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Scientists Confirm Radiation Levels at Hiroshima

They match earlier estimates

(HealthDay is the new name for HealthScoutNews.)

WEDNESDAY, July 30, 2003 (HealthDayNews) -- New research effectively retires a 15-year controversy over how much radiation people were exposed to during the atomic bomb attack at Hiroshima in 1945.

The findings confirm that long-standing estimates of cancer risk from exposure to radiation from the bombs were accurate.

The study appears in the July 31 issue of Nature.

Because humans are almost never exposed to the high radiation levels that were experienced after atomic bombs were dropped on Hiroshima and Nagasaki, historic estimates are all scientists had to deduce cancer risk. Estimates of the radiation exposure at that time have been used for years to calculate cancer risk from radiation and to set safety standards for both industry and medicine.

"From a public health perspective, what you really want to know is the risk of getting cancer per unit of [radiation] dose," says Jonathan M. Links, professor of environmental health sciences at Johns Hopkins Bloomberg School of Public Health. "If you're going to answer that question, the two obvious things you need to know is how many cancers did they get and what was the dose. If you mess up in either, your risk estimate is wrong."

Two kinds of radiation followed the explosion of the atomic bombs -- gamma rays and neutrons. The gamma ray estimates were validated in the mid-1980s. However, a 1987 report reevaluating the neutron dose exposure said the original estimates might be wrong.

"There was controversy over what fraction of the radiation was neutron versus gamma," Links explains. "Even within the neutrons, there was controversy over what fraction was so-called fast neutrons versus slow neutrons." The reason this even mattered is that the different types of radiation had different effects.

As a result of that report, scientists made new measurements of low-energy neutron exposure in samples from both Hiroshima and Nagasaki, mainly because they could be measured.

"Methods were available to measure low-energy neutrons, but not fast neutrons, this many years after the bombing," says study author Tore Straume, a professor of radiobiology at the University of Utah School of Medicine.

That still left the question of exposure to fast neutrons.

Japanese scientists did measure fast neutrons weeks after the explosion, but the methods available at the time could not detect particles more than 700 meters from ground zero. Most survivors who received high doses of radiation were located between 900 and 1,500 meters from the epicenter.

"It has been well known that fast neutrons would be better [to measure] because they are more directly related to neutron dose," Straume explains. "During the early 1990s we came up with the idea that it may be possible to measure fast neutrons using modern technologies that had just recently emerged. The development of accelerator mass spectrometry during the 1980s and construction of such a facility at Livermore [Lawrence Livermore National Laboratory in California] in the early '90s made possible the detection of Ni-63 in copper samples."

The Ni-63, or 63Ni, that Straume refers to is an isotope of nickel that is produced when fast neutrons hit copper. Straume and his colleagues obtained copper samples from unshielded locations in Hiroshima, including copper lightning conductors, rain gutters and the roof of a Buddhist shrine. These samples came from locations ranging from 380 meters to more than 5,000 meters from the blast. The samples were sent to labs at Livermore and in Munich, Germany, so the minute amounts of 63Ni could be measured.

Then they used these measurements to re-estimate the number of fast neurons at these same distances. This was the first time since 1945 that fast neuron levels had been measured.

The calculations showed that the fast neutron exposure at between 900 and 1,500 meters from ground zero was similar to previous estimates.

"The principal conclusion is that our fast neutron measurements are consistent with neutron doses in Hiroshima between 900 and 1,500 meters from the hypocenter, which are the most important distances for the A-bomb survivor data," Straume says.

"The big deal about this paper is they really pushed the sensitivity of the analytic technique in a way that made them able to accurately and precisely detect these transformed atoms in building materials at relevant distances from the blast," Links says.

In order to come up with a risk estimate, scientists are really dealing with four unrelated sources of uncertainty, of which the type of radiation is only one. "This article does not mean that now we're just totally certain of the cancer risk estimates, but it means one of those four things that we have to worry about, we no longer have to worry about," Links says.

Which means the information the world has been relying on is sound.

More information

For more on radiation, visit the Radiation Effects Research Foundation or the Board on Radiation Effects Research.

SOURCES: Tore Straume, Ph.D., professor, radiobiology, University of Utah School of Medicine, Salt Lake City; Jonathan M. Links, Ph.D., professor, environmental health sciences, Johns Hopkins Bloomberg School of Public Health, Baltimore; July 31, 2003, Nature
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