1. Department of Physics, University of Niš, PO Box 224, 18000 Niš, Serbia
2. Institute of Occupational Health, Department for radiological protection, Vojislava Ilića bb, 18000 Niš, Serbia
3. Faculty of Science, Department of Physics, University of Kragujevac, 12 R. Domanovića Str., 34000 Kragujevac, Serbia