Mg-codoped Lu 3 Al 5 O 12 :Ce single crystal scintillators were prepared by a micropulling down method in a wide concentration range from 0 to 3000 ppm of Mg codopant. Their structure and chemical composition were checked by X-ray diffraction and electron probe microanalysis techniques. Absorption and luminescence spectra, photoluminescence decays, and thermoluminescence glow curves were measured together with several other scintillation characteristics, namely, the scintillation decay, light yield, afterglow, and radiation damage to reveal the effect of Mg codoping. Several material characteristics manifest a beneficial effect of Mg codopant. We propose a model explaining the mechanism of material improvement which is based on the stabilization of a part of the cerium dopant in the tetravalent charge state. The stable Ce 4+ center provides an additional fast radiative recombination pathway in the scintillation mechanism and efficiently competes with electron traps in garnet scintillators.

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New materials are studied for fast and heavy scintillators, which are increasingly used and/or demanded in coming medical and industrial applications. Among others, Ce-doped Lu-containing compounds are under study and development, especially LSO:Ce 3+ and LuAP:Ce 3+ crystals. However, the growth of single crystals of these compounds appeared very difficult, especially the latter system has not been offered by any industrial company yet, even if the first papers dealing with this promising scintillator were published in 1995 by several groups [for review, see [1]).Contrary to LuAP perovskite phase, the garnet one Lu 3 Al 5 O 12 (LuAG) appears as rather stable and can be thus grown more easily. LuAG single crystals have a density of 6.73 g/cm 3 (94% of BGO) and the leading fast emission/scintillation decay component is about 50 ns (six times faster than BGO) peaking between 500 and 550 nm (similar spectral region as BGO emission). Despite of the fact that such material can appear as serious competitor of BGO scintillator, little attention was paid to it in the literature up to now [2].This note provides a direct comparison between YAP:Ce 3+ and LuAG:Ce 3+ scintillators grown in the same laboratory 1 ) as for radioluminescence light output, scintillation decay and thermoluminescence characteristics. A correlation between the occurence of thermo-luminescence (TSL) peaks round RT and of very slow components in the scintillation decay of LuAG:Ce 3+ is revealed. Importance of understanding of the nature of such TSL active trapping states and their removal for further optimisation of LuAG:Ce 3+ scintillator is concluded.The experimental apparate used for scintillation and TSL measurements are described elsewhere [3,4]. We have grown YAP:Ce and LuAG:Ce crystals by Czochralski method. For scintillation decay measurements part of the grown ingot was used with polished front faces (about 1 30 Â 15 mm), while for the other measurements polished plates about 1 8 Â 1 mm were prepared. The composition was checked by electron beam excited X-ray analysis, which confirmed the appropriate stoichiometry of the perovskite and garnet phases, respectively. The concentration of Ce ions in the crystals was measured by the same method to be of about 7000 ppm and comparable amount of Ce 3+ centres in both materials were evidenced also from the optical absorption measurements (amplitude of the Ce 3+ -related absorption bands in UV-visible spectral region). The absorption spectrum of LuAG:Ce 3+ features wide Ce 3+ absorption bands around 450, 343 and 217 nm. The lowest Ce 3+ absorption bands coincide with the excitation bands (450 nm and 345 nm) of the Ce 3+ emission peaking round 510±550 nm at room temperature (RT) with the characteristic double peak structure (512 nm and 547 nm) due to the splitting of the Ce 3+ 4f ground state. The same emission appears also under X-ray excitation , see Fig. 1. The same experimental conditions of the radioluminescence measurements in Fig. 1 allowed to compare relative light output in both materials. After ...

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