1. Instituto de Pesquisas Energéticas e Nucleares, S ão Paulo, SP 05508-000, Brazil
2. Universidade Estadual Paulista, Júlio de Mesquita Filho, UNESP, Botucatu, SP 18618-970, Brazil
3. Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark
4. Institut de Ciència de Materials de Barcelona, Campus de la UAB 08193 Bellaterra, Spain