Effect of γ-Irradiation on the Semitransparent Gray Color Obsidian in the 5 to 500 kGy Range

Semitransparent gray color obsidian samples (bulk and powder) were irradiated at room temperature by Co⁶⁰ radiation source with the γ-photon average energy of 1.25 MeV and different doses from 5 to ~500 kGy. It is known that γ-irradiation influences the optical properties of glass materials depending on the composition as well as on the presence of defects in the glass matrix. Analysis of obsidian samples was carried out by absorption and reflection spectroscopy in the UV, visible, and IR ranges, as well as by EPR measurements for characterization of semitransparent gray color obsidian depending on the γ-irradiation of different doses. The difference in transmittance spectra between the pristine sample and those irradiated with different doses made it possible to distinguish three absorption bands at 368, 386, and 442 nm, which are responsible for Fe³⁺ ions in different environments. The content of hydroxyl OH groups, determined from the absorption in the band at ~4500 cm^–1, does not change in the range of used doses. The EPR measurements showed three signals with g-factors of ~6.0, ~4.2, and ~2.0 characteristic for Fe³⁺ ions, the intensities of which increase with increasing irradiation doses. Obsidian turned out to be resistant to the formation of NBOHC paramagnetic defects in the region of the indicated doses of γ-irradiation.