Speaker
Description
The vertical Bridgman method is widely used in research of various scintillator crystals, including garnets, in order to improve their performance [1]. Conversion of a part of Ce3+ ions to Ce4+ by divalent co-doping is presently considered as one of the most efficient ways to improve the performance of Ce-doped garnet scintillators [2]. While cations with unit charge difference and nearly the same size are easily tolerated in garnet hosts [2,3], the case of monovalent cations is less evident and insufficient information is available to consider their substitution preferences and functional role. Most of reported studies were performed on crystals grown by Czochralski and micro-pulling techniques.
In this presentation we shall discuss incorporation of monovalent impurities (Li+ and Na+) and related effects in YAG:Ce and LuAG:Ce single crystals grown by Bridgman. Measurements of lattice constant (ao), optical absorption, radioluminescence and pulsed X-ray decays were performed to reveal the tendencies of impurity incorporation and localization within the lattice, charge balance maintenance and impact of impurities on optical properties and performance. Annealing and gamma-ray irradiation effects have been examined in comparison to those in crystals with divalent co-doping and without any co-doping.
Measurements of ao in series of ceramic samples prepared by solid phase reactions did not give clear evidence for substitution at any cation site within the lattice. In most cases the slopes of measured and calculated dependences of ao on Li or Na concentration (assuming incorporation in dodecahedral sites) are very different suggesting, among others, substitutions at interstitial positions. In contrast to crystals with divalent co-doping, which show an increase of absorption in the UV range due to conversion of a part of Ce3+ to Ce4+, there is no such effect in most of the studied crystals with Li or Na. Presence of impurities is however evidenced in acceleration of the fast decay component and decrease of radioluminescence intensity. Radiation hardness of crystals is high showing no induced absorption bands in the range of emission after gamma-ray irradiation with 1 kGy. The results will be compared with available data reported for crystals prepared by other techniques. The advantages and limitations of the co-doping approach with various species will be discussed basing on observed tendencies of incorporation and crystal growth and perfection.
This work was performed in the frame of the International Associated Laboratory (CNRS–France & SCS–Armenia) IRMAS and European Union Horizon 2020 Program under grant agreement no. 644260 (Intelum).
[1] A.G. Petrosyan, K.L. Ovanesyan, R.V. Sargsyan, et al, J. Crystal Growth 312 (2010) 3136.
[2] M. Nikl, K. Kamada, V. Babin, et al, Cryst. Growth Des., 14 (2014) 4827.
[3] A.G. Petrosyan, K.L. Ovanesyan, M.V. Derdzyan, et al, J. Crystal Growth 430 (2015) 46.
