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Traces of “black rain” discovered at six locations, nine to 22 kilometers from the A-bomb hypocenter

by Tomomitsu Miyazaki, Senior Staff Writer

A team of researchers from Hiroshima University and other institutions, investigating traces of the “black rain” which fell in the aftermath of the atomic bombing of Hiroshima, has issued its findings. The results indicate that Cesium 137, a radioactive substance believed to be derived from the black rain, was found in soil beneath the floors of six sites, including residences in Yuki Town (Saeki Ward, Hiroshima City) and Akiota Town (Hiroshima Prefecture). These locations lie roughly nine to 22 kilometers from the bomb’s hypocenter. This is the first time that traces of black rain have been confirmed in these areas, and all were found outside of the designated zone known as the “heavy rain area.” Those within the “heavy rain area” at the time of the bombing are eligible for relief measures put in place by the central government.

The research team, which includes Masayoshi Yamamoto, a professor in radiochemistry at Kanazawa University, investigated the soil under the floors of 20 homes built between 1946 and 1948. After digging to a depth of 30 centimeters, they discovered the traces of Cesium 137 in the soil.

It is believed that Cesium 137, produced by the uranium fission that occurred in the atomic explosion, was also contained in the black rain and remains present in the earth to this date. However, Cesium 137, as well as other radioactive substances like plutonium, were also scattered throughout the world as a result of the atmospheric nuclear tests conducted repeatedly by the United States and then the former Soviet Union until the early 1960s.

For this reason, the group determined that 14 locations, where the amount of plutonium detected was higher than a certain level, were substantially affected by the nuclear tests--since only small amounts of plutonium were emitted by the atomic bomb dropped on Hiroshima--and so the Cesium 137 found in those locations could not be confirmed as a byproduct of the Hiroshima bomb. They therefore concluded that only the Cesium 137 found at the other six sites is highly likely to have come from the black rain that fell on Hiroshima in the wake of the bombing.

Among the six locations, three sites lie within the area officially termed the “light rain area”: Yasuhigashi, about nine kilometers from the hypocenter; Asa Town, about 16 kilometers from the hypocenter; and Akiota town, about 22 kilometers north of the hypocenter. The other three spots, located in Yuki Town, lie about 20 kilometers to the northwest and are not even within the bounds of the designated “light rain area.”

The researchers, employing backward calculations to estimate the radiation level of the Cesium 137 at the time of the atomic bombing, deduced a level of 40 to 100 becquerels per square meter. According to the team, this is equivalent to the level of radiation suffered by those who were directly exposed at a distance of about three kilometers or so from the hypocenter.

(Originally published on May 24, 2012)  

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