~ 8 —
following an exposure of only a few hours.
Small detonations,
such as are fired at the Nevada Proving Ground, produce contamination extending for only a few miles and the dangerously
active areas are confined to tne controlled bombing range.
The dimensions and shape are determined by the whole complex
of wind patterns but cnaracteristically there is a narrow
fan in which the area of highest contamination has a somewhat elliptical shape.
Up-wind and cross-wind contaminations are Limited in extent and are far less spectacular
in intensity.
For either an air burst or a surface burst, the
radioactive material which does not descend quickly is
carried by the winds prevailing at the various altitudes so
that the fallout of this finely divided particulate material
is determined by the wind trajectories and the particle size,
the latter greatly influencing the velocity of fall.
When
such material descends sufficiently to enter the rain cloud
bearing level, usually below 20,000 ft., the fallout to the
earth may be accelerated by rain or snow, and it is common to
find, following a detoiation, that rainwater from regions of
cloud passage shows readily detectable activity.
Other than
such incidental concentrations the general fallout tends to
be remarkably uniformly distributed over the earth's surface.
Radioactivity resulting from detonations decays
with time in accordance with the following equation:
-1.2
I = Iot
where I is intensity at time t and I, is the
intensity at unit time.
The significance of any measured
activity is dependent upon the time since detonation.
Within the first few hours and days most of the activity is
due to radioisotopes of short half-life so that radioactive
elements such as iodine, molybdenum, ruthenium and many others
may be demonstrated.
The activity of such elements is soon
gone and the exposure due to them is of transient character.
It may be of general interest that the average
total fallout over the United States from the CASTLE series
of tests in the Pacific this past spring has amounted to
approximately 100 millicuries per square mile as of this
week (September 23, 1954).
While this amount of radio-
activity is minute, it is possible with techniques available
today to detect and measure it accurately.
This has been
done for all regions of the country.
The gamma radiation
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