Radiation and Environmental Biophysics

, Volume 44, Issue 2, pp 75–86 | Cite as

The Hiroshima thermal-neutron discrepancy for 36Cl at large distances. Part I: New 36Cl measurements in granite samples exposed to A-bomb neutrons

  • Thomas Huber
  • Werner Rühm
  • Kazuo Kato
  • Stephen D. Egbert
  • Florian Kubo
  • Vitali Lazarev
  • Eckehart Nolte
Original Paper

Abstract

The long-lived radioisotope 36Cl (half-life: 301,000 years) was measured in granite samples exposed to A-bomb neutrons at distances from 94 to 1,591 m from the hypocenter in Hiroshima, by means of accelerator mass spectrometry (AMS). Measured 36Cl/Cl ratios decrease from 1.6×10−10 close to the hypocenter to about 1–2×10−13, at a distance of 1,300 m from the hypocenter. At this distance and beyond the measured 36Cl/Cl ratios do not change significantly and scatter around values of 1–2×10−13. These findings suggest that the 36Cl had been predominantly produced by thermalized neutrons from the A-bomb via neutron capture on stable 35Cl, at distances from the hypocenter smaller than about 1,200 m. At larger distances, however, confounding processes induced by cosmic rays or neutrons from the decay of uranium and thorium become important. This hypothesis is theoretically and experimentally supported in a consecutive paper. The results are compared to calculations that are based on the most recent dosimetry system DS02. Close to the hypocenter, measured 36Cl/Cl ratios are lower than those calculated, while they are significantly higher at large distances from the hypocenter. If the contribution of the cosmic rays and of the neutrons from the decay of uranium and thorium in the sample was subtracted, however, no significant deviation from the DS02 calculations was observed, at those distances. Thus, the Hiroshima neutron discrepancy reported in the literature for 36Cl for samples from large distances from the hypocenter, i.e., higher measured 36Cl/Cl ratios than predicted by the previous dosimetry system DS86, was not confirmed.

Notes

Acknowledgements

The authors would like to thank Prof. Haruhiko Morinaga and Dr. Kenjiro Yokoro for their continuous support during the early phase of this project. Drs. Yasukazu Yoshizawa and Taizo Muta provided helpful advice and suggestions concerning the samples from the building of Faculty of Science, Hiroshima University. Without the samples that were provided by the priests from various temples this study would not have been possible. Drs. Tore Straume and Shoichiro Fujita provided material from the Red Cross Hospital building and the Postal Savings Office building, respectively. Support by Silke Merchel to develop the carrier-free chemistry is greatly acknowledged. The authors are also grateful to Dr. Gensei Yoshino for his advice and suggestions throughout the geological investigations and to Herbert Reithmeier for his help during the AMS beam times. This work was partially supported by BMBF (German Ministry of Education and Research) and DFG (German Research Council).

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Thomas Huber
    • 1
  • Werner Rühm
    • 4
    • 5
  • Kazuo Kato
    • 2
  • Stephen D. Egbert
    • 3
  • Florian Kubo
    • 1
  • Vitali Lazarev
    • 1
  • Eckehart Nolte
    • 1
  1. 1.Physics DepartmentTechnical University of MunichGarchingGermany
  2. 2.Department of Radiological SciencesHiroshima Prefectural College of Health SciencesMiharaJapan
  3. 3.Science Applications International CorporationSan DiegoUSA
  4. 4.Institute for Radiation BiologyUniversity of MunichMunichGermany
  5. 5.Institute of Radiation ProtectionGSF–National Research Center for Environment and HealthNeuherbergGermany

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