The
of principal concern, because of their greater penetrating power.
most energetic beta particles travel only a few yards in air and are of
concern only when the fallout materials remain in contact with or in|
very close proximity to the skin, or when the emitting materials find
their way into the body.
The amount of alpha emitting isotopes
—
associated with fallout material is considered to be of relatively
™
-
minor consequence.
EXTERNAL GAMMA EXPOSURE
The gamma radiation dose that one may actually receive and
the biological effects are dependent upon a number of factors, as
follows:
1.
Radiological decay.
The decrease in radioactivity of fallout material roughly
follows the relationship of (time)-1-2,
This means that, for every
sevenfold lapse of time after a nuclear explosion, there will be a tenfold reduction in dose rate.
For example, if fallout occurs one hour
after a detonation, such as might occur for twenty or thirty miles
around ground zero of a high yield weapon, the dose rate will be one-
tenth of its initial value by the seventh hour. - An additional tenfold
reduction would require seven times seven hours or approximately two
additional days of waiting.
The theoretical* dose accumulated from
the first to seventh hour after detonation would be approximately the
* Calculations of theoretical doses are based on (a) the radio-
activity decreasing according to (time}-1-2, (b) there is no loss”
of activity by weathering effects, and (c) the person is out-ofdoors for the time considered.
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