[1] Ist Nazl Fis Nucl, Lab Nazl Sud, I-95123 Catania, Italy
[2] Univ Bari, Ist Nazl Fis Nucl, Sez Bari, I-70126 Bari, Italy
[3] Univ Bari, Dipartimento Interateno Fis, I-70126 Bari, Italy
[4] Univ Bologna, Ist Nazl Fis Nucl, Sez Bologna, I-40127 Bologna, Italy
[5] Univ Bologna, Dipartimento Fis, I-40127 Bologna, Italy
[6] Ist Nazl Fis Nucl, Sez Catania, I-95123 Catania, Italy
[7] Catania Univ, Dipartimento Fis & Astron, I-95123 Catania, Italy
[8] Ist Nazl Fis Nucl, Sez Genova, I-16146 Genoa, Italy
[9] Univ Genoa, Dipartimento Fis, I-16146 Genoa, Italy
[10] Ist Nazl Fis Nucl, Lab Nazl Frascati, I-00044 Frascati, Italy
[11] Univ Naples Federico II, Ist Nazl Fis Nucl, Sez Napoli, I-80126 Naples, Italy
[12] Univ Naples Federico II, Ist Nazl Fis Nucl, Dipartimento Sci Fisiche, I-80126 Naples, Italy
[13] Univ Pisa, Ist Nazl Fis Nucl, Sez Pisa, I-56127 Pisa, Italy
[14] Univ Pisa, Dipartimento Fis, I-56127 Pisa, Italy
[15] Univ Roma La Sapienza, Ist Nazl Fis Nucl, Sez Roma 1, I-00185 Rome, Italy
[16] Univ Roma La Sapienza, Dipartimento Fis, I-00185 Rome, Italy
[17] Univ Catania, Dipartimento Fis & Astron, I-95123 Catania, Italy
[18] Osserv Astron Capodimonte, INAF, I-80131 Naples, Italy
[19] Univ Wisconsin, Dept Phys, Madison, WI 53711 USA