1. Department of Physics, Okayama University, Okayama 700-8530, Japan
2. Institute for Cosmic Ray Research, University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8582, Japan
3. Institut für Physik, Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany
4. Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai, Naka, Ibaraki 319-1195, Japan
5. Département de Physique, École Polytechnique, 91128 Palaiseau Cedex, France
6. Universitá di Padova and INFN, Dipartimento di Fisica, Padova 35131, Italy