ig uniform from the sea surface to the bottom of the layer, or thermocline, beyond which the
temperature decreases rapidly with increasing depth. This layer owes its existenceto the
stirring action of wind and waves and is often referred to as the “mixed layer.”
When a substance of soluble or colloidal nature or one having about the same density as
water falls on the ocean surface, it becomes distributed into the mixed layer rapidly, often within
Upon reaching the thermocline, however, it virtually ceases to penetrate downward,
because of the sharp increase in density, and thence stability, at this boundary.
It is this phenomenon that permits the success of the survey, because most of the radioactive
afew hours.
fallout is retained in the upper layer and subjected to uniform mixing long enough for the survey
vessels to measure surface values of dose rate throughout the fallout area.
Since the depth of
mixing of fallout is known and because the dose rate is uniform to this depth, one can mathematically squeeze alt of this activity into a layer only 1 meter thick, and thence onto the surface of
a hypothetical plane.
2.3.1 Dose Rate.
are of two types:
The measurements of radioactivity in water made by the survey vessels
1, The underwater Geiger counter (henceforth this instrument will be referred to as a probe)
is towed just below the surface of the water by the survey vessel and a continuous trace of its
output versus time is recorded. This is later reduced to dose-rate values and plotted against
the geographic location that corresponds to the recorded time. This yields a general pattern of
dose-rate values over the entire area traversed by the survey vessel.
2. During the survey, the vessel stops at selected locations and measures the depth to which
the radioactive fallout has mixed into the water. This is accomplished by lowering the probe,
by use of a hydrographic winch, until the output signal from the probe indicates that it is sur-
rounded by clean water.
The final dose-rate values desired are those that would have been measured at a heightof
3 feet had the same fallout occurred on flat ground, instead of on the ocean. The derivation of
the factor necessary to make this transition from the dose rate in water to dose rate at 3 feet is
presented in Appendix A. In the treatment of the data, two other correction factors must be
utilized before the information is ready for final presentation.
2.3.2 Decay Coefficient. The radioactive-decay constant used for fission products is generally accepted as being t7’**; this decay value was used in Reference 2. For measurements taken
under ordinary circumstances, this value is a sufficiently close approximation. However, the
measurements taken from the survey vessels are far from ordinary. The fission products that
fall on the ocean surface are subjected to fractionation, both in air and in water, This alone
may give rise to a shift in the decay exponent. Furthermore, the measurements are being made
under water, and the energy spectrum as seen by the instrumentis subject to degradation by the
scattering in water. The instrumentitself does not have uniform response to incident gamma
radiation of varying energy from a distributed source.
Asa result of these considerations and
because the energy spectrum of the fission products changes rapidly during the first few days
following their formation, it was felt a direct measurement by the probe of the decay of radioactive fallout would be valuable for Operation Redwing. This measurement was carried out —
following each of the shots, and the method used is presented in the section on instrumentation.
2.3.3 Current Drift. The survey vessels cannot accomplish their tasks rapidly enough to
prevent the ocean currents from distorting the fallout pattern considerably. Before a meaningful
Picture can be presented, all measurements must be corrected for this distortion. This must
be done so that radiation values will be plotted where the material fell out, rather than where
they were measured. The method of obtaining this factor is presented in the section on results
and discussion.
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