down ty the Larger particles.
It consists largely -£ matter
drawn «up intc the fireball from the surface which i either
totally cr partially varorized. Under these condit:ons so
mucn matter is vaporized by virtue of the fireball't. touch-
ing the ground that the particle sizes formed in the freshly
cooled vapor are large.
The second and third types of radioactive fallout
are world-wide in nature and consist of finer material and
are divided according to whether the material happens to lie
n the lower part of the atmosphere, the troposphere, where
rain and weather phenomena occur, or the higher part of the
atmosphere, the stratosphere, which is free of such precipitating mechanisms. The tropospheric fallout occurs ina
matter of two or three weeks or a month or so.
It occurs
largely as a result of rain and snow, and water precipitaticn in general, and falis in the general latitude of the
test site. The stratossheric fallout, in contrast, takes
years, We are not compistely certain, but it appears that
an average tine of something like ten years, or perhaps
somewhat less, is a reasonable figure, and during this time
the distribution becomes nearly world-wide. When the
stratospheric fallout manages finally to pass into the
troposphere 2% is quickly removed by the same type of
mechanism that brings down the world-wide tropospheric fall-
out, namely rain and moisture.
The precipitating mechanisms consist in general
of the collision of the tiny particles with moisture droplets
in clouds, tcgether with the interception of particles by
falling raindrops. The first mechanism was recently suggested by Dr. Greenfield in connection with Sunshine problems--~the study of world-wide fallout is called Project
Sunshine.
In addition to the scavenging action of rains and
‘fogs, there is definite evidence for a considerable prob-
ability of pick-up on direct contact of air with surfaces
such as the leaves of grass and trees.
Frequently, grasses
are found to have higher strontium-90 content than would
correspond tc the soils in which they grow, and this is
due undoubteciy to direct pick-up.
The dissemination of strontium-90 and all fallout
is greatly dependent upon the firing conditions.
There is
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