Authors
Galina Yordanova
Konstantin Preslavski University of Shumen, Faculty of Natural Sciences, Department of Physics and Astronomy, Shumen, Bulgaria
Dragomir Marchev
Konstantin Preslavski University of Shumen, Faculty of Natural Sciences, Department of Physics and Astronomy, Shumen, Bulgaria
Borislav Borisov
Konstantin Preslavski University of Shumen, Faculty of Natural Sciences, Department of Physics and Astronomy, Shumen, Bulgaria
Teodora Atanasova
Konstantin Preslavski University of Shumen, Faculty of Natural Sciences, Department of Physics and Astronomy, Shumen, Bulgaria
Samuil Petrov
Konstantin Preslavski University of Shumen, Faculty of Natural Sciences, Department of Physics and Astronomy, Shumen, Bulgaria