pefore the maximum radiation could be read. This resulted in the flattened peaks, as seen in
Figures 2.14 and 2.15. Since this penetration meter represents a single point, further interpretation of the results seems unnecessary; however, the amountof information obtainable from
this type of radiation-recording device is considerable, and the general methods with certain
modifications could be an important means of evaluating fallout from future tests.
9.6.6 Probe Penetration Depth.
In reducing the measurements made during Operation Castle
(Reference 2) it was assumed that the depth of mixing corresponded to that of the top of the thermocline. By making this assumption, it then became only necessary to gather together all of
the bathythermograms taken during the test period and to compute the average depth of the mixed
layer (top of the thermocline) to determine the depth of mixing.
In order to check the validity of the above asumption and to actually measure the penetration
depth at various points throughout the fallout pattern, the probes were constructed with pressuresensing elements. These have been described in the section covering instrumentation.
A comparison between penetration depth and depth of the mixed layer is shown in Figure 2.16.
The penetration depth is defined as follows: if dose rate is plotted against depth, the penetration
depth is the depth which, when multiplied times the surface dose rate reading, would yield the
same area as graphic integration of the area under the curve from the surface to the depth at
which the sea-water background is attained.
In Figure 2.16, the penetration depth is 93 meters.
When this is multiplied by the dose rate
reading of 0.041 mr/hr# at the surface, a value of 3.82 (mr/hr#)-meters is obtained.
Graphic
integration of the area above the curve yields 3.9 (mr/hr#) meters.
It may be seen from Figure 2.16 that some of the fallout products penetrate below the mixed
layer. This is particularly true for Shot Zuni, which was fired over land. The percentage lost
from the mixed layer in this fashion cannot be determined from the measurements that were
made.
This is a function of particle size and character that varies both with the particular shot
and with direction and distance from ground zero. For purposes of calculating the 3-foot dose
rate, it has been assumed that none of the fallout penetrates below the calculated penetration
depth. This assumption yields values of 3-foot dose rate which can only be less than the actual
.
case.
The penetration depths, from probe measurements, for each shot are listed in Tables 2.1
through 2.4 for each ship. Some of the stations are not listed. In those cases, either the penetration depth could not be determined from the probe record or, as in the case of Horizon Stations
13 through 17 during Shot Navajo, the ship was measuring radioactive water flowing out of Bikini
Lagoon. For purposes of comparison, the depth of the mixed layer, as determined from BT
measurements at each station, is listed.
The above information is plotted graphically in Figures 2.17 through 2.20, wherein penetration
depth and mixed layer depth are plotted against hours since arrival of fallout. At early times,
while downward mixing was, still taking place, the plot of penetration depth versus time results
ina sloping line. After mixing is complete, a constant penetration depth is assumed to hold for
the remainder of the survey.
Table 2.5 summarizes the penetration measurements.
its probable error, is listed for each shot.
mixed layer.
The average penetration depth, with
The same information is shown for the depth of the
The ratio of penetration depth to mixed-layer depth is also listed.
Only in the
case of the land shot, Zuni, does this ratio exceed one. The other three shots give about the
same ratio, 0.75.
Part of the probable error in penetration measurements arises from the presence of internal
waves, which cause fluctuations as great as 20 meters in the depth of the mixed layer at any
given geographic location (Reference 6). Another cause is the variations in the depth of the
mixed layer from one region of the fallout pattern to another (Reference 6). The penetration
depth was plotted against mixed-layer depth and against geographic location; no direct correlation could be found. Because of this, the average penetration depth for each shot (Table 2.5)
is used to determine the 3-foot dose rate after mixing is complete. Prior to completion of mixing, the penetration-depth curves shown in Figures 2.17 through 2.20 are used.
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