-3ence that important factors include not cn iv contact
of the Sirebail with the surface, but the nature of ‘ie
surface, whether it be land or water and the type of soil and
the composition of the water, whether fresh or sea water.
Also, the height to which the fireball rises is important, in
particular the heisht relative to the tropopause, the dividing
layer between the troposphere and the stratosphere. Yield is
the main consideration here. A rough rule is that megaton
weapons push through the tropopause into the stratosphere,
and kiloton weapons stay below the tropopause in the troposphere.
Thus, we see immediately that kiloton weapons deposit their fission products much more quickly than do megaton
weapons.
Of course this is of less importance in so far as
the long-lived fission products, such as strontium-90 and
cesiun-137, are concerned, but it is of more importance fer
the shorter-lived fission products.
As a general rule, an
air-fired kiloton weapon will deposit its radioactive fall-
out in a period of between two weeks and one month on the
average after the c¢tonation, whereas an air-fired megaton
weapon will deposit its radioactive fallout over many years~-
on the average about ten years. Thus, the effects wnich are
due to the short-lived fission products are larger for a
given anount of fission energy release in kiloton weazons
than they are for air-fired megaton weapons. Considering
the average age of the kiloton fission products to be 1
month, the external gamma ray exposure from one megaton
of fission fired as say 50 bombs of 20 kilotons each would
be 30 times that for a single bomb giving one megaton of
fission energy--if both were fired well up in the air. The
fission products from the small bombs fired in Nevada would -
fall in the latitudes 10°N to 60°N in about one month,
while the larger bomb would give fallout over essentially
the whole earth in about ten years.
For strontium-90 effects there is relatively little difference per unit fission
yield.since even the residence time in the stratosphere is
small compared so the 28 year half-life of radioactive
strontium and the 27 year half-life of radioactive cesiun,
which is produced at slightly higher yield than stronvium-90,
and which appeérs to be disseminated in about the same way.
The content of radiostrontium and radiocesiun in
the stratosphere is by direct measurement shown to be roughly
the same thougn the radiocesium is somewhat higher possibly
(more)
i