1. Department of Physics, Okayama University, Okayama 700-8530, Japan
2. Universitá di Padova and INFN, Dipartimento di Fisica, Padova 35131, Italy
3. Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai, Naka, Ibaraki 319-1195, Japan
4. Institut für Physik, Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany
5. Département de Physique, École Polytechnique, 91128 Palaiseau Cedex, France