1. Istituto di Struttura della Materia, CNR., Frascati, Italy
2. Dipartimento di Energetica dell' Università "La Sapienza", Roma, Italy
3. Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland
4. Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland
5. Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA