1. National Institute for Lasers, Plasma and Radiation Physics, Electron Accelerators Laboratory, 409 Atomistilor St., P.O. Box MG-36, 077125, Bucharest-Măgurele, Romania.
2. Corresponding author. Phone: +40-21-457-45-07. Fax: +40-21-457-42-43. E-mail: monica.nemtanu@inflpr.ro
3. National Institute of Materials Physics, 105 bis Atomistilor St., 077125 Bucharest-Măgurele, Romania.