[1] Univ Florence, INFM, I-50125 Florence, Italy
[2] Univ Florence, Dipartimento Astron & Sci Spazio, I-50125 Florence, Italy
[3] Terza Univ Roma, Dipartimento Ingn Elettron, I-00146 Roma, Italy
[4] Univ Roma Tor Vergata, INFM, Dipartimento Sci & Tecnol Fis & Energet, I-00133 Rome, Italy
[5] Univ Firenze, Dipartimento Ingn Elettron, Lab NDT, I-50100 Firenze, Italy
[6] Ist Nazl Fis Nucl, Sez Firenze, I-50125 Florence, Italy