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RADIATION STANDARDS, INCLUDING FALLOUT
THE GEOGRAPHICAL DISTRIBUTION OF FALLOUT
There are two prime means of monitoring deposited radioactivity ; soil sampling and pot or funnel collectors. The preferable method is soil sampling
because it is nature’s measure of accumulated radioactivity. The results of
the latest available analyses in 1960 are given in figure 5. The isolines delineate locations with equal amounts of strontium 90 deposition and the darkened
areas show the areas of heavier fallout. The dots indicate the locations at
which soil samples were collected by Dr. Alexander of the U.S. Department of
Agriculture and analyzed by the Health and Safety Laboratory of the AEC. The
interpolation between sampling places has been performed by the Weather Bureau
assuming that there is a relationship between fallout and rainfall; the more
rainfall in a given climatic region the more the fallout. It is evident that levels
of fallout are greatest in the North Temperate Zone.
Fallout is greater over oceanic than over land areas because rainfall is
greater. There is a suggestion from data collected in the oceans and seas both
by Dr. Bowen of Woods Hole and from certain Soviet studies, that there may
be additional mechanisms over large water bodies which further enhance the
fallout. One such is the capture of the radioactive aerosols by heavy salt particles which then settle out into the ocean. The problem of oceanic deposition is
among the main unresolved scientific questions on fallout. The magnitude of
the removal by impaction on herbage and other vertical surfaces is another
source of present-day ignorance on fallout. The research in the areas of the
rainout mechanism under Federal sponsorship has, however, made notable gains
since 1959.
The somewhat heavier fallout in the Midwest United States is probably attributable, in part, to the extra fallout from Nevada atomic tests. However, a
comparison of the north-south profiles of the fallout during the interval mid-1959
to mid-1960 when there was no Nevada fallout, showed the same general peak
at about 40° N. Thus, it is probably that not all of the Midwest fallout excess
can be attributed to the Nevada tests.
For the most part, except immediately downwind of a proving ground, the
strontium 90 fallout is derived from powerful tests which lift their nuclear clouds
into the stratosphere.
Figure 6 displays a north-south cross section of the accumulated strontium 90
fallout. The vertical axis has the deposition increasing upward. The uppermost line shows the cross section of the total] strontium 90 fallout from all tests
before mid-1961. This is derived from the soil picture of the previous placard
on which the accumulated fallout was presented up to mid-1960 plus the fallout
for the ensuing year. The increment since mid-1960 is obtained from the second
method of measuring fallout; collecting precipitation in pots and funnels each
month. The Atomic Energy Commission, whose data have been here used, has
a worldwide network of approximately 125 stations making such incremental
collections.
This line brings out more clearly the peak in the 30° to 60° N. band and the
presence of a secondary peak in the 30° to 60° S. band, with equatorial and
polar minima. It should be mentioned that virtually no samples are taken south
of 40° §. latitude. The amount of precipitation decreases toward the poles and
this may well account for the decrease in fallout toward each end of the graph
but this is not the ease in the equatorial regions. The equatorial minimum
results from lower air concentration, confirmed by the extensive ground level
U.S. Naval Research Laboratory measurements and limited aircraft observations
in the troposphere. It is concluded by virtually all scientists in the fallout field
that the temperate or polar regions are the part of the earth’s atmosphere where
the stratospheric-tropospheriec exchange takes place. This high latitude exit
from the stratosphere accounts for the lower air concentrations in the troposphere near the Equator.
The middie line shows the fallout from the Soviet October 1958 tests during
the year 1959. This 1-year period has been chosen because the fallout is likely
to be of stratospheric origin (the tropospheric component having been largely
washed out by 1959) and because the end of 1959 is about as late as one can
reasonably distinguish between Soviet October 1958 fallout and other sources.
It should be recalled that by the end of 1959, about 80 percent of the Soviet
October 1958 strontium 90 had already been deposited.
This line is believed to lie entirely within the Northern Hemisphere and
peaks in the temperate zone. It is for this reason that predictions of fallout
from Soviet 1961 nuclear tests were forecast to affect the Northern Hemisphere
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