1. School of Earth and Environmental Sciences/RIO, College of Natural Sciences, Seoul National University, Seoul 151-742, Korea, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan, Radioactivity Group, National Institute of Standards and Technology, United States Department of Commerce, Building 245, Room C114, Gaithersburg, Maryland 20899, and Environmental Radioactivity Measurement Facility, Department of Oceanography, Florida State University, Tallahassee,...