1. National Research Institute of Nuclear Physics and Engineering "Horia Hulubei", P. O. Box MG-6, Bucharest-Magurele, Romania
2. Research Center of Optical Engineering and Photonics, Polytechnical University of Bucharest, 313 Spl. Independentei, 77206 Bucharest, Romania
3. Center of Advanced Studies in Physics, Romanian Academy, Calea Victoriei 125, Bucharest, Romania