[1] Univ Pavia, Dept Nucl & Theoret Phys, I-27100 Pavia, Italy
[2] Univ Brescia, Dept Mech Engn, I-25123 Brescia, Italy
[3] Ist Nazl Fis Nucl, Sez Padova, I-27100 Pavia, Italy
[4] Univ Padua, Dept Phys, I-35131 Padua, Italy
[5] Ist Nazl Fis Nucl, Sez Padova, I-35131 Padua, Italy