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Measuring fast neutrons in Hiroshima at distances relevant to atomic-bomb survivors

An Addendum to this article was published on 22 July 2004

Abstract

Data from the survivors of the atomic bombs serve as the major basis for risk calculations of radiation-induced cancer in humans1. A controversy has existed for almost two decades, however, concerning the possibility that neutron doses in Hiroshima may have been much larger than estimated. This controversy was based on measurements of radioisotopes activated by thermal neutrons that suggested much higher fluences at larger distances than expected2,3,4,5,6. For fast neutrons, which contributed almost all the neutron dose, clear measurement validation has so far proved impossible at the large distances (900 to 1,500 m) most relevant to survivor locations6. Here, the first results are reported for the detection of 63Ni produced predominantly by fast neutrons (above about 1 MeV) in copper samples from Hiroshima. This breakthrough was made possible by the development of chemical extraction methods7,8 and major improvements in the sensitivity of accelerator mass spectrometry for detection of 63Ni atoms (refs 8–11). When results are compared with 63Ni activation predicted by neutron doses for Hiroshima survivors6, good agreement is observed at the distances most relevant to survivor data. These findings provide, for the first time, clear measurement validation of the neutron doses to survivors in Hiroshima.

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Figure 1: Measured and calculated 63Ni produced in copper samples by the bomb neutrons in Hiroshima.

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Acknowledgements

We thank I. Proctor and A. M. Kellerer for their essential support of this project. We also thank S. Fujita and K. Shizuma for providing copper samples for this study. We thank the following organizations for supporting this work: the US Department of Energy, the US National Academy of Sciences, the European Commission, the German Federal Ministry of Environment, Nature Conservation and Nuclear Safety.

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Straume, T., Rugel, G., Marchetti, A. et al. Measuring fast neutrons in Hiroshima at distances relevant to atomic-bomb survivors. Nature 424, 539–542 (2003). https://doi.org/10.1038/nature01815

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