1. Institute of Physics, University of Tartu, Riia 142, 51 014 Tartu, Estonia
2. Institute of Experimental Physics, Hamburg University, Luruper Chaussee 149, 22761 Hamburg, Germany
3. Permanent address: Department of Physics, University of Science and Technology of China, 230026 Hefei, China
4. Institute of Scintillation Materials, Lenin Avenue 60, 61001 Kharkov, Ukraine
5. Kagami Memorial Laboratory of Material Science and Technology, Waseda University, Nishiwaseda 2‐8‐26, 169051 Tokyo, Japan