Effect of thermic annealing on gamma-induced color centers and activator luminescence in Lu$_2$SiO$_5$:Ce scintillator crystals

Cerium doped lutetium orthosilicate Lu$_2$SiO$_5$:Ce is used as a scintillation detector for nuclear radiation. However, after $^{60}$Co-gamma-irradiation to a high dose 5 $\cdot$ 10$^6$ Gy at 310 K the yield of gamma-luminescence (GL) decreases by 25%. For revealing reasons and mechanism of such degradation, we studied isochronous thermal anneal of the induced optical centers and recovery of the activator GL in the temperature range from 373 to 773 K in air. The irradiation was shown to generate absorption bands at 238 nm ($F$-center), 263 (Ce$^{3+}$/Ce$^{4+}$) and 293 nm (Ce$^{3+}$/F$^+$) and also broad band at 422 nm, which is supposed to relate to Ce$^{4+}$+V$_\mathrm{O}$-defect center and yellow color, causing the decrease in GL bands 399 and 420 nm (Ce1-center) and 450 nm (Ce2-center). At temperatures $>$ 573 K $F$-centers and Ce$^{4+}$+V$_\mathrm{O}$ complexes anneal and the GL recovers up to the origin level, that is due to migration of an interstitial oxygen to an anion vacancy followed by healing the radiation defect.