New research into radiation absorbed by survivors of the Japanese atomic bombs has reaffirmed the accuracy of earlier studies.
At issue was a scientific reevaluation of calculated neutron doses, undertaken in the so-called Dosimetry System of 1986, or DS86. It suggested that survivors had actually absorbed much higher doses of neutrons than previously estimated.
"Data from Hiroshima and Nagasaki serve as the world's primary basis for estimating radiation-induced cancer risk in humans," said Tore Straume, the study's lead author. "But there were discrepancies between estimates and measurements of neutrons, which called into question the credibility of the entire dosimetry system."
The newest findings, published this week in the journal Nature, are based on research by a team from Lawrence Livermore National Laboratory, the University of Utah, and two facilities in Germany: the Technical University (TUM) in Garching, and the Ludwig Maximilians University (TUM) in Munich.
In the weeks after the bombs were dropped on Hiroshima and Nagasaki in August 1945, Japanese scientists were able to measure certain types of radiation near ground zero.
Those early gamma ray estimates have been validated. But detectors in those days were not capable of detecting neutron doses at a distance greater than about 700 meters, about the same altitude as the bomb when it detonated. Since most survivors were farther away from the center of the blast, direct measurements were not possible.
"It became clear that we should really be measuring fast neutrons since they contributed essentially all of the neutron dose," said Straume, who spent 18 years at LLNL before moving to the University of Utah in 1997. "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."
The first task in the project was to identify a nuclear reaction resulting in a signal of the high energy (fast) neutrons that could still be detected ï¿½ and quantified ï¿½ more than 50 years later. The requirements for this reaction were considerable. Besides numerous physics and chemistry concerns, the reaction had to take place in a material common in an urban environment in the 1940's, and the histories, both before and after the event, as well as the location at the time of the bombing, of any sample must be precisely known.
This task was undertaken by Alfredo Marchetti, a postdoc in LLNL's Biology and Biotechnology Research Program at the time, and presently a researcher in the Energy and Environment Directorate.
Marchetti made an exhaustive search of known neutron-induced reactions, looking at expected signals and the decay of those signals since 1945. Marchetti's computer-based search was made possible by his access to the LLNL nuclear data libraries. Of the hundreds of reactions he examined, less than ten would show any signal, and of these, one reaction stood out as the best choice.
According to Marchetti: "In many respects, the reaction of neutrons on copper to produce Nickel-63 is the perfect detector for this work. Nickel-63 has a 100 year half-life, so most of the signal would still be there; purified copper, in the form of electrical conductors, is very common in an urban setting; and the nickel produced would remain trapped in the copper."
Perfect, except for some daunting technical challenges in the measurements. For copper samples exposed at Hiroshima, the Nickel-63 signal is less than one million atoms per gram of copper. The radioactivity of Nickel-63, a low-energy beta emitter, is also quite weak, so the measurements can only be made by counting the individual atoms with a mass spectrometer.
"When Alfredo contacted me in 1994 about this project, it sounded very exciting. But before the end of the first phone call, we realized we would be trying to measure Nickel-63 at 1 billionth of a part per billion ï¿½ 18 orders of magnitude," said Jeff McAninch, previously a postdoc at the Lab's Center for Accelerator Mass Spectrometry (CAMS), and now a physicist in the Physics and Advanced Technology Directorate. "To make matters worse, copper and nickel look almost exactly the same to a mass spectrometer."
Over a three-year period, Marchetti and McAninch, developed chemical extraction methods that can isolate Nickel-63 from copper materials exposed to atomic bomb neutrons, and mass spectrometric measurements using the CAMS Van de Graaff accelerator to quantify the Nickel-63.
The researchers made the first measurements of Hiroshima samples (lighting rods from a Hiroshima bridge at the hypocenter, and the roof of a nearby building) in May 1998. These first measurements were made in LLNL's Building190 under the glare of lights from a Japanese public television crew. The crew was preparing a documentary, aired in Japan later that year, reporting on the scientific studies surrounding the atomic bomb survivors.
Ultimately, limitations in the CAMS machine prevented the LLNL team from extending the measurements to samples at greater distances from the hypocenter.
To complete the measurement series, Straume and his team invited researchers at TUM and LMU to join the collaboration. The TUM accelerator runs at higher voltages, allowing better discrimination of Nickel-63 from copper, and the German team had for many years been involved in other Hiroshima and Nagasaki dosimetry measurements.
The results reported in Nature cover samples collected in Hiroshima, including lightning rods, rain gutters and copper roofing materials located as far as 5,000 meters from the epicenter of the blast.
The measurements are consistent with recent calculations of the Nickel-63 activation. For the most part, the newer calculations agree with the earlier findings from DS86, except at the closest distances. The study suggests that "these results may be consistent with a slightly underestimated height-of-burst for the Hiroshima bomb."
"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, the U.S. National Academy of Sciences, the European Commission, the German Federal Ministry of Environment, and Nature Conservation and Nuclear Safety.
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