July 30, 2003 — Previous estimates of the amount of neutron dose received by Hiroshima atomic-bomb survivors are essentially accurate and the existing standards for estimating risk for radiation-induced cancer need not be changed, according to a study published in the July 31 issue of Nature.
The research was conducted by a team of scientists from the United States and Germany, led by Tore Straume, Ph.D., professor in the Radiobiology Division of the University of Utah School of Medicine.
The study’s findings will finally end a long-standing controversy on whether neutron doses received by Hiroshima survivors might have been much higher than previously calculated. Health effects data from the survivors have been used to set the standards for safety in the nuclear industry and in medicine, where radiation is used.
“Data from Hiroshima and Nagasaki serve as the world’s primary basis for estimating radiation-induced cancer risk in humans,” said Straume, principal investigator. “But there were discrepancies between estimates and measurements of neutrons, which called into question the credibility of the entire dosimetry system.”
The controversy began almost two decades ago during reevaluation of survivor radiation dose estimates, called Dosimetry System of 1986 or DS86. DS86 provided individual dose estimates based on information regarding each survivor’s location and shielding at the time of the explosion.
Two kinds of radiation–gamma rays and neutrons–were emitted from the explosion of the atomic bombs dropped by the United States on the Japanese cities of Hiroshima and Nagasaki in August 1945 to end World War II. Estimates of gamma rays were adequately validated by measurements, but calculations for neutrons were not.
The Radiation Effects Research Foundation acknowledged in a 1987 report on the reassessment of DS86 that calculated neutron doses for survivors in Hiroshima might be inaccurate. The report generated a lot of interest and resulted in studies measuring low-energy (thermal) neutron activation in Hiroshima and Nagasaki. The focus was initially on low-energy neutrons because they could be measured using existing detection technologies.
Straume is among those who measured low-energy neutrons in Hiroshima and Nagasaki samples. Prior to coming to Utah in 1997, he was a leader of a team of researchers studying radiation effects at the Lawrence Livermore National Laboratory (LLNL) in California.
“It became clear that we should really be measuring fast neutrons since they contributed essentially all of the neutron dose,” said Straume. “Unfortunately, a method that could detect fast neutrons more than half a century after the bombing was not available and would have to be developed.”
Although measurements of fast neutrons were conducted in Hiroshima by Japanese scientists within weeks of the explosion using a short half-life isotope, the methods available at the time could not detect bomb neutrons beyond about 700 meters from ground zero. Most survivors were at distances farther than that, so those measurements could not be used for direct validation of neutron dose at the survivor locations.
In the early 1990s, Straume’s group at LLNL began the development of a new chemical-extraction technique to isolate an isotope of nickel (Ni-63), which is produced when fast neutrons hit copper. Dr. Alfredo Marchetti developed the method separating nickel from copper, while Dr. Jeff McAninch, developed the accelerator mass spectrometry (AMS) method required to detect individual atoms of Ni-63. This effort was later extended to include an AMS facility in Munich, Germany, which provided measurement capabilities at larger distances than possible at LLNL.
The new method detected trace amounts of Ni-63 in copper samples ranging from 380 meters to more than 5,000 meters from ground zero in Hiroshima.
Straume said his team’s study is the first since 1945 to measure fast neutrons. The group’s measurements validated that fast neutron exposure between 900 and 1,500 meters from the bombing — the distance at which most survivors were found and, therefore, the most relevant — was consistent with estimates based on the DS86 doses for survivors.
“With our study, we can finally say that a large discrepancy in neutron dose to Hiroshima survivors does not exist,” said Straume.
The study was funded by the U.S. Department of Energy, U.S. National Academy of Sciences, European Commission, German Federal Ministry of Environment, and Nature Conservation and Nuclear Safety.
The investigators in the study are:
T. Straume, University of Utah
G. Rugel, Technische Universitat Munchen, Garching, Germany, and Ludwig Maximilians Universitat Munchen, Munchen, Germany
A.A. Marchetti, Lawrence Livermore National Laboratory, Livermore, Calif.
W. Ruhm, Ludwig Maximilians Universitat Munchen, Munchen, Germany
G. Korschinek, Technische Universitat Munchen, Garching, Germany
J.E. McAninch, Lawrence Livermore National Laboratory, Livermore, Calif.
K. Carroll, Lawrence Livermore National Laboratory, Livermore, Calif.
S. Egbert, Science Applications International Corp., San Diego, Calif.
T. Faestermann, Technische Universitat Munchen, Garching, Germany
K. Knie, Technische Universitat Munchen, Garching, Germany
R. Martinelli, Lawrence Livermore National Laboratory, Livermore, Calif.
A. Wallner, Technische Universitat Munchen, Garching, Germany, and Ludwig Maximilians Universitat Munchen, Munchen, Germany
C. Wallner, Technische Universitat Munchen, Garching, Germany.