1. Institute of Nuclear Physics, PAS, Radzikowskiego 152, 31-342 Kraków, Poland
2. AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
3. Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland
4. INFN - Laboratori Nazionali di Frascati, Via E. Fermi 40, I-00044 Frascati (Rome), Italy
5. Universita' Roma Tre, Dip. Scienze Geologiche, L. go S. Leonardo Murialdo 1, 00146 Rome, Italy