1. Department of Physics, University of Ioannina, P.O. Box 1186, GR-451 10 Ioannina, Epirus, Greece, IOM-CNR, Laboratorio Nazionale TASC, S.S. 14, km 163.5, I-34012 Trieste, Italy, and Sincrotrone Trieste, S.S. 14 km 163.5, Area Science Park-Basovizza, 34012 Trieste, Italy