1. a University of Athens, Physics Department , Section of Solid State Physics, Panepistimiopolis, GR 157 84 Zografos, Athens , Greece
2. b J. J. Thomson Physical Laboratory, University of Reading , Whiteknights, Reading , RG6 6AF , England
3. c Institute of Nuclear Technology and Radiation Protection , NCSR ‘Demokritos’, 153 10 Ag. Paraskevi, Attiki , Greece