[1] Univ Turin, Dept Phys, Turin, Italy
[2] Univ Turin, NIS Ctr Excellence, Turin, Italy
[3] Ist Nazl Fis Nucl, Sez Torino, I-10125 Turin, Italy
[4] Consorzio Nazl Interuniv Sci Fis Mat CNISM, Sez Torino, Turin, Italy
[5] Ist Nazl Ric Metrol INRIM, Turin, Italy