonite to calcite, thereby enhancing crystallinity and the stability of CO 2 − radicals. A strong linear correlation was established between irradiation dose, EPR signal intensity (notably R2, linear from 0 to 10 kGy), and total color difference (ΔE, linear up to 30 kGy). The dosimetric response is stable at 300 °C regardless of the dose rate, providing a significant advantage for ensuring the reliable dosimeter. These results imply that natural pearls, especially after thermal treatment at 300 °C, provide an affordable, environmentally friendly, and high-performing substitute for synthetic dosimeters. They also show promise for use in radiation processing, including industrial sterilization, and environmental monitoring.