[1] INFN,GENOA,ITALY
[2] LAB NAZL GRAN SASSO,ASSERGI,ITALY
[3] RUHR UNIV BOCHUM,INST PHYS IONENSTRAHLEN,D-44780 BOCHUM,GERMANY
[4] INFN,PADUA,ITALY
[5] INFN,TURIN,ITALY
[6] DIPARTIMENTO FIS SPERIMENTALE,TURIN,ITALY
[7] ENEA,FRASCATI,ITALY
[8] DIPARTIMENTO FIS,MILAN,ITALY
[9] INFN,FERRARA,ITALY
[10] DIPARTIMENTO FIS,FERRARA,ITALY