1. Forschungszentrum Karlsruhe, Institute for Technical Physics, Tritium Laboratory Karlsruhe P.O. Box 3640, D-76021 Karlsruhe, Germany (+49) 7247 82 2675, e-mail: robert.michling@itp.fzk.de
2. European Commission Joint Research Centre, Institute for Transuranium Elements P.O. Box 2340, D-76125 Karlsruhe, Germany