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Many of the samples collected in the 1959-1970 background survey were also
analyzed for their 241am content.
Aircraft sampling was normally conducted in
the vicinity of three latitudes:
70°N, 35°N, and 10°N.
Stratospheric samples
were collected quarterly at altitudes ranging from 40,000 to over 60,000 feet.
For brevity, the details of the individual sample collection are omitted in
this manuscript. The measurements were performed at McClellan Central Laboratory and Tracer Lab West--now called L.F.E. Environmental Laboratories.
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It ig interesting to note that the 24tam/239pu atom ratio in the atmosphere
increased yearly from 1963-1967.
This was the result of intermixing of debris
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to 733py were superimposed over the previous inventory.
The net effect is an
increase in the total atmospheric 235pu relative to 24tam,
CONCLUSIONS
The 240py/2 35py isotope ratios in the atmosphere have changed with time. A
possible explanation for this variation is that recent injections of debris
Play a major role in determining the characteristics of the transuranic elements
in the atmosphere.
Measurements of stratospheric 2"!Am from 1963-1970 seem to indicate that as a
first approximation, the 24lam burden of the atmosphere may be considered as a
The 2"1Am concentration of the atmosphere
decreased from a high of 5.92 x 10° atoms/SCM in 1963 to 0.27 x 10° atoms/SCM
Table 3.
during 1967 and subsequent years containing smaller quantities of 744m relative
241 ay Background Summary 1963-1970
To a lesser extent, the
241am/233pu ratio was influenced by the ingrowth of 2%!am from the decay of
24lpy,
From 1967-1970, the americium/plutonium ratio decreased in contradiction
with expectations. The reason for this anomaly is that much of the old debris
that was injected from 1956-1963, containing relatively large quantities of
“lam, has been removed from the atmosphere.
The fresh injections of debris
260
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When sets of 74!Am data are averaged, general trends become apparent. ‘Table 4
shows the average annual @"?am and 24!4m/23%Pu levels obtained by averaging
the data reported in Table 3 on a yearly basis.
The data seem to indicate
that as a first approximation, one may assume that all the atmospheric americium
was injected as a point source in 1963.
This burden decreased with an apparent
half-life of 1.6 years.
point source injection during 1963.
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It is interesting to note that the concentration of 74 JAm
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as well as the ?"lam/23%pu ratios vary from sample to sample.
The amount of
241am formed is a function of both the age and irradiation history of the
debris.
containing different relative amounts of 74am.
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Table 3 shows the results of the study for samples whose measurement uncertainty
were less than 25%,
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