[1] Univ Roma Tre, Dipartimento Sci, Via Vasca Navale 84, I-00146 Rome, Italy
[2] Casaccia RC, ENEA FSN FISS SNI, Via Anguillarese 301, I-00123 Rome, Italy
[3] Univ Modena & Reggio Emilia, Dipartimento Ingn E Ferrari, Via Vivarelli 10, I-41125 Modena, Italy
[4] Consiglio Nazl Ric IOM CNR, Ist Officina Mat, Str Statale 14,Km 163-5 AREA Sci Pk, I-34149 Trieste, Italy
[5] Univ Johannesburg, Dept Phys, POB 524, ZA-2006 Johannesburg, South Africa