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- 16 -
pot and the gummed film tend to concentrate fallout, particularly
on dry, windy days.
It is also possible that some of the Sr-90
has been leached beyond the sampling depth.
At the present time,
there is insufficient knowledge to explain the difference between
the two methods.
One megacurie, being intermediate between the
two estimates, is perhaps the most reasonable approximation of
the total amount of Sr-90 deposition on the earth by delayed
fallout in mid~-1956.
The test firings in Nevada only partly account for the
relatively elevated deposition in the North Temperate Zone.
There
is evidence that much of the differentiation betreen the North
Temperate Zone and the rest of the world can be attributed to the
preferential fallout in this region fram 1954 series in the
Pacific (Operation CASTLE).
By mid-1956 fallout from the USSR
tests did not account for a major fraction of the total observed
fallout.
Stratospheric Reservoir
An estimate of the future distribution of Sr-90 may be obtained
from the above data plus knowledge of the amount of Sr-90 suspended
in the upper atmosphere, and the rate at which it precipitates
to earth's surface.
The direct method of measuring the straos-
pheric reservoir is to obtain samples, using balloons or high
flying eircraft.
This hes been done intermittently since 1953
but the data are too few to permit one to estimate by this method,
with any degree of confidence, the totel inventory of stratospheric Sr-90.
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