Electron and Hole Trapping in Ce3+ - and Pr3+ -Doped Lutetium Pyrosilicate Scintillator Crystals Studied by Electron Paramagnetic Resonance

Electron and hole trapping is studied in the ${\mathrm{Ce}}^{3+}$- and ${\mathrm{Pr}}^{3+}$-doped ${\mathrm{Lu}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$ scintillation single crystals (LPS:$\mathrm{Ce}$ and LPS:$\mathrm{Pr}$) by electron paramagnetic resonance (EPR). Detailed EPR measurements of the x-ray irradiated LPS crystals reveal that holes generated by irradiation are predominantly trapped at oxygen lattice ions creating ${\mathrm{O}}^{\ensuremath{-}}$ centers. The same x-ray irradiation creates also electron-type centers, which are attributed to ${\mathrm{Lu}}^{2+}$ ions, where the trapped electron at the Lu lattice ion is stabilized by a nearby defect, such as the oxygen vacancy and ${\mathrm{Ir}}^{3+}$ impurity ion. Both the hole and electron centers can be thus considered as a bound small polaron, which makes the charge trapping in a scintillation mechanism quite competitive. The hole ${\mathrm{O}}^{\ensuremath{-}}$ and electron ${\mathrm{Lu}}^{2+}$ centers show thermal stability well above room temperature. Thermal decays of their concentrations correlate well with the appearance of the thermally stimulated luminescence glow peaks at 470--550 K. The presence of the same intrinsic traps in the $\mathrm{Ce}\text{\ensuremath{-}}$ and $\mathrm{Pr}$-doped LPS crystals suggests that the difference in the light yield of these crystals is an intrinsic property of the ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Pr}}^{3+}$ activator centers in the LPS lattice. An origin of charge traps in this pyrosilicate structure and their role in the scintillation mechanism is compared with the results previously described in the literature on orthosilicates.