1. Univ. of Bucharest, Doctoral School on Physics, P.O. Box MG-11, 077125 Magurele, (Ilfov), Romania.
2. Univ. of Bucharest, Dept. of the Structure of Matter, Earth and Atmospheric Physics and Astrophysics, P.O. Box MG-11, 077125 Magurele, (Ilfov), Romania (corresponding author).