Various units have been used to express exposure to
radiation such as the roentgen, rep, rem, and rad. All are intended to express some relationship between the radiation
energy absorbed and biological effects. Since it is not critical
for the following discussions to understand the technical differences among the units, only the “roentgen” will be used. To
provide some perspective as to the magnitude of the ‘“roent-
gen” table I is included.
2. Sources and Nature of Fallout
The major source of radioactive materials in fallout is the
fissioning or splitting of atoms of uranium and plutonium that
gives rise to a large numberof unstable radioisotopes. Inthe
fusion process hydrogen nuclei are joined together. Induced
radioactive
products
result when
inert materials capture
neutrons that are released during either the fission or fusion
process.
Generally, these induced radioactive materials are
. relatively short-lived and contribute only in a minor way to
radiation exposures to man. The principal exception is carbon
14 described in section I F (page 16).
Someof these radioactive materials escape as gases and are
dispersed and diluted in the atmosphere. Most of the fission
products, however, become incorporated into or attached onto
minute inert particles of dust and debris from the immediate
environment of the bomb.
The dust particles, together with
the associated radioactive nuclides, are swept high into the air
by the heat and force of the nuclear explosion. The larger
particles and those in the lower levels of the cloud fall nearby.
Smaller particles in the upper levels are carried away to be
spread worldwide. The worldwide distribution of these radio-
active particles follows the same pattern as would occur with
any other small particles injected into the same regions of the
atmosphere —-radioactivity has essentially no effect on the
pattern of distribution.
Roughly, a nuclear detonation of one-half million tons or
less, fired at a low altitude—but high enough so the fireball
does not intersect the ground—results in most of the fission
products remaining in the lower atmosphere, the troposphere.
They are deposited on the earth’s surface at a rate such that
one-half of the amount remaining inthe atmosphere at any one
time falls in 2-4 weeks (called tropospheric residence half-
time). As the energy yields of the nuclear detonations increase, more and more of the fission products are swept higher
and higher into the stratosphere—the layer above the tropo-
sphere (fig. 2).
The residence half-time here is more like onehalf a year for injection into the lower stratosphere in the polar
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