1. Service de Métrologie Nucléaire (CP 165/84); Université libre de Bruxelles; 50 av. F. D. Roosevelt, B-1050 Brussels Belgium
2. Instituto de Ciencia de Materiales de Sevilla, Univ. Sevilla - CSIC; av. Américo Vespucio 49, E-41092 Sevilla Spain
3. Department of Physics, Chemistry and Pharmacy; University of Southern Denmark; DK-5230 Odense M Denmark