b. The gaseous mixture of fission products, etc., condensed on liquid droplets of earth
which never became hot enough to vaporize. Particles formed in this manner were partially
black or green, glassy, and with the activity rather uniformly distributed throughout the
particle.
c. The gaseous mixture of fission products, etc., condensed on solid earth particles
which were passing through the incandescent cloud. Only the surfaces of such particles
were active.
d. Various agglomerations of the foregoing particle types.
Measurements have been performed on collected debris to determine if fractionation takes
place in the relative abundance of the various fission product isotopes. The following partial
picture can be constructed from present evidence:
a. Nearly all airborne debris samples collected within a few days after a detonation
have shown comparatively little fractionation. For example, Sr-89 and Ba-140 abundance
has nearly always lain between 0.4 and 1.5 relative to Mo—99 despite the noble gas precursors
of these isotopes which might reduce condensation. However, examples of fractionation
have been observed after both kiloton and megaton range surface shots, in which airborne
debris is enriched and close in fallout depleted in Sr—89, 90 by a factor of 10-100. The
picture is not yet entirely consistent.
b. Fallout within the U.S. from Nevada tower and air bursts has not been seriously fractionated in Sr—89 and Sr-90. Within a factor of a few, the abundance of these isotopes
relative to each other and to the gross fission activity appears to have been as expected
assuming no fractionation.
c. Residual high altitude air activity after a few months appears from limited data to be
enriched many fold in Sr-89, 90 relative to gross fission product activity.
B. Dependence of fallout on meteorological conditions:
Fallout depends on meteorological conditions in 2 respects: Location of fallout and quantity
of fallout. Considerable information obtained both theoretically and experimentally is available
on these points.*
1. Close-in fallout:
The typical close-in fallout pattern for tower shots in Nevada has been an elongated plume
downwind from the detonation site. Often the point of maximum dose rate, except at the site
itself, has been found many miles downwind,as a result of debris having fallen from tropopause
altitude in about 3 hours. Attempts have been made to correlate fallout pattern with local
meteorological and debris cloud height conditions. One method has been to trace the path of fall
of typical particles (e.g., 125 micron diameter at one test site) from cloud to ground assuming
Stoke’s law. However, vertical convection distorts the predictions of this analysis. Another
method has been to establish purely empirical correlations between wind conditions and fallout
pattern observed in past shots. The results of one such analysis indicated maximum surface dose
rate was proportional to wind velocity shear with altitude. However, it is very likely that this
overall correlation depends upon other interactions of meteorological conditions which may have
only local validity.
2. Statistical estimate of long range cloud transport:
In order to predict the position to which a particular point in a debris cloud will be trans-
ported by the wind, statistical calculations have been carried out for Nevada shots over a 2-day
period at the 300 milli-bar air pressure level (c.30,000 ft.) in the spring season.
The mean
Footnote references on pp. 42-43
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