of principal concern, because of their greater penetrating power.
The
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 ino
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 onetenth 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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