Scientists calculate radiation dose in bone remains of Hiroshima bomb victims

Publicado em 30 abril 2018

The bombing of the Japanese cities of Hiroshima and Nagasaki by the United States in 1945 was the first and only use of nuclear weapons against civilian targets. The researchers subsequently conducted a series of studies to measure the impact of fallout on both the radiation dose to which the victims were exposed and the impact of this exposure on DNA and health in general.

The research, begun in the 1980s by the physicist Sérgio Mascarenhas at the University of São Paulo (USP), led to an article in the journal PLOS ONE which describes a method for precise measurement

"We used a technique known as electron spin resonance spectroscopy to perform retrospective dosimetry, and there is currently a new interest on this type of methodology because of the risk of terrorist attacks in countries like the United States, "said Professor Oswaldo Baffa of the University of São Paulo.

"Imagine someone in New York is planting an ordinary bomb on the explosive with a small amount of radioactive material, and techniques like these can help identify who has been exposed to radioactive fallout and needs treatment."

Angela Kinoshita , A professor at the Sagrado Coração University of Bauru, São Pa Ulo State, said the study is unique in that it used samples of human tissue from victims of the bomb dropped on Hiroshima.

"There were serious doubts about the feasibility of this method for determining the radiation dose deposited in these samples because of the processes in the episode," she said. "The results confirm their feasibility and open up various possibilities for future research that could clarify details of the nuclear attack."

In the 1970s, when he taught at the São Carlos Physics Institute of the University of São Paulo (IFSC-USP) Mascaren has discovered that X-rays and gamma rays make human bones weakly magnetic. The phenomenon, known as paramagnetism, occurs because the hydroxyapatite (crystalline calcium phosphate) in the mineral part of the bone tissue absorbs carbon dioxide ions, and when the sample is irradiated, the CO loses 2 electrons and becomes CO 2 -. This free radical serves as a marker for the radiation dose absorbed by the material.

"I discovered that we could use this property for beam dosimetry and began to use this method in archaeological dating," recalls Mascaren.

His goal At that time, the age of the bones absorbed in Sambaquis (shamans created by the original inhabitants of Brazil as rubble mountains, skeletons of prehistoric animals, human bones, stone or bone crockery and other wastes) was calculated Natural radiation was found centuries ago through contact with elements such as thorium, which are present in the sand at the seashore.

Because of this research, he was invited to teach at Harvard University in the United States. However, before leaving for the United States, he decided to go to Japan to receive bone samples from the victims of the atomic bombs and test his method.

"They gave me a jawbone, and I decided to measure the radiation directly at Hiroshima University," he said. "I had to prove experimentally that my discovery was real."

Mascaren was able to show that a dosimetric signal could be obtained from the sample, even though the technology was still rudimentary and there were no computers that could process the results. The research was presented at the annual March meeting of the American Physical Society, where it made a strong impression. Mascarenhas brought the samples to Brazil, where they stay.

"There have been significant improvements in instrumentation to make it more sensitive over the past 40 years," Baffa said. "Now you can see digitally processed data in spreadsheets and charts on the computer screen, and basic physics has also evolved to simulate and manipulate the signal from the sample with the help of computer techniques."

Thanks to these advances, he added, in the new study, it was possible to have the signal corresponding to the radiation dose absorbed during the nuclear attack by the so-called background signal, a kind of noise scientists have suspected, due to overheating of the material during the Explosion had arisen.

"The background signal is a broad line that can be generated by different things and has no specific signature," said Baffa. "The dosimetric signal is spectral, and every free radical vibrates at a certain point in the spectrum when it is exposed to a magnetic field."


To perform the measurements, the researchers removed the millimeter scale pieces of the jawbone used in the previous study. The samples were re-irradiated in the laboratory using a technique called additive dosing.

"We added radiation to the material and measured the rise in the dosimetric signal," Baffa explained. "We then constructed a curve and extrapolated the starting dose when the signal was probably zero, and using this calibration method we were able to measure different samples because each bone and part of the same bone has a different sensitivity to radiation, depending on its composition."

Thanks to this combination of techniques, they were able to measure a dose of about 9.46 Gray (Gy), which is high in Baffa's view. "About half of this dose or 5 Gy is fatal if the whole body is exposed to it," he said.

The value was comparable to the doses used by other techniques applied to non-biological samples, such as the measurement of the luminescence of quartz grains in brick and roof tile fragments found at the bomb sites. According to the authors, they were also similar to the results of biological measurement techniques used in long-term studies, using changes in survivor DNA as parameters.

"The measurements we have received in this latest study are more reliable and up to date than the preliminary result, but I currently evaluate a method that is about a thousand times more sensitive than spin resonance Months have news, "said Mascarenhas.