ence 6 may be . _d to deduce the dose-rate contribution{. an amount, in curies, equal to that
for the mixed fission products. The dose-rate measurement at 300 feet is more sensitive to
this isotope by a factor of 3.6 because of its increased roentgen conversion from curies, and the
larger volume of water contributing to the surface radiation flux.
1.3.4 Distribution of Fallout.
To estimate the distribution of fallout, the equation relating
gamma dose rate above the Surface to contamination density in a volume of Sea water may be
used in conjunction with the isodose distribution charts and depth of mixing measurements. The
contamination density in a thin layer at the surface may be estimated from the average gamma
dose rate in the various 1sodose defined areas. Summation of the estimated contamination would
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Outer
Boundory
A
4
8
~-eowmty
Distonce —=
—
Minimum
~~
Delectable
Limit
0
Outer
Boundary
Figure 1.3 Determination of estimated outer boundary.
yield an estimate of the megacuries of surface radioactivity in the fallout area. This may be
correlated with the depth of mixing and the total fallout activity computed.
If the fallout is deposited in the sea, the equation in Section 1.3.1 indicates that a contamination
density of 1 megacurie per naut mi? would produce a gamma dose rate of 2.5 mr/hr at 3 feet
from the surface. The same contamination density, on land, would produce 2,800 mr/hr (Section
1.3.2). For rough estimates, 1 mr/hr at 3 feet over water is equivalent to a 1,000:1 increase
in activity per naut mi? when compared to 1 mr/hr on land.
The calculations for land and water are summarized as follows: on land, 1 mr/hr at 3 feet is
equivalent to 2.1 * 107 (dis/min)/ft? or 3.56 x 1074 negacuries/naut mi’; on water, 1 mr/hr at 3
feet is.equivalent to 4.43 x 10° (dis/min}/liter or 4.04 x 107! megacuries/naut mi*® where depth of
mixing is 60 meters.
,
When the fission product falls intc the sea, the outer boundary of the contamindted erSeean! ¥ ES
be indicated by gamma-radiation readings that are only slightly above the background gamma
dose rate. Figure 1.3 illustrates the radiation profile across a contaminated area. The esti-
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mated outer boundary (EOB) from a shot with a high-fission yield is indicated at A and D. A
shot with the same total energy yield, but producing a smaller quantity of fission products, will
have an EOB at B and C. Both shots may have the same actual outer boundary, yet the minimum detectable limit of radiation of the instrumentation will result in a low estimate for the
area. For material-balance calculations, the quantity of radioactivity outside the EOB will be
Small in relation to the quantity located in the higher-intensity areas.
15
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