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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.
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