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Radioactive materials contained in black rain detected outside the Health Examination Special Designated Area

by Uzaemonnaotsuka Tokai, Staff Writer

On February 1, a group led by Professor Masaharu Hoshi at the Hiroshima University Research Institute for Radiation Biology and Medicine announced that it has detected, under the floor of a private house located about eight kilometers north of the hypocenter, radioactive materials believed to have been contained in the "black rain" that fell in the aftermath of the atomic bombing of Hiroshima. The house stands outside the Health Examination Special Designated Area, which is considered the same area as the "heavy rain area." Professor Hoshi said, "The finding can serve as scientific evidence in pursuit of the expansion of the Health Examination Special Designated Area."

The group embarked on its research last February, focusing on the area within about 30 kilometers of the hypocenter. The members of the group have been checking for the presence of Cesium 137, a product of fission, in the soil under the floors of private houses that were built between 1945 and 1949, which were minimally impacted by post-war nuclear tests. The team has already collected soil from 15 sites in the city of Hiroshima and finished analyzing the soil from seven of these sites.

As a result of the analyses, the group has detected Cesium 137 in two sites: 1) former Yasu Village (now, Aita in Asaminami Ward), which is located about eight kilometers north of the hypocenter and not in the "heavy rain area" and 2) former Tomo Village (now Tomo, Numata Town in Asaminami Ward), which is located about seven kilometers northwest of the hypocenter and is inside the "heavy rain area." By March, Professor Hoshi will estimate the radiation doses of that time with the cooperation of the Kyoto University Research Reactor Institute in Osaka and explore the impact of the radiation on the human body.

Regarding the black rain, the former Ministry of Health and Welfare released the results of a survey on the soil within 30 kilometers of the hypocenter in 1976. But it stated that the area of the black rainfall could not be identified due to the effects of radioactive fallout caused by nuclear tests conducted by such countries as the former Soviet Union and China.

On the other hand, researchers, including those at Hiroshima University, announced in 1996 that they detected Cesium 137 in three sites that were inside the former area of the city of Hiroshima and outside the "heavy rain area." These results were produced after measuring the radiation of the soil that was collected by a research group of the former Japanese Imperial Army from about 30 sites within a five-kilometer radius from the hypocenter in the aftermath of the atomic bombing. The soil was returned to the city of Hiroshima in 1992.

This time, Professor Hoshi succeeded in detecting Cesium 137 in sites farther from the hypocenter than in the sites of the radiation measurement in 1996. Now, the team will increase the number of sites for collecting soil, seek to grasp the full picture of the rainfall area, and aim to complete the final analysis within fiscal 2011. The City of Hiroshima, which has been cooperating in this research, intends to make use the results for consultations with the government in pursuit of the expansion of the Health Examination Special Designated Area, which is cited in the Atomic Bomb Survivors Relief Law.

Keywords

Black rain
The black rain, which is believed to have contained radioactive fallout, fell immediately after the atomic bombing. In 1976, the Japanese government defined an oval area stretching northwest from the hypocenter for about 19 kilometers from north to south and for about 11 kilometers from east to west as the "heavy rain area" and designated the area as the Health Examination Special Designated Area. Concerning the area stretching northwest from the hypocenter for about 29 kilometers from north to south and for about 15 kilometers from east to west, which includes the above-mentioned "heavy rain area," the Japanese government defined the other area of the oval as the "light rain area" and did not include it in the Health Examination Special Designated Area.

(Originally published on February 2, 2010)

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