[1] Ist Nazl Fis Mat, Dipartimento Fis, I-50125 Florence, Italy
[2] LENS, I-50125 Florence, Italy
[3] Ist Nazl Fis Mat, Dipartimento Energet, I-50139 Florence, Italy
[4] Ist Nazl Fis Nucl, I-50125 Florence, Italy
[5] Dipartimento Fis, I-50125 Florence, Italy
[6] CNR, MASPEC, I-43100 Parma, Italy