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Traces of Hiroshima
Measuring Atomic Radiation 58 Years Later
Edward R. Winstead

Featured Article.

Most of what the world knows about the dangers of radiation comes from the Japanese people who survived the atomic bombs in 1945 and the researchers who study them.

Hiroshima, October 1945.

Since the 1950s, scientists have tracked the health of some 80,000 survivors and their children. The data have been the primary source for developing estimates of risk from radiation exposures, as well as for safety standards for medical procedures and nuclear industries. The data are used worldwide.

Until recently, however, one aspect of the atomic bomb survivor study has been in doubt. Since 1987, questions have lingered about the dose of neutron radiation that many survivors received in Hiroshima. If those dose estimates were in fact flawed, then so might be the estimates of damage people suffer from various doses of radiation.

Now, on the 58th anniversary of the bombings, those doubts are finally being put to rest.

The new research is a “tour de force experimentally,” says Warren K. Sinclair, who was president of the National Council on Radiation Protection and Measurements from 1977 to 1991 and is now president emeritus.

The new estimates are based on analyses of copper found in structures around Hiroshima—a flag pole, for instance, or copper roofs, rain gutters. When neutron beams from the bomb hit the copper 58 years ago, they created nickel particles, which can now be isolated and measured for the first time.

With these results, published in Nature, it’s clear that current estimates of radiation exposure with respect to the occurrence of cancer and other diseases based on the atomic bomb survivor study do not need to be changed. Only slight refinements in the neutron dose estimates are needed.

Tandem accelerator and mass spectrometer at Lawrence Livermore National Laboratory.

“This study will reduce the anxiety about the dose estimates in Hiroshima,” says Mark P. Little of the Department of Epidemiology and Public Health, Imperial College Faculty of Medicine, London, who wrote a commentary accompanying the new findings.

The key to the study is a technique for extracting trace amounts of an isotope of nickel (Ni-63), which is produced when fast neutrons hit copper. The researchers then used mass spectrometry analysis to detect individual atoms of Ni-63.

When the project, led by Tore Straume of the University of Utah School of Medicine in Salt Lake City, began a decade ago at Lawrence Livermore National Laboratory in Livermore, California, the tools for assessing the amount of neutron radiation released more than a half-century earlier did not exist.

“We began to explore ways to measure fast neutrons in 1992,” Straume recalls. “We had to identify radioactive nuclides that would have been produced by the bomb that have a long half-life and would be measurable today.”

The challenge was to detect nickel atoms in a sea of copper within structures in Hiroshima, says Straume, noting that the nickel and the copper have the same mass. Researchers in Livermore and Munich (at the Technische Universität and Ludwig Maximilians Universität) used different accelerator mass spectrometry machines to verify their approach.

To estimate the doses, they used computers to model the location of the copper fragments relative to the explosion. One of the fragments came from the lightning rod above the old Hiroshima city hall.

The copper fragments—provided by scientists at the Radiation Effects Research Foundation and Hiroshima University—came from distances relevant to the atomic bomb survivor study, between 900 and 1,500 meters from the blast. Most people who were less than 900 meters away from the “hypocenter," just below the exploding bomb, did not survive.

However, the dose of radiation received by most people who were more than 1,500 meters away from the hypocenter was generally too low to produce clearly detectable health effects.

Fast neutrons hitting copper produce an isotope of nickel.

The results of the study are being incorporated into a new system for estimating radiation doses. Known as DS02 (for Dosimetry System 2002), it improves upon the 1986 version, says Warren K. Sinclair, who helped oversee the development of the new system.

“DS02 will change risk estimates only a little, but the confidence in the new system will go up enormously,” Sinclair says. “Because all such risk estimates for cancer are based on this study, this is a very important step in radiation protection.”

It is also reassuring to know that doses of radiation used for the treatment of cancer and other diseases are as safe as they can be while still being effective.

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Radiation Effects Research Foundation

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Straume, T., et al. Measuring fast neutrons in Hiroshima at distances relevant to atomic-bomb survivors. Nature 424, 539-542 (July 31, 2003).
Little, M. P. Radiation: a dose of the bomb. Nature 424, 495-496 (July 31, 2003).

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