1. Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City 84112, U.S.A.
2. University of Adelaide, Adelaide, S.A., Australia
3. Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, Brasil
4. Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
5. Faculty of Engineering, Kanagawa University, Yokohama 221-8686, Japan
6. Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan
7. Center for Cosmology and Particle Physics, New York University, 4 Washington Place, New York, NY 10003
8. Institut für Kernphysik, Karlsruher Institut für Technologie, Postfach 3640, 76021 Karlsruhe, Germany
9. Institut für Physik, Universität Siegen, Walter-Flex-Str. 3, 57068 Siegen, Germany