1. Karlsruhe Institute of Technology, Institute for Applied Materials, P.O. Box 3640, D-76021 Karlsruhe, DE
2. Technische Universität München, FRM II and Physik-Department E21, James-Franck-Str. 1, D-85748 Garching, DE
3. Neutron Imaging and Activation Group, Paul Scherrer Institut, CH-5232, Villigen, CH