Residual radiation, however, from such a
getonation may be
expected to affect very large areas for @ considdrable period of
time.
Fallout is the phenomenon responsible forlthe major part of
the residual radiation hazard,
A. Description of Fallout
The term "fallout" is used to describe thal
radioactive
particles produced by a nuclear detonation when they fall back
upon the earth from the upper air.
It is compose—t
of fission
troducts, particles of the bomb itself, substance
made radioactive
by neutrons, and material from the surface of thefearth carried
aloft by the explosion.
In the case of an air bufst, where the
fireball does not come into contact with the earth, the radioactive
products of the detonation are carried high into
fhe atmosphere
as very smell particles and are scattered widely
the winds.
The great bulk of this material will undergo radidective decay before the particles have fallen to the earth.
Whe, however, the
detonation is such that the fireball rests upon t
ground, great
amounts of earth are drawn into the rapidly rising
fireball
resulting in coarse, highly radioactive particles
PWhich tend to
fall rapidly while being carried along by the wind
The cloud of a thermonuclear explosion risek
highest levels of the atmosphere and spreads over
miles in the firet hours.
rapidly to the
fundreds of square
During this time the pafticles are being
acted upon by the winds, including those up to 60,800 or 80,000
feet, which may vary greatly in direction and velogity at
different heights.
Particle size will affect the
te of fall, and
as the material descends through the rain cloud bedring levels the
fallout may be slightly accelerated by rain or sno
For a considerable distance around the point
of detonation,
radioactive particles will be distributed up-wind
d cross-wind,
as well as down-wind.
this close-in
The actual distance to which]
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