The air dose outside of buildings due to primary and scattered radiation released by atomic bombs in Hiroshima and Nagasaki was estimated as a function of distance from the hypocenter. Neutron dose was estimated from 6oCo activity in iron imbedded in concrete and gamma dose from thermoluminescence in bricks and tiles.The precision (coefficient of variation) of estimation was less than 0.11 for y-rays and less than 0.15 for neutrons. The results as compared with York's values(l) show only a minor difference for y and agree with that for neutrons in Nagasaki, but a large difference of approximately 50 per cent for neutrons and 30-70 per cent for y-rays at distances from 500 to 1500 m from the hypocenter for Hiroshima. Therefore the total air dose was almost equal to York's value in Nagasaki, but less than half in Hiroshima.
The gamma-ray dose from the radioactivity induced in soil and building materials by neutrons was estimated on the basis of the experimental data. l'he chemical compositions of soil and building materials collected from Hiroshima and Iiagasaki were determined by an activation analysis. It was found that 24Xa and 5"Mn contributed mainly to the gamma-ray dose from the radioactivity induced in soil. The dose received by a person who entered the hypocenter area in Hiroshima one day after the bombing and remained there 8 fir would have been 3 rads. The doses at distances of 500 m and I000 m from the hypocenter in Hiroshima were 18% and 0.07% respectively, of that at the hypocenter. The accumulated gamma-ray dose from immediately after the explosion of the A-bombs to infinity was about 80 rads at the hypocenter in Hiroshima, and about 30 rads in Nagasaki.
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