assist the ships’ personne! in tracking the low-level boundaries of the fallout area.
In its capacity as a safeguard, the instruments were successful. All three survey ships
contacted direct fallout from Shot Zuni, and inthe case of all three, the build-up of shipboard
radiation was detected by the Nav-rad immediately. This gave the personnel timeto take the
necessary radiation safety precautions and to head the ship out of the fallout before the levels
pecame hazardous.
In its capacity as an aid in tracking the low-level boundaries of the fallout area, the Nav-rad
was singularly unsuccessful. Its failure in this respect was partly caused by the high ships’
background from the Zuni fallout, but it is not implied that this is the only reason.
Even though
the detection elements are extremely sensitive, the height of the device above the sea surface
(over 30 feet in all cases) and its shielding from the back and sides reduces its effective sensitivity to the radioactive water surrounding the ship. The probe detects a change in activity when
the Nav-rad shows none. AS an above-the-surface detector of water contamination, it is an
excellent device, but not when compared to a sensitive probe in the water surrounded by the
radioactivity.
2.6.2 Ship Surveys. The surveys following each shot involved over 1,000 miles of travel for
each of the destroyer escorts. In addition, during each survey the M/V Horizon spent over 4
days inside the fallout area, taking detailed radiological and oceanographic measurements.
The ships’ tracks for each shot are shown in Figures 2.5, 2.6, 2.7, 2.8, and 2.9. The tracks
are based on position determined by hourly loran fixes, which have an estimated accuracy of
1 mile.
2.6.3 Reliability of Probe Measurements. At intervals during each fallout survey, the probe
on each Ship was removed from the water and calibrated against a Co® source of known strength.
This was done to ensure that probe response would be known for all measurements.
In addition to the individual shipboard calibrations, the ships were brought together inside
the fallout pattern whenever possible. By this method, the probe readings were intercalibrated
between ships while the probes were all in water containing the same concentration of fission
products.
As a result of these controls, the calibrations of the probes are known for all readings, and
the intership comparisons show agreement within 5 percent for all shots except for a 6-hour
period following Shot Navajo. During that time the probe of one of the ships showed a 70 percent
disagreement with those of the other two ships. The cause of the discrepancy has not been discovered, but the readings of the errant probe have been brought into agreement for that period
of time.
2.6.4 Instrument Contamination.
Previous experience had shown that probes became contam-
inated when towed through water containing fission products.
It had been found (Reference 2)
that metal surfaces were notorious in this respect. In an effort to reduce the amount of contamination, the probes were wrapped with polyethylene tape.
In spite of the precaution taken in taping the probes, they occasionally became contaminated.
Since it was not possible to detect this contamination in the highly radioactive water where it
occurred (above 10 mr/hr), the probe was often towed for many hours before its condition was
noticed and corrected by re-taping.
For the final data reduction, the contamination of the probes has been estimated for each ship
curing each of the fallout surveys. The estimates are based on the following: (1) the minimum
probe reading recorded at the bottom of each cast— it was not always assumed that the water at
the bottom of the cast was clean, however, this procedure gave a maximum for the amount of
contamination; (2) comparison between the NRDL counts of the Nansen bottle samples, reduced
to dose rate, and the probe readings; (3) use of inter-ship calibrations, wherever applicable;
(4) reduction in the probe reading after re-taping as a direct measure of the amount of contamination; and (5) knowledge of the radiation levels traversed by the ship for information by which
the contamination curves could be estimated
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