Centers responsible for the TL peaks of willemite mineral estimated by EPR analysis

The mineral willemite (Zn₂SiO₄) exhibits five thermoluminescence (TL) peaks approximately at 160, 225, 260, 310 and 400 °C. Electron paramagnetic resonance (EPR) studies were carried out to study the defect centers induced in the mineral by gamma irradiation and also to identify the centers responsible for the TL process. Room temperature EPR spectrum of irradiated mineral is a superposition of at least four distinct centers. One of the centers (center I) with an isotropic g factor 2.0114 is attributable to an intrinsic O^- type center and the center correlates with the TL peak at 160 °C. Center II exhibiting hyperfine lines is also tentatively assigned to an O^- ion and is related to the low temperature TL peak at 160 °C. Center III is characterized by an axially symmetric g-tensor with principal values g_||=2.0451 and g⊥=2.011 and is identified as an O₂^- ion. This center appears to be related to 160, 225 and 260 °C TL peaks. Center IV with principal g-values g_||=2.0025 and g⊥=2.0088 is attributed to an F⁺-type center (singly ionized oxygen vacancy) and is the likely recombination center for TL peaks between 160 and 310 °C.