[1] Louisiana State Univ, Dept Phys & Astron, Baton Rouge, LA 70803 USA
[2] Ist Nazl Fis Nucl, I-00185 Rome, Italy
[3] Univ Padua, Dipartimento Fis, I-35131 Padua, Italy
[4] Ist Nazl Fis Nucl, I-35131 Padua, Italy
[5] Univ Roma Tor Vergata, Dipartimento Fis, I-00133 Rome, Italy
[6] Ist Nazl Fis Nucl, I-00133 Rome, Italy
[7] Univ Western Australia, Dept Phys, Nedlands, WA 6907, Australia
[8] CNR, ITC, Ctr Fis Stati Aggregati, I-38050 Trento, Italy
[9] Ist Nazl Fis Nucl, I-38050 Trento, Italy
[10] CNR, Ist Fis Spazio Interplanetario, I-00133 Rome, Italy
[11] Univ Aquila, Dipartimento Fis, I-67100 Laquila, Italy
[12] Ist Nazl Fis Nucl, I-67100 Laquila, Italy
[13] Univ Roma La Sapienza, Dipartimento Fis, I-00185 Rome, Italy
[14] Ist Nazl Fis Nucl, Lab Nazl Frascati, I-00044 Frascati, Italy
[15] Univ Ferrara, Dipartmento Fis, I-44100 Ferrara, Italy
[16] Ist Nazl Fis Nucl, I-44100 Ferrara, Italy
[17] Univ Trent, Dipartimento Fis, I-38050 Trento, Italy
[18] Ist Nazl Fis Nucl, Grp Collegato Trento, I-38050 Trento, Italy
[19] Ist Nazl Fis Nucl, Lab Nazl Legnaro, I-35020 Legnaro, Padova, Italy
[20] Ist Nazl Fis Nucl, I-35131 Padua, Italy