S i ona 7 ae contamination will extend depends on the yield off the weapon, and may be expected to cover at least the area boundeR limits of the D-ring of >iast damage. It will p by the outer bably be of a high degree of radioactivity. Outside the zone of close-in contamination] radioactive fallout can be expected to occur progressively inj the direction of the effective wind over a number of hours and Bver an area of @ great many square miles. The radioactive material may or may not be visible but can be detected with radiologifal monitoring instruments. Falling dust or ash, if visible, will most likely be radioactive, The intensity of the radioactivity is very high immediately after the burst, but "decays" or diminishes rapidly. Therefore, since not much time will have elapsed, the partidles reaching the ground near the burst will be very highly radioadtive while those which are carried a long distance will have lost Pmuch of their radioactivity before they alight. The fallout material on the ground, of cotirse, continues its radioactive decay. After 24 hours the rate @f radiation per hour based on the decay rate formula given in "Tie Effects of Atomic Weapons", published by the U.S. Atomic En@rgy Commission, September 1950, 1s about 2% of the rate at one h@ur after the burst. This may still bevery dangerous at somefpoints, however. B. Energy Range of Residual Radiation Radiation emitted from fallout is of a vorer penetrating power than the initial radiation produced at theftime of bomb detonation. Therefore, the effectiveness of @ given thichness of shielding will be greater for residual than for [nitiel nuclear radiation. -6- DPE ARCHIVES -— 7 an . hai UNCLE?