Investigation on Main Radiation Source at Operation Floor of Fukushima Daiichi Nuclear Power Station Unit 4Hideo Hirayama, Kenjiro Kondo, Seishiro Suzuki, Shimpei Hamamoto and Kohei IwanagaEPJ Web Conf., 153 (2017) 08011DOI: https://doi.org/10.1051/epjconf/201715308011