[1] Univ Turin, CNR, Inst Cosmo Geophys, Turin, Italy
[2] Ist Nazl Fis Nucl, Turin, Italy
[3] Univ Bologna, Bologna, Italy
[4] Ist Nazl Fis Nucl, I-40126 Bologna, Italy
[5] Univ Campinas, Campinas, SP, Brazil
[6] Russian Acad Sci, Inst Nucl Res, Moscow, Russia
[7] Univ Aquila, I-67100 Laquila, Italy
[8] MIT, Cambridge, MA 02139 USA
[9] Ist Nazl Fis Nucl, Lab Nazl Frascati, I-00044 Frascati, Italy