[1] Azienda Osped Maggiore carita, Dept Med Phys, I-28100 Novara, Italy
[2] Univ Milan, Dipartimento Fis, I-20133 Milan, Italy
[3] Ist Nazl Fis Nucl, Sez Milano, I-20133 Milan, Italy
[4] Univ Milano Bicocca, Dipartimento Sci Mat, I-20125 Milan, Italy
[5] Univ Southampton, Optoelect Res Ctr, Southampton SO17 1BJ, Hants, England