1. Institute of Physics, Maria Sklodowska-Curie University, Lublin 20-031, Poland
2. A.A. Galkin, Donetsk Institute of Physics and Technology Kiev, Ukraine
3. Warsaw Polytechnic University, 75 Koszykowa Street, Warsaw 00-662, Poland
4. Belarusian State Agrarian Technical University (BSATU), Prospekt Nezavisimosti 99, Minsk 220023, Belarus