Paper ATOMIC BOMB INDUCED 152 Eu: RECONCILIATION OF DISCREPANCY BETWEEN MEASUREMENTS AND CALCULATION K. Komura,* M. Hoshi, † S. Endo, † T. Imanaka, ‡ S. D. Egbert, § W. Ru ¨hm,** H. Fukushima, †† and S. Fujita ‡‡ Abstract—In order to resolve the discrepancy between the measured and calculated 152 Eu activity induced by the atomic bomb at Hiroshima, extremely low background gamma-ray spectrometry was performed for 17 granite samples collected from 134 m to more than 3 km from the hypocenter. Measure- ments agreed well with theoretical calculations based on DS02 up to 1.4 km from hypocenter. Health Phys. 92(4):366 –370; 2007 Key words: atomic bomb; 152 Eu; neutrons; spectrometry, gamma INTRODUCTION IN 1976, residual 152 Eu (half-life = 13.3 y) was first recognized by in situ gamma-ray spectrometry per- formed inside of the “Genbaku Dome” (“A-bomb Dome”) in Hiroshima (Sakanoue and Komura 1977). 152 Eu was a very important nuclide for evaluating the thermal neutron fluences from the atomic bombs in Hiroshima and Nagasaki. A great number of 152 Eu measurements have since been made from samples ex- posed to either the Hiroshima or Nagasaki atomic bomb (Hasai et al. 1987; Hoshi et al. 1989; Nakanishi et al. 1983, 1987, 1991; Okajima and Miyajima 1987; Saka- noue et al. 1987; Shizuma et al. 1992, 1993, 2003). The “Dosimetry System 1986” (DS86) was formulated for the evaluation of individual radiation dose to the survi- vors based on a computational model that was verified by the use of experimental data from Hiroshima and Na- gasaki (Roesch 1987). When DS86 was being developed, 152 Eu data were not sufficient to support or contradict the DS86 calculated values because 152 Eu data beyond 1 km from the hypocenter were not available at that time. Therefore, some problems remained unresolved when the DS86 Final Report was published. One of the important problems was the discrepancy between thermal neutron measurements and calculations, i.e., measured 152 Eu agreed rather well with the calculations between 0.5 and 1 km; however, closer than 0.5 km the measured values were lower than the calculations and beyond 1 km measured values became higher than the calculations: at around 1.5 km the discrepancy amounted to 10 –15 times higher (Loewe et al. 1987). In 2000, a U.S.–Japan Working Group was orga- nized to reassess DS86 (Young and Kerr 2005). One of the main objects was to find a solution to explain the discrepancy between measured and calculated activities both for 60 Co and 152 Eu. The U.S. group took the lead role in the theoretical (calculations) approach, and the Japa- nese group took the lead in the experimental (measure- ments) approach. Besides critical examinations of past 152 Eu data, Accelerator Mass Spectrometry (AMS) of 36 Cl (half-life = 3.0  10 5 y) of atomic bomb-exposed samples had already been performed in the U.S. (Straume et al. 1992) and Germany (Kato et al. 1990; Ru ¨hm et al. 1992). The 63 Ni induced in copper samples by the fast neutron reaction 63 Cu(n,p) 63 Ni was also discussed in workshops (Shibata et al. 1994; Straume et al. 1994; McAninch et al. 1997; Ru ¨hm et al. 2000). During the last years, joint efforts to detect 36 Cl by means of AMS included groups from the University of Tsukuba * Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Wake, Tatsunokuchi-machi, Ishikawa 923-1224, Japan; † Research Institute for Radiation Biology and Medicine, Hiroshima University, Kasumi 1-2-3, Minami-ku, Hiroshima 734-8553, Japan; ‡ Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan; § Sci- ence Applications International Corporation, 10260 Campus Point Drive, San Diego, CA 92121; ** Technical University of Munich, Physics Department, Garching, Germany; Ludwig Maximilians Uni- versity, Radiobiological Institute, Munich, now GSF Federal Center for Environment and Health, Institute for Radiation Protection, 85764 Neuherberg, German; †† Japan Chemical Analysis Center, Sannocho, Inage-ku, Chiba 263-0002, Japan; ‡‡ Radiation Effects Research Foundation, Hijiyama Koen 5-2, Minami-ku, Hiroshima 732-0815, Japan. For correspondence contact: K. Komura, Low Level Radioactiv- ity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Wake, Tatsunokuchi-machi, Ishikawa 923- 1224, Japan, or email at komura@yu.incl.ne.jp. (Manuscript accepted 19 October 2006) 0017-9078/07/0 Copyright © 2007 Health Physics Society 366