3.
The third section contained the battery packs for the pulse a: .plifier. The filament ba:teries were two 1.3-volt RM-42 mercury cells in parallel, and the plate voltage was supplied
py forty-two 1.3-volt RM-1 mercury cells. This plate voltage was also additive to the high
yoltage pack, giving a total of 730 volts across the G-M tubes.
4, The radiation-sensing element was built into the switching (or terminal) end of the probe.
Since the anticipated variations in dose rate were large, four combinations of G-M tubes were
used to cover four different ranges of dose rate levels. The most systematic way of describing
these combinations is to consider the vessels which utilized each type. The M/V Horizon and
the two destroyer escorts were equipped with two interchangeable sensing heads for each probe.
In order to change ranges on these heads, the instrument had to be brought on deck and the selector switch turned by hand.
For the fourth section, sensitive and medium ranges were combined into one head. The
sensitive range consisted of eight Anton 315 G-M tubes wired in parallel. A pulse amplifier
was used both for amplification of the signal and for range selection. Using this combination,
four ranges of sensitivity were covered between 0.0005 and 25 mr/hr. Also included in this
head was a single Anton BS-2 G-M tube, whose pulse was put through the amplifier. This tube
covered the range from 5 to 100 mr/hr. The other head contained two Anton BS-213 G-M tubes,
whose output was measured directly. This combination covered the range from 100 mr/hr to
100 r/hr.
Since the function of the YAG’s was to be present under the fallout and all personnel were
closeted in a shielded control room, the range selection had to be made remotely. In the medium
and low sensitive head, a relay was installed. When the range required changing, the relay was
tripped by a pulse from the control room. The medium sensitivity range in this head was a
single Anton 315, whose output was measured directly. The range covered by this tube was
from 1to 100 mr/hr. The low-sensitivity range was the same as that used on the M/V Horizon
and the two destroyer escorts. The other replaceable head was the high-sensitivity head similar to those on the Horizon and destroyer escorts, but lacking the medium-range BS-2.
All of these instruments were calibrated periodically by the use of point sources of Co.
Since a drop in battery voltage would result in a calibration change, it was necessary to take
calibration checks before and during each shot to guard against any shift in calibration.
2.5.2 Towing Cable.
The probe was connected to the ship by use of a special, three-
conductor armored cable.
The conductors entered the probe first through a rubber packing
gland and then through Stupakoff (glass-insulated) connectors. This ensured that, even if the
packing gland leaked, no water could enter the probe through the electrical connection. The
Output signal traveled through the conductors and was picked up at the winch on a mercury slipring assembly.
2.5.3 Recorders. From the slip-ring assembly, the signal was transmitted to the recorder
through a coaxial cable. Leeds and Northrup X-Y recorders were used. These were especially
adapted so that a single recorder could plot radiation either against time or against depth. While
the probe was being towed across the pattern, the recorder was placed on time drive and a continuous trace of surface radiation was recorded. When the ships stopped on station to make a
Penetration measurement, the recorder was switched to Y-axis drive and a trace of depth versus
tadiation was recorded.
2.5.4 Calibration of Probe. All calibrations of the towed probes were made with point sources
of Co. This meant that the calibration was strictly accurate only for hard gammaradiation
that struck the probe normalto the axis.
_ In order to relate these Co®calibrations to underwater measurements of uniformly mixed
fission Products and subsequently to the dose rate at 3 feet above a hypothetical infinite plane,
2 Series of special calibrations were made (Appendix A).
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