the pulse-repetition rate was so great that the recorded marks overlapped and could
not be resolved. At that time, Stylus No. 2 would be used, with each mark representing
11 pulses from the detector head. The chart drive that supplied the time base was
calibrated with a Watchmaster before each event. By means of the Watchmaster, the
chart drive could be set to have a maximumerror of 1 minute in 24 hours, or +0.069
percent. This was not the optimum recording system for use with this detector but
rather a compromise forced by a lack of funds and time.
2.2.3 Initial Instrument System, Gustave I Detector.
For the high-range, fast-
resolution detector, the basic circuit of Figure 2.1 was used with a scintillation detector
as the sensing element.
The latter consisted of an RCA 929 phototube and a National
Radiac Scintillon Branch plastic phosphor mounted in an electron-equilibrium thickness
of bakelite to provide an air-equivalent response (Reference 8). The purpose of the
electron-equilibrium layer was to present a source of electrons that might be scattered
into the crystal to replace those electrons produced by radiation absorbed nearthe crystal surfaces and lost without being detected. These detectors were constructed to cover
three ranges: 10° to 10° r/hr, 10° to 10’ r/hr, and 104 to 108 r/nr.
The overall detector response is given approximately by:
f=kr
(2.3)
Where: f= the pulse repetition rate
r =the gamma-exposure rate in r/hr
k = a parameter chosen to meet specific design objectives
The maximum pulse-repetition rate of these instruments was 1,000 pulses/sec, the
maximum rate that could be resolved by the recorder (a Cook Research Laboratory
MR-33 eight-channel magnetic-tape recorder). Typical calibrations for these detectors
are shown in Figure 2.3. Figure 2.4 shows the energy dependenceof the Scintillon-
phosphor Gustave I detector, relative to Co™ gammaradiation at a rate of 100 r/hr.
To reduce the errors due to flutter and wow, a 1,900-cycle-time base was recorded on
the tape simultaneously with the gamma-exposure-rate data. An American Time Products transistorized-frequency »tandard with an accuracy of +0.02 percent was used to
provide the time base.
2.2.4 Photomultiplier Feedback Circuit, Initial Instrument System. This system was
essentially the same as that used during Operation Castle (Reference 2). The detecting
element, a Scintillon phosphor 2.75 inches in diameter by 0.5 inch 1n height mounted in
a bakelite block for electron equilibrium, was placed inside a blast-resistant housing
at the top of a light pipe. The output of the crystal after passing through the light pipe
was detected by an RCA 6199 photomultiplier tube. The photomultiplier tube was used
in a 100-percent-feedback circuit which held the photo-multiplier-tube-anode current
nearly constant, regardless of the incident light flux, by reducing the dynode voltage
(Figure 2.5). The gain of a photomultiplier tube with constant anode current was approximately proportional to the antilog of the dynode voltage. In this manner, a useful
dynamic range of about a factor of 10" was realized.
2.2.5 Calibration.
Three radiation sources (a 250-kv X-ray generator, a 2.5-Mev
Van de Graaff generator, and a 200 -curie Co™ source) were used in the calibration of
22