[1] Univ Ferrara, Dept Phys, I-44100 Ferrara, Italy
[2] INFM, I-44100 Ferrara, Italy
[3] INFN Ferrara, I-44100 Ferrara, Italy
[4] Ist Nazl Fis Nucl, Lab Nazl Legnaro, I-35100 Legnaro, PD, Italy
[5] Univ Brescia, INFM, Dept Chem & Phys Mat, I-25133 Brescia, Italy