Abstract
Scientific records of Nagasaki "black rain" are limited. According to the records, the black rain began to fall about 20 minutes after the explosion in Nishiyama area located about 3 km east of the hypocenter. Wind at the time of explosion was reported about 3 m/s from south-west. But significant 239,240Pu and 137Cs contaminations were found mainly in the area due east of the hypocenter. 239,240Pu concentration and external dose rate of Nishiyama area was very much higher than the "Black rain" area of Hiroshima. Rainfalls were reported at many places near around the hypocenter. Rainfall in the sunshine was reported from about 10 minutes after the explosion at Omura Navy Hospital about 19 km north-east of the hypocenter. But the rainfall was not black.
Falls of dry "black ash" and light fine dust were reported in the area due east of the hypocenter and they reached over 10 km from the hypocenter. It was reported that farms turned dimly white with the dust and characters could be written by a finger on leaves of taro.
People who were not officially acknowledged as atomic bomb victims have been accusing Japanese government to request acknowledgement by asserting that there was very strong radioactivity of the fallout immediately after the explosion in a large area including Nishiyama area. The maximum accumulated external dose for a person from 1 hour after the explosion in Nishiyama area was estimated about 0.4 Gy. And no signs of high dose internal exposure were found from whole body counting and chromosome study. However, the direct measurements of the external dose started from 48 days after the explosion and the whole body counting and chromosome study started after 1969. Many of inhabitants of Nishiyama district at the time most likely ate vegetables growing in the district and drank water of Nishiyama water reservoir. A resident of those days who drank water of Nishiyama water reservoir every day says that his throat began to swell and pain from one year after the explosion. Estimation of internal exposure of early days is important.
