Color and luminescence centers in LuAG and LuAG : Pr crystals before and after $^{60}$Co $\gamma$-irradiation

The absorption and thermoluminescence spectra and their relaxation in pure and praseodymium-doped LuAG crystals are comparatively studied to determine the contribution of intrinsic and impurity defects before and after irradiation by $^{60}$Co $\gamma$-quanta with an average energy of 1.25 MeV, dose rate of 0.8 Gy/s, and dose of 10$^3$ Gy at 310 K. The existence of intrinsic $F^+$ defects with a typical absorption band at 375 nm and impurity Cr$^{4+}$ centers absorbing at 285, 420, 490, and 595 nm in both LuAG and Pr:LuAG samples before irradiation is related to the crystal-growth technology and the impurity composition of the charge. An increase in the concentration of $F^+$ and Cr$^{4+}$ centers after $\gamma$-irradiation is caused by charging of neutral $F$ centers and changing of the electronic state (valence) of Cr$^{3+}$ impurity centers. Formation of antisite defect complexes Lu$^{3+}_{\mathrm{Al}}$–V$_{\mathrm{O}}$, which are responsible for a luminescence band at 335 nm, is suggested. The first broad peak at 340 K on the thermoluminescence curves of both samples after irradiation with a dose of 10$^3$ Gy is related to liberation of charge carriers from shallow trapping levels; thermal activation and radiative recombination with emitting centers begin at 310 K, which places the upper limit on the temperature range of application of these scintillation crystals.