  We investigated the thermal stability of scintillation and the luminescence
performances of (La, Gd)${}_{2}$Si${}_{2}$O${}_{7}$ single crystals at various
Ce concentrations. We prepared (La${}_{0.25-x}$, Ce${}_{x}$,
Gd${}_{0.75}$)${}_{2}$Si${}_{2}$O${}_{7}$ (x = 0.0001, 0.001, 0.005, 0.01,
0.02, 0.05, and 0.1; unit: molar concentration) single crystals by the
Czochralski and micro-pulling-down methods. With increasing Ce concentration,
the photoluminescence emission and photoluminescence excitation spectral bands
shifted to low energies and the activation energy $\mathrm{\Delta }E$ for
thermal quenching decreased. For Ce $\mathrm{<}$ 0.5 at.% samples, the
photoluminescence emission background value calculated in the exponential
approximation started to increase at temperatures greater than 320 K, which is
probably because of Ce${}^{3+}$ 5$\textit{d}$ excited-state ionization.
However, the effect was weaker for the Ce $\ge $ 0.5% samples, which may
indicate a comparatively larger contribution from other nonradiative
relaxations. Thus the main reason for the thermal quenching of the Ce${}^{3+}$
emission in (La, Gd)${}_{2}$Si${}_{2}$O${}_{7}$ is the combination of the
5$\textit{d}$1 excited-state ionization and nonradiative relaxation via
thermally excited crossover from the 5$\textit{d}$ excited state to the
4$\textit{f}$ ground state. The temperature dependence of the scintillation
light yield was similar irrespective of the Ce concentration, with Ce 1.0%
exhibiting the best performance within the temperature range 300 K to 450 K.
