Effect of gamma-induced defects on the activator glow in Lu$_2$SiO$_5$ : Ce scintillator crystals

Correlations were studied between the optical absorption (OA) spectra and the integral curves of thermal glow (TG) in 300–600 K after irradiation of Lu$_2$SiO$_5$ : Ce scintillation crystals with $^{60}$Co gamma-quanta (1.17 and 1.33 MeV) at the dose rate 1.1 Gy/s in the dose range 70–5 $\cdot$ 10$^7$ Gy at 310 K and their gamma-luminescence (GL). There are intrinsic defects caused by technological process, such as neutral V$_{\mathrm{O5}}$-centers with OA band at 193 nm and charged $\equiv$ Si-V$_{\mathrm{O5}}$ – 213 nm, Lu1–F$^+$–Si – 238 nm, Ce$^{3+}$/Ce$^{4+}$ – 263 nm, and Ce$^{3+}$/F – 295 nm centers. Irradiation to the dose 5 $\cdot$ 10$^4$ Gy resulted in decreasing in V$_{\mathrm{O5}}$-center concentration, but did not influenced on others. While, after doses $>$ 5 $\cdot$ 10$^4$ Gy concentrations of all other mentioned defects grew. The observed recovery of OA at 193 nm and decrease in TG peak at 335 K with the ageing time (1, 3 and 10 hours) at 305 K, and also the correlated growth of OA at 238 nm and TG peak at 540 K after serial irradiations to doses from 70 to 2.3 $\cdot$ 10$^6$ Gy are due to releasing electrons from these color centers followed by radiative recombination at Ce1-centers. However Се$^{3+}$ GL yield decrease at 400 and 420 nm at doses $>$ 10$^5$ Gy is possible related with increasing concentrations of $\equiv$ Si–V$_{\mathrm{O4}}$, Lu1–F$^+$–Si and Се$^{3+}$/F centers, which compete with Ce1 ones in trapping electrons. Thus, the upper limit for stable Lu$_2$SiO$_5$ : Ce gamma-scintillation is 10$^5$ Gy.