1. Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, 28049 Madrid, España
2. Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria
3. Institut Laue Langevin, B. P. 156X, 38042 Grenoble, France