A technique originally intended for dating archaeological artefacts has been used to estimate the amount of radiation produced by the nuclear bomb dropped on Hiroshima, Japan in 1945. It marks the first time a bone fragment from a victim of the explosion has been used successfully for such an analysis.
The jaw fragment used in the study. Image: Sergio Mascarenhas IFSC-USP
Somewhere between 90,000 to 120,000 civilians were killed when the United States detonated a 15-kiloton nuclear weapon above the Japanese city of Hiroshima on 6 August 1945. Many civilians were killed instantly, but a significant portion survived the initial blast only to succumb to their injuries during the days, weeks and months that followed. Many died from the effects of severe radiation poisoning.
It's been 73 years since the Hiroshima bombing, and scientists are still trying to assess the full impact of the explosion on human health. Over these seven decades, researchers have sought to determine how much radiation was produced from the fallout, the range of radiation inflicted onto the population, and how that radiation affected physical wellbeing.
New research published in PLoS One is improving our picture of this grim episode, revealing the startling - but altogether unsurprising - extent to which the population was exposed to radiation at Hiroshima.
To do their analysis, a team of scientists used a technique dating back to the early 1970s with origins in archaeology. The researchers say it's now the first successful attempt to assess radiation dosage in a sample of human tissue taken from a victim of the Hiroshima attack.
The technique is called electron spin resonance spectroscopy (ESR), and it's used to measure the transition frequency between different electron spin rates, or for studying materials with unpaired electrons.
In the 1970s, study co-author Sérgio Mascarenhas from the University of São Paulo, Brazil, discovered that X-ray and gamma-ray irradiation causes human bones to become weakly magnetic, a process known as paramagnetism. This phenomenon causes the mineral portion of bone to lose electrons at a measurable rate, leading to the idea that ESR could be used for "retrospective dosimetry" - a fancy way of saying the technique could be used to create a timeline of an object's exposure to radiation.
At first, Mascarenhas used the technique to do archaeological dating, measuring the age of bones, shellfish, ancient tools and so on. ESR works as a dating technique because materials naturally absorb radiation over time, such as through long term exposure to thorium, for example.
It soon occurred to Mascarenhas that the technique could also be used to measure the amount of radiation absorbed during a nuclear incident - such as the bombing of Hiroshima. Armed with this idea, he went to Japan in 1972.
"They gave me a jawbone, and I decided to measure the radiation right there, at Hiroshima University," he said. "I needed to prove experimentally that my discovery was genuine."
Mascarenhas managed to obtain a dosimetric signal, which suggested he was onto something. But given the weak signal, and with no computers to help with the analysis, his team wasn't able to meaningfully move forward. But Mascarenhas's idea has now finally been validated.
"There have been major improvements in the 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. Basic physics has also evolved to the extent that you can simulate and manipulate the signal from the sample using computational techniques."
The updated technique also allowed the researchers to separate two signals trapped within the bone, namely the signal indicative of Hiroshima radiation, and the so-called background noise created by superheating during the explosion.
For the new analysis, Mascarenhas sampled the same jawbone he used in the original analysis - a bone that was found about 1.2 to 1.5km away from the centre of the explosion. The researchers measured a dose of 9.46 grays (Gy), which is very, very high. Even at half that dose of exposure, at around 5 Gy, a person who had their entire body exposed would be killed. One Gy is the absorption of one joule of energy (in the form of ionising radiation) per kilogram of matter.
Revealingly, these results are consistent with genetic studies done of Hiroshima survivors, and of physical objects taken from the site, including irradiated fragments of bricks and house tiles.
"There were serious doubts about the feasibility of using this methodology to determine the radiation dose deposited in these samples, because of the processes involved in the episode," said co-author Angela Kinoshita, a professor at Universidade do Sagrado Coração. "The results confirm its feasibility and open up various possibilities for future research that may clarify details of the nuclear attack."