1. Faculty of Hydrotechnics Department of Physics University of Architecture, Civil Engineering and Geodesy Hristo Smirnenski Blvd. 1 1046 Sofia Bulgaria
2. Faculty of Forest Industry University of Forestry Kl. Ohridsky Blvd. 10 1756 Sofia Bulgaria
3. Institute of Physics Martin-Luther-University Von-Seckendorff-Platz 1 06099 Halle Germany
4. Department of Physics University of Sofia Blvd. J. Bouchier 5 1164 Sofia Bulgaria