c. Surface shot.—Thepicture is about the same as in the tower shot, except that more surface
dust and sand is entrained; therefore the fraction falling is probably greater.
2. Dry fallout in the first few hours:
a. Airburst.—Practically no fallout is observed, even in the case of low air bursts where the
dust column catches and joins with the mushroom in the course of its rise. The fallout curtain
constitutes less than 1%, and does not begin to reach the ground until several hours after
the burst.
b. Tower shot.—The height of tower and yield have an effect on the fraction which falls in
the first few hours, the trend being towards larger fractions being scavenged for the larger yields
and the lower burst altitudes, as one would expect. The observed early fallouts in Nevada range
from 5% to 30%, with the most probable fraction about 15%.
Most of these shots have been over
sandy desert. A few detonations have been over significantly finer grained soil, and in these
cases close-in fallout has been considerably lower.
c. Surface shot.—The only data available for early fallout from surface shots are from a low
kiloton device in Nevada and a megaton-range device on a Pacific coral atoll. Information on
neither of these was as accurate as could be wished, but estimates for close-in fallout (within
a few hundred miles for the Pacific shot) were in the range of 50% to 90%.
3. Deposition after the first few hours on the continent from rain and dry fallout:
a. Airburst (low yield).—The yield of a bomb determines the height to which the debris is
carried. When the yield is in the order of 5 KT orless a large fraction remains down in the rainbearing level, where it can be scavenged by rain. In such cases about 15% of the total yield falls
on the North American continent from tests in Nevada.
b. Airburst (high yield) .—vYields of 10 KT or more result in the mushroom cloud and 99%
of the material being carried above the top of the rain-bearing level (about 20,000 ft. in summer).
Thus, the rainout and fallout from such a burst can come only from the wake and fallout curtain
(amounting to about 1%) or from an occasional thunderstorm or towering cumulus cloud which
results in downward mixing of the material. The continental fallout from such bursts in Nevada
has generally accounted for only about 1% of the total activity.
c. Tower shot.—Here, as stated above, the radioactive debris is spread over all altitudes to
some extent, and the continental fallout from Nevada tests has usually amounted to about 5%
of the total activity. These have all been from “high yield” tests, which in this context means
greater than about 10 KT, so that most of the material is carried above the rain-bearing level.
4. World-wide fallout:
There are essentially no data available as yet which permit a correlation of very distant
fallout with conditions of debris formation. At large time and distance, fallout has not been
unambiguously identifiable with any single shot in a series of unlike detonations.
5. Physical and chemical composition of debris:
Data available on physical and chemical composition of bomb debris do not give a comprehensive picture. The type of particle collected for study depends so strongly on the nature of
the collecting apparatus that representative sampling has not been achieved. Therefore, only
qualitative descriptions of the particles are possible.
The height of burst has a marked influence on the method of formation of the particles,
and therefore on their chemical, radiochemical, and physical properties. The following methods
of formation and properties have been observed:
a. The gaseous mixture of fission products, earth, and bomb casing condensed into
droplets and solidified before coming into contact with particles of liquid or solid earth.
These particles from fission products were black, glassy, ferro-magnetic, and highly radio-
active. They contained considerable iron, probably as magnetite.
4
* Qn
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