RELATIONSHIP BETWEEN THE AIR CONCENTRATION OF
RADIOACTIVE FISSION PRODUCTS AND FALLOUT
Perhaps the most widely used methods for the evaluation of atmospheric and deposited
radioactivity are the air-filter and gummed-paper techniques; both methods have the advantage
of simplicity. In general, effects associated with meteorological factors, especially rain,
makeit difficult to compare one method with the other.
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The concentration of radioactivity in the ground-level air at Washington, D. C., was
measured with the NRL filter equipment (1). The pumping rate was 30 cubic feet per minute,
using Army Chemical Corps Type 5 filter paper.
Daily values for the fission product
concentration per unit volume of air were obtained from the rate of decay of the filter
collections, The gummed-paper collections were made with the standard arrangement
used by the Weather Bureau and the AEC. The deposited radioactivity per unit area was
found from the counting rate of the ignited residue of the gummedpapers. Measurements
were made from December. 1954 through May 1955,
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A comparison of daily concentrations of fission products collected by gummed papers
and air filters is made in Fig. 1. The rainfall during the period also is shown. As may
. be seen from the figure, the ground concentration usually rises with rain or snow. This
seemsto be true for old fission products, that is, those collected between December 1954
and February 22, 1955, as well as for the younger material collected from the TEAPOT
tests.
In order to obtain a more accurate comparison, the gummed-paper residues and the
daily filter collections for each week were combined and counted as nearly simultaneously
as possible. The results are shown in Table 1. The rates of fallout listed in the table
were obtained by assuming a uniform distribution of radioactivity with altitude. The expression “rate of fallout” is used throughout this discussion as a convenient means for comparing
the fallout activity per unit area per day with the activity per unit volume of air. Examples
of the effect of nonhomogeneity in the atmospheric concentration are shown by the high rates
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of fallout for 4/1P-4/18 and 5/2-5/9.
The former collection accounts for 65% of the total
fallout for the whole period of the measurements. In these cases, activity was apparently
rained out of high-level clouds from relatively recent atomic tests. In the period of the
5/16-5/31 collections, however, rain caused a proportionately larger increasein the air
‘concentration than in the fallout.
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It will be observed in Table 1 that the rates of fallout vary from 0.7 to 400 10° ft/day.
The average over the five- month period was 43 10° ft/day. The rates of fallout obtained
from a similar tabulation of the daily readings are presented in Table 2 for each month from
December 1954 through May 1955. The average rate of fall for days without rain was
2.7X 10° ft/day and, for rainy days, 30% 10° ft/day.
The extent to which the average rates of fallout can be relied upon to be a measure of
the deposition even in the Washington area is by no means clear. Previous measurements (2)
on natural radioactive particulates indicated the mean lifetime of these substances in the
- lower atmosphere to be about 15 days. The present data seem to show a considerably faster
rate of fall. Undoubtedly, the distance from the tests at which measurements are made, the
frequency of rainfall, particle size, efficiency of rainfall in washing the atmosphere, and
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height of rainfall are important factors.
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