*
4
1955 so mach of the debris originated from the Spring 1954 test series
(Castle) that one could reasonably essign an origin to the fission products
and deal with the ercss activity, if fractionation could be neglected.
After 1955, the number of tests were so numerous that almost all samples
contained fission products from many tests.
The method of estimating
individual isotopes from the gross fission product activity and burst assignment was no longer as useful.
Aireraft filtering flights reaching into the lower stratosphere have
been reported by Stewart et ar(") by Aler, Bjornerstedt, Edvarson and Low
in Sweden, and by Hvinden (7) in Norway.
(6)
All showed significant increases in
the long-lived radioactivity of the air with increasing altitude, and a
relatively sharp increase at or near the tropopause.
The results are shown
in Figure 1.
tshii'®) |
in Japan, employed a technique utilizing balloon-borne Geiger
counters to attain altitudes of 50,000 feet, compared to the 40,000 feet
attained by the aircraft.
However, this technique had the drawback of
measuring the total gamma activity in situ, and it was necessary to subtract
the cosmic ray contribution based on a knowledge of the mean distribution of
cosmic-ray activity with altitude.
The two contributions were of about the
same order of magnitude in the lower stratosphere.
While these flights
showed higher concentrations of fission products at the tropopause level,
they did not show the sharp gradient at the tropopause or the large differences
between the troposphere and stratosphere.
IV.
Models of Stratospheric Storage and Fallout
MN A 6S
mes
Utilizing the estimates of stratospheric injection, the scanty upperair measurements available and the observations of surface deposition,
several estimates of the probable residence time of Sr-90 particles in the
stratosphere have been made.
Libby! 3) estimates about 10 + 5 years, Stewart
et a4) 5-10 years, vachtal?) , about 5 years, and the U.N. Scientific
Committee (10) a value of about 8 years with a range of 5-10 years.
All of
these estimates imply a stratospheric structure at variance with known
meteorological principles (this has been recognized by the last two references
cited).
The concept of an unvarying mean residence time for stratospheric
debris would imply uniform mixing in the stratosphere with a semi-permeable
membrane at the tropopause and would be independent of the actual altitude
4
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