1. Department of Physics, Tohoku University, Sendai, 980-8578, Japan
2. Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826, Japan
3. Department of Electrical Engineering, Ichinoseki National College of Technology, Ichinoseki, 021-8511, Japan
4. Department of Electrical and Electronic Engineering, Akita University, Akita, 010-8502, Japan