Author(s):
1. Plamen Tsankov, Technical University of Gabrovo, Department of Physics, Chemistry and Ecology 4 Hadzhy Dimitar str., Bulgaria
2. Deshka Markova, Technical University of Gabrovo, Department of Physics, Chemistry and Ecology 4 Hadzhy Dimitar str., Bulgaria
3. Stoyo Platikanov, Technical University of Gabrovo, Department of Physics, Chemistry and Ecology 4 Hadzhy Dimitar str., Bulgaria
4. Milko Yovchev, Technical University of Gabrovo, Department of Physics, Chemistry and Ecology 4 Hadzhy Dimitar str., Bulgaria

Abstract