correction factor. The altitude error at the 300-foot level has a maximum value of 15 feet based
on the APN-1 specification. The altitude correction-factor error will be less than 4 percent.
The absolute value assigned to an isodose depends on the calibration of the radiation detector
and altimeter, and on the altitude-correction factor.
The major assumption of an average
gamma-emission energy of 500 kev in evaluating the altitude absorption derivation is supported
by the gamma-spectrometer results (Section 3.1.2), and the ratio of the radiation readings of an
energy-dependent detector and the Top Hat detector during a survey over the Eniwetok Atoll
(Section 3.2).
Examination of the radiation dose-rate relations between various altitudes over land and water
during Operation Redwing (Section 3.2), during previous operations (Appendix C), and during Op-
eration Pluinbbob (Reference 12) indicate the validity of the assumptions and the accuracy of the
calculated altitude-correction values,
4.2.2
Contamination-Density Determinations.
As indicated in Section 1.3.4, fallout on a land
surface is expected to produce, at 3 feet from the surface, a gamma dose rate about 1,100 times
higher than the dose rate resulting from the same fallout density in the sea. Agreement of data
with the theoretical derivation primarily depends on the accuracy of three factors: (1) the depth
of vertical mixing, because material below the surface of the sea will not contribute to the gamma field, (2) the average gamnia-emission energy, which determines the thickness of the surface
layer that does contribute tothe gamma field, and (3) the air absorption, which determines the
surface area viewed by the radiation detector. The equivalent depth of mixing was estimated as
60 meters (Section 3.4.2). This is in essential agreement with measurements made during Op-
cration Castle.
The experimental work was based on only a few stations and did not necessarily represent
the conditions throughout the fallout area. However, variation in mixing will introduce variations
in the area enclosed by an isodose contour; this is discussed in Section 4.3.1.
The average gam-
ma energy and the altitude absorption characteristics assumptions are supported by several
measurements as discussed in Section 4.2.1.
There is one direct comparison of the land and water equivalence based on the fallout follow-
ing Tewa (Figure 3.23), The isodose pattern encloses Parry Island, Eniwetok Atoll.
This island
:s located between the 25 and 250 mr/hr land-equivalent isodose lines (0.025 and 0.25 mr/hr water
sodose),
Radsafe measurements indicate a gamma dose rate between 100 and 125 mr/hr on
Parry at 24 hours following Shot Tewa.
The contamination density calculations are based on the factors discussed above, and on the
relationship between beta and gamma curies. A direct comparison of the conversion between
gamma dose rate and beta specific activity is discussed in Appendix D. The measurements are
not conclusive. However, the general trend of this data does agree with the theoretical calculations (Section 1.3.1).
4,3 DISTRIBUTION OF CONTAMINATION IN THE SEA
The fallout estimates based on the aerial-survey charts show a definite relation to the fission
yield. However, the distribution of this material is not related to the total energy yield, because
the conditions of the shot—— water, land, or air- —affect the fallout. Meteorological conditions
also play a major part in determining the area of contamination.
4.3.1
Stability of Contaminated Area.
Fallout deposited in the sea is acted upon by the ocean
currents, producing a horizontal translation of the location of the material, and a vertical dis-
placement based on the mixing of the material in the sea volume.
To obtain a measure of the
stability over a period covered by the aerial surveys, measurements were repeated from day to
day.
All gamma radiation measurements were referred to 3 feet from the surface and to H+24°
hours so that a conymon comparison could be made for any particular isodose area. The hortzontal translation is clearly indicated by the positional shift of the isodose pattern, The vertical
mixing is indicated by the amount of area enclosed within the described pattern.
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