Lutetium oxyorthosilicate Lu₂SiO₅ (LSO) and pyrosilicate Lu₂Si₂O₇ (LPS) activated by Ce³⁺ or Pr³⁺ are known to be effective and fast scintillation materials for the detection of X-rays and γ-rays. Their performances can be further improved by co-doping with aliovalent ions. Herein, we investigate the Ce³⁺(Pr³⁺) → Ce⁴⁺(Pr⁴⁺) conversion and the formation of lattice defects stimulated by co-doping with Ca²⁺ and Al³⁺ in LSO and LPS powders prepared by the solid-state reaction process. The materials were studied by electron paramagnetic resonance (EPR), radioluminescence spectroscopy, and thermally stimulated luminescence (TSL), and scintillation decays were measured. EPR measurements of both LSO:Ce and LPS:Ce showed effective Ce³⁺ → Ce⁴⁺ conversions stimulated by Ca²⁺ co-doping, while the effect of Al³⁺ co-doping was less effective. In Pr-doped LSO and LPS, a similar Pr³⁺ → Pr⁴⁺ conversion was not detected by EPR, suggesting that the charge compensation of Al³⁺ and Ca²⁺ ions is realized via other impurities and/or lattice defects. X-ray irradiation of LPS creates hole centers attributed to a hole trapped in an oxygen ion in the neighborhood of Al³⁺ and Ca²⁺. These hole centers contribute to an intense TSL glow peak at 450–470 K. In contrast to LPS, only weak TSL peaks are detected in LSO and no hole centers are visible via EPR. The scintillation decay curves of both LSO and LPS show a bi-exponential decay with fast and slow component decay times of 10–13 ns and 30–36 ns, respectively. The decay time of the fast component shows a small (6–8%) decrease due to co-doping.

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