1. ICEMS, Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal
2. Depart. Eng. Quimica, Instituto Superior Engenharia, P-3031-199 Coimbra, Portugal
3. KFKI Res. Inst. for Nucl. and Part. Physics, P.O. Box 49, H-1525 Budapest 114, Hungary