/ 14 August 2003

The day the sky exploded

Fifty-eight years later, using recently discovered large-scale Japanese maps, sophisticated computer models and new radiation measurements, scientists have completed a painstaking reconstruction of events in Hiroshima.

Up to 140 000 people are thought to have been killed when an atomic bomb was dropped on the Japanese city of Hiroshima on August 6 1945. As many as 80 000 died when a second device exploded in the skies above Nagasaki three days later.

It was the first time nuclear weapons had been used to kill people and for almost six decades since, physicists have been trying to piece together what happened during and in the immediate aftermath of the attacks.

Weapons like the bomb used on Nagasaki were tested after the war, so scientists knew a fair amount about what must have happened there during the blast. But with Hiroshima things were more complex, partly because the bomb was a one-off — nothing like it was ever used again.

Now, 58 years after the Hiroshima bomb was dropped, scientists think they have all the answers they will ever get about events that day. Using recently discovered large-scale Japanese maps from the time, sophisticated computer models and new radiation measurements, the scientists from Japan and the United States have completed a painstaking reconstruction of events in Hiroshima.

This is more than mere scientific curiosity. The reconstruction is being used to better estimate the doses of radiation received by the survivors of the attack. This information is used to set everything from their financial compensation from the Japanese government, to safety limits on modern-day exposures to radiation.

Human exposure to dangerous levels of radiation is rare, so the survivors provide the best evidence of what the effects are. By comparing the radiation doses the survivors received with the illnesses they later developed, scientists try to work out how lower exposures to radiation may trigger cancer.

Every time you have an X-ray, for example, the safety data used to set your dose of radiation can be directly traced back to the events at Hiroshima. Likewise for patients receiving radiotherapy and for those people working in nuclear power stations.

Over the years, the Japanese-US Radiation Effects Research Foundation (Rerf) has followed the health of 86 500 people exposed to radiation in Hiroshima and Nagasaki, for whom doses could be estimated. About half of these people have since died, many of them from diseases blamed on the radiation released by the bombs.

About 50 000 of the survivors followed by Rerf received high radiation doses because they were within 2,5km of the hypocentre, the point on the ground directly beneath the explosion (both bombs were detonated in the air to cause maximum devastation). The other survivors studied were further away and received lower doses.

Worryingly, perhaps, the system previously used to determine the radiation exposure of these survivors is riddled with problems.

There are also holes in the science: experts could not be sure how powerful the Hiroshima bomb was, or where exactly it exploded. Some measurements of radioactivity on the ground were also thought to be inaccurate and, until recently, researchers assessing radiation exposure didn’t even have access to good maps of the two cities as they were in 1945.

The new reconstruction, however, should give a clearer picture of what happened. ”At this point what we have is a physics-based analysis,” says Nolan Hertel, a nuclear physicist at the Georgia Institute of Technology, who was part of the review group that has pieced together the latest evidence.

The physics-based analysis, as used until now, calculates doses without accounting very well for buildings or where people were. The new reconstruction, however, is able to take more accurately into account whether a survivor was shielded from the main blast and how this may have affected the dose of radiation they received.

Thousands of Hiroshima residents, for example, were partly protected behind a hill called Hijiyama in the south-east of the city. This means the doses received by Hiroshima survivors can be revised, depending on where they were.

”Our initial work with the new system indicates that there are small changes in individual dose estimates [of] typically 10% to 15% increases, though dose estimates are decreased for some individuals,” says Dale Preston, a statistician with Rerf. This will hopefully make the data used to calculate X-ray doses more accurate.

The new system being used by Rerf is called DS02, and is essentially a complicated series of formulae and equations that predict the radiation flow from a bomb blast. It replaces DS86 that has been in place since 1987.

The new version is superior in almost every respect. Aside from the improved shielding effects, DS02 also better accounts for the way radiation travels through the air. But the biggest improvement is in the measurement on the ground around Hiroshima of evidence of a type of radiation called fast neutrons. These particles made up only about 1% to 2% of the radiation emitted from the Hiroshima bomb, but are believed to have caused about a fifth of all cancers subsequently blamed on radiation.

The fast-neutron count was the most controversial part of the old DS86 system. Samples taken on the ground just after the blast appeared to show a level of radiation much higher than the DS86 calculations and estimates suggested. The uncertainty threw doubts over the whole DS86 analysis.

This is no longer a problem. Using chemical analysis techniques only developed in the past few years, scientists have now pinpointed the precise amount of fast neutrons released by the Hiroshima bomb. It turns out that the early measurements were indeed too high, by as much as 35%.

The new measurements of fast-neutron dose take advantage of the fact that the radiation is so powerful that it can literally change one metal into another. Evidence of such transmutations are scattered around modern-day Hiroshima.

The team scraped metal samples from lightning conductors and rain gutters that were blasted by the bomb. They then looked for a form of nickel produced when fast neutrons strike copper: the more nickel, the more radiation. It turns out that in the region where most of the strongly exposed survivors were found, the earlier fast neutron estimates from DS86 were pretty accurate.

This is good news because it means that the risk assessments based on the old system should be more reliable than some thought. In other words, the danger from X-rays has probably not been miscalculated.

The scientists developing the new reconstruction have also been able to follow the angle and direction of the radiation striking buildings and the ground back to their point of origin: the bomb. In this way they have worked out that the Hiroshima device was more powerful than believed: 16 kilotons rather than 15 kilotons (a one kiloton blast is equivalent to 900 tonnes of conventional TNT exploding). They have also revised the exact point of the explosion over the area of the city where a memorial park now stands — saying it was 15m further west and 20m higher at 600m.

Such details may seem irrelevant next to the human tragedy that unfolded below, but the researchers say it is important that all uncertainties are ironed out.

Sixty years after their forebears helped to build the bomb, the modern-day scientists say it is time to close the book on what happened at Hiroshima. — Â