RADIATION STANDARDS, INCLUDING FALLOUT
55
points on the earth’s surface and in the atmosphere. An especially serious source
of error is due to possibly greater fallout over oceans. This will be discussed
shortly. There is a large uncertainty in the fraction of the U.S. Pacific test
strontium 90 which deposited as local fallout and was thus unavailable for
worldwide fallout. The numbers in the lower row assume that about a third of
all the strontium 90 was deposited as local fallout. This number, 3 megacuries,
as evidenced by the question mark, is uncertain and there is some reason to
believe that it may be'too high. Assuming that the fission yield information,
the 9.2 megacuries, is approximately correct, a smaller local fallout could create
a larger and possibly significant discrepancy between the two approaches. If
there really is more worldwide fallout available than has been estimated from
the 3 megacuries of local fallout, this extra amount is most likely in the oceans
where we have been unable to properly measureit.
2. The Soviet 1958 and 1961 inventories
The Soviet Union conducted a large nuclear test series in the autumn of 1958.
The AEC estimated that between 1.25 and 1.50 megacuries of Sr” were added
to the atmosphere. By dating from short-lived isotopes, it was possible to distinguish with some confidence between the radioactivity from this and other
sources. The numbers in the first column show that of the approximately 0.80
megacurie accounted for from actual observations (fig. 3), 80 percent had fallen
out by the end of 1959. There is a discrepancy between the 0.80 megacurie and
the AEC release, however. The most likely explanation is the greater oceanic
fallout which would raise the 0.63 and, if true, would argue that even more
than 80 percent had already fallen out.
The second column, the inventory for the 1961 Soviet tests, eannot yet be
properly completed. As will be seen later, it appears that fallout in 1962 is
roughly the same as that in 1959 and on this basis, one can estimate about 0.2
megacurie of Soviet strontium 90 deposition up to March 1962. The stratospheric
inventory to 70,000 feet has been computed by the Defense Atomic Support
Agency and their contractor, Isotopes, Inc., based on U-2 aircraft sampling. The
sum of the two numbers, 0.2 and 1.3, is 1.5 megacuries. Again, there is a discrepancy but here, the explanation is more likely to be inadequate stratospheric
sampling as seen in the next placard.
Figure 4 shows a north-south cross section of the atmosphere. The horizontal axis is so arranged that equal lengths cover equal areas of the earth’s
surface. The vertical axis is altitude in feet. The average position of the
tropopause, with the troposphere below it and the stratosphere aboveit, is indicated at about 50,000 feet south of 30° N. and at about 30,000 feet north of
30° N. It has at least one break in each hemisphere and, particularly in the
stormy temperate zone, undergoes large day-to-day changes in height. The
placard shows that the construction below 70,000 feet reaches a maximumin the
polar regions and near the latitude of the Novaya Zemlya Soviet proving grounds.
The zone of greatest radioactivity concentration slopes upward toward the
equator where the radioactivity becomes less concentrated. Some of the Russian debris has reached the equator and passed into the Southern Hemisphere
by April 1962. This has been confirmed by the AEC balloon flights in southern
Australia at 50,000 feet from February through April 1962. This picture, it must
be remembered, preceded any U.S. atmospheric testing and subtraction of the
pre-Russian 1961 radioactivity was accomplished with little difficulty.
At present only balloons are capable of sampling above about 70,000 feet. The
AEC has a station which operates an air filtration unit with the cooperation of
Air Force at San Angelo, Tex., and the profile of Russian debris for the
month of February 1962 is given as the horizontal lines emanating from the
vertical line located at about 30° N. latitude. The peak coincides with the peak
concentration detected by the aircraft and at 70,000 feet, it appears as though
the nuclear cloud has been topped.
But at 90,000 and 100,000 feet, it is evident
that here is a tendency for increasing concentrations once again.
Dr. Gustafson,
of Argonne National Laboratory, with AEC, Air Force, and Weather Bureau
participation flew a different kind of monitoring instrument at Thule, Greenland, in April and the horizontal bars at about 80° N. show the results of his
measurements. Although this peak does not exactly coincide with the peak in
the aircraft profile there is again evidence of increasing radioactivity at higher
altitudes. It must be concluded that the inventory calculation to 70,000 feet has
neglected those Soviet clouds which rose to great heights, a condition fully recognized by the Defense Atomic Support Agency group constituting the inventory.
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