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4. Atmospheric storage of debris:
The length of storage of debris in the atmosphere influences both the uniformity of geographical distribution of fallout and the rate at which it becomes available for oral uptake.
The significance of height of burst and yield have already been mentioned. Storage is greater
for airbursts than for tower shots than for surface shots.
Also, it is greater for those shots
whose cloud penetrates into the stratosphere than for those whose cloud remains in the tropo-
sphere than for those whose cloud remains in the rain-bearing layer.
The mixture of the various
types of shots in any one weapon test series has madeit difficult to interpret observed storage
in terms of these parameters. However, certain limits may be set from the experimental data
now available.
Reasoning from present observed geographical distribution of fallout to storage time, one
obtains a certain amount of information on this time. For tower or air shots in Nevada (spring,
1953), it is shown below that fallout per unit area in the eastern half of the United States
was only a few times as great as that in western Europe. When allowance is made for spread
in latitude of cloud trajectories at increased distance from detonation site, it appears that half
life of storage under these conditions is greater than one week.
It is also shown below that an estimate of at least 15% of the debris from this same test
series had fallen out by June 14, 1953. Allowing for gummed paperinefficiency and insufficient
time for fallout from the final large shot, it seems probable that the half time for fallout was
less than 2 months.
Direct measurements have been performed on air activity collected at various altitudes.
These suggest an upper limit for the fraction of weapon debris, from past tests, which has been
stored in the atmosphere over long periods. The New York laboratory has analyzed air filter
samples from surface level in New York, air filter samples collected by jet planes at 40,000 feet
in New Mexico, and balloon borneelectrostatic precipitator collections at 80,000 feet over New
Mexico.
The 40,000 feet and 80,000 feet samples proved to be 50-100% Sr-—89 and Sr-90.
At the
beginning of 1954, Sr-90 alone represented 5-15% of the total; during the spring tests it was a
lower percentage. The results are shown in Table 1.
While these data are too few and variable to permit extensive interpretation, probably at
least one important conclusion can be drawn. By comparison with soil Sr-90 measurements
reported below, it seems likely that less than half of the total Sr-90 produced prior to the test
series of the spring, 1954, was still in the air at the start of 1954.
TABLE 1
Surface
|
46,000 feet
80,000 feet
Dec., °53 through
- March, *54.
Av. 0.005 dpm/ft?*. . | Av. 0.017 dpm/ft?.....
|
(range: 0.001-0.05)
Av. 4.3 x 10°? dpm/ft?
April, '54 through
June, ’54.
Av. 0.01 dpm/ft?... | No measurements: Av. 5x 107 dpm/ft
(range: 0.001-0.02)|
made.
|
(range: 0.4-30 x 10-4)
C. Present distribution of weapon debris:
The present geographical distribution of weapon debris can be estimated from three types
of measurement: Survey meter data on close-in fallout taken immediately after detonation;
gummed paper measurements at United States and, in recent tests, foreign stations; and analysis
of soil for Sr-90 in Project SUNSHINE.
1. Close-in fallout:
The nature of the fallout patterns in the neighborhood of the test site has been discussed
earlier. Within a radius of 200 miles of the Nevada test site approximately 10% of all debris
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