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.
A series of studies began afterwards to measure the impact of radioactive fallout, in terms of the radiation dose to which the victims were exposed and the effects of this exposure on DNA and on health in general.
Research that began in the 1980s continues under the leadership of the physicist Sérgio Mascarenhas, Professor at the University of São Paulo (USP), Brazilian scientists have published an article in the journalPLOS ONE that describes a method of accurate measurement of the dose of radiation absorbed by the bones of the victims of nuclear bombs dropped on Japan.
The research was conducted during the postdoctoral research of Angela Kinoshita, currently a professor at the University of the Sacred Heart in Bauru, State of São Paulo. His supervisor was then Oswaldo Baffa, Professor at the School of Philosophy, Science and Literature of Ribeirão Preto of the University of São Paulo (FFCLRP-USP).
"We use a technique known as electronic resonance spectroscopy to perform retrospective dosimetry, and there is currently a renewed interest in this type of methodology due to the risk of terrorist attacks in countries like the United States," Baffa said.
"Imagine someone in New York planting an ordinary pump with a small amount of radioactive material attached to the explosive. Techniques like this can help identify who has been exposed to radioactive fallout and needs treatment."
As Kinoshita explained, the study is unique insofar as it used samples of human tissue from the victims of the bomb dropped on Hiroshima.
There were serious doubts about the feasibility of using this methodology to determine the radiation dose deposited in these samples, due to the processes involved in the episode, "he said." The results confirm its viability and open several possibilities for future research that they can clarify details of the nuclear attack. "
The equipment used in the investigation was purchased during a project coordinated by Baffa and supported by the São Paulo Research Foundation – FAPESP.
Origins In the 1970s, when he taught at the São Carlos Institute of Physics at the University of São Paulo (IFSC-USP), Mascarenhas discovered that X-rays and gamma radiation made human bones weakly magnetic. known as paramagnetism, occurs because hydroxyapatite (crystalline calcium phosphate) in the mineral portion of the bone tissue absorbs carbon dioxide ions Ono, and when the sample is irradiated, the CO 2 loses electrons and becomes CO 2 -. This free radical serves as a marker of the radiation dose received by the material.
"I discovered that we could use this property to perform radiation dosimetry and we started using the method in archaeological dating," Mascarenhas recalled.
His objective at that time was to calculate the age of the bones found in sambaquis (dumps created by the original inhabitants of Brazil such as mounds of shellfish remains, skeletons of prehistoric animals, human bones, stone utensils or other debris) in the natural radiation absorbed for centuries through contact with elements such as thorium that are present in the sand on the shore of the sea.
With the strength of this research, he was invited to teach at Harvard University in the United States. Before leaving for the United States, however, he decided to go to Japan to try to obtain bone samples from the victims of the nuclear bombs and test their method with them.
"They gave me a jaw and I decided to measure the radiation right there at the University of Hiroshima," he said. "I needed to demonstrate experimentally that my discovery was genuine."
Mascarenhas managed to demonstrate that a dosimetric signal could be obtained from the sample even though the technology was still rudimentary and there were no computers to help process the results. The research was presented at the March Annual Meeting of the American Physical Society, where it made a great impression. Mascarenhas brought the samples to Brazil, where they remain.
"There have been significant improvements in instrumentation to make it more sensitive in the last 40 years," said Baffa. "Now, you see digitally processed data in tables and graphs on the computer screen, and basic physics has also evolved to the extent that you can simulate and manipulate the sample signal using computational techniques."
Thanks to these advances, he added, in the new study, it was possible to separate the signal corresponding to the dose of radiation absorbed during the nuclear attack of the so-called background signal, a kind of noise that scientists suspect may have resulted from the overheating of the material during The explosion.
"The background signal is a broad line that can be produced by several different things and lacks a specific signature," said Baffa. "The dosimetric signal is spectral, each free radical resonates at a certain point in the spectrum when exposed to a magnetic field."
To perform the measurements, the researchers removed parts of scale millimeter of the jaw used in the previous study. The samples were irradiated again in the laboratory using a technique called the additive dose method.
"We added radiation to the material and measured the increase in the dosimetric signal," Baffa explained. "Then we built a curve and extrapolated from that the initial dose, when the signal presumably was zero." This calibration method allowed us to measure different samples, since each bone and each 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 approximately 9.46 grays (Gy), which is high in Baffa's view. "Approximately half of that dose, or 5 Gy, is fatal if the whole body is exposed to it," he said.
The value was comparable with the doses obtained by other techniques applied to non-biological samples, such as the measurement of the luminescence of the quartz grains present in the fragments of bricks and tiles found in the sites of the pumps. According to the authors, he also approached the results of biological measurement techniques applied in long-term studies that use alterations in the DNA of the survivors as a parameter.
"The measure we obtained in this last study is more reliable and up to date than the preliminary finding, but I am currently evaluating a methodology that is approximately one thousand times more sensitive than spin resonance, we will have news in a few months," he predicted. Mascarenhas.