1. Department of Physics and Mathematics Surface Analysis Laboratory; University Roma Tre; via della Vasca Navale 84 00146 Rome Italy
2. Department of Engineering; University Roma Tre; via Vito Volterra 62 00146 Rome Italy
3. INFN; University Roma Tre; Rome Italy
4. Istituto Superiore per la Conservazione ed il Restauro; Via di San Michele 23 00153 Rome Italy