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RADIATION STANDARDS, INCLUDING FALLOUT
Poleward from the middlelatitudes the total deposition of stratospheric debris decreases with decreasing precipitation (rain and
snow). It should be noted however that there does not appearto be
a strongly decreasing trend in air concentration and concentration 1n
precipitation from the middle latitudes poleward.
;
Making
use of planned and incidental tracers in debris from the
specific detonations, and taking advantage of changing Sr**/Sr?
ratios with time from different series of tests, we can now with considerable certainty makes estimates of residence time in the stratosphere for debris introduced at certain locations in the stratosphere.
1. More than 50 percent of the debris from the 1958 U.S.S.R. Arctic
tests in the range of a few hundred kilotons to a few megatons injected
into the lower stratosphere (30,000 to 90,000 feet) was deposited in the
Temperate Zone of the Northern Hemisphere during the spring fol-
lowinginjection.
2. Debris from equatorial tests in the low megaton range exhibited
a half-residence time of the order of 1 year during thefirst year and
increasing somewhat in half-residence time after 1 year.
3. Debris injected at a very high altitude (greater than 130,000
feet) near the Equator began to appear in the lower altitudes after 1
year and has been deposited at a rate corresponding to a half-residence
time of at least 3 years.
There is still to be determined therelative roles of horizontal transport and vertical transport in the holdup of high altitude equatorial
debris. Also, we have yet to learn whether the fallout pattern from
a debris cloud, such as that from the U.S.S.R. 55 to 60 megaton detonation of October 1961 which reached an altitude of 130,000 to 170,000
feet in the Arctic, will behave more like very high altitude equatorial
debris, like low stratospheric arctic debris, in some intermediate
fashion or in somedifferent fashion altogether.
So much for the transport and distribution of fallout. Let us turn
now to techniques for estimating fallout rates in a given geographic
location. Surface air sampling techniques have changed little in the
past 3 years. These samples, when held for 4 to 5 days to permit
decay of natural radioactivity and then analyzed by gammaray spectroscopy, give an index of the concentration in the air at groundlevel
of important gamma-emitting fission products (Cs*", I! and
zirconium-niobium isotopes) in the trophospheric or relatively fresh
fallout and of Cs*’ in stratospheric fallout. These readings do not
relate directly to final deposition on the ground. I think it is important to keep this in mind.
The latter information must be derived from analysis of soil and
vegetation or from material collected in pot or funnel systems as the
greater part of the radioactive debris is brought to earth in rain or
snow.
In the 1957 hearings it was suggested that Cs’ in milk and in
humans might prove to reflect fallout rate more than accumulated
fallout. Data presented in 1959 made this even moreplausible. Today
there can be no question but that this is the case. The milk levels
which peaked in the spring of 1959 had fallen by later summerof 1961
to about one-eighth that value. Levels in humanstend to lag behind
milk levels by about 6 months.
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