Appendix A
GITR INSTRUMENT
Instruments to record gamma radiation as a function of time had been developed and used during previous
field operations (References 3 and 4).
However, this earler instrumentation was entirely unsuitable for
use during Operation Hardtack. wherein high time resolution, wide radiation-intensity ranges. improved
detector geometry. simplified and unattended operation. rugged watertight performance. and improved
capability for data reduction were required. These requirements were the basis for the development of
the GITR Model 103 (Figures A.1 and A.2).
The instrument developed was a dose-increment recorder consisting of:
(1) two concentric ionizanon
chambers with recycling electrometers. (2) magnetic-tape recorder. (3) mechanical] timer, and (4) contro!
circuit and battery power suppiy (Figure 4.3). These components were packaged in a watertight aluminum
case 21 by 16 by 13 inches in size and had an overall weight of 55 pounds.
The externally mounted detector
unit was connected to the main instrument assembly by means of a watertight cable.
tector could be plugged into the main instrument assembly within the case itself.
A...
Optionally, the de-
DETECTOR UNIT
The detector consisted of a low-range ionization chamber constructed around a high-range ionization
chamber. with each chamber connected to a recycling electrometer circuit (Figure A.4}.
The recycling
electrometer consisted of a CK 5886 electrometer tube connected as a cathode-coupled blocking oscillator
with the interelectrode capacity of the ionization chamber in the first grid. Initially, the ionization chamber was charged. and the voltage on the first grid was below the predetermined triggering level of the
electrometer. Yonizing radiation discharged the chamber and caused a positive voltage shift on the first
grid. When a predetermined voltage level] was reached. the circuit was triggered and generated a pulse
of fixed amplitude at the cathode. The pulse caused the first grid to conduct and to transfer a constant.
predetermined charge to the chamber. Simultaneously, the pulse was recorded on magnetic tape. The
pulse terminated at the cathode in approximately 500 psec, and the tube was left nonconducting with a
negative voltage on the first grid, thus completing the cycle.
The gamma-dose increment required to discharge the ionization chamber was directly proportional to
the amount of charge transferred to the chamber (Figures B.1 and B.2, Appendix B). The charge transferred during each cycle was constant but dependent upon the triggering level of the electrometer, which
was controlled by the adjustable bias voltage of the second grid. Calibration of detectors was achieved
by adjustment of the bias voltage until a predetermined dose increment caused the electrometer to cycle
(Appendix B). The calibration control for each chamber was located on the moistureproof electrometer
housing attached to the base of the chamber assembly.
The ionization chambers were constructed of thin-walled aluminum spinnings mounted concentrically.
Cylindrical and hemispherical surfaces were used wherever possible to establish optimum voltage gradi-
ents for efficient charge collection. The chambers werefilled with pure argon at 7.5 psi and sealed by
soft-soldering techniques over nickel-plated surfaces. The volumes of the two chambers were 1.475 cc
and 14.0 cc for the low-range and high-range chambers, respectively. The sensitivity ratio of 1,000 between the two ranges was achieved by the design value of the input capacity of the electrometer circuits.
A lead-tin filter over the entire outer surface of the detector provided reasonably uniform energy response
from about 100 kev to 2 Mev (Figure B.3).
A.2.
RECORDER SYSTEM
The recording medium was 900-foot lengths of instrumentation-quality magnetic tape spooled on standard 5-inch reels.
The tape was 0.25 inch wide and had a polyester backing 0.001 inch thick.
A Brush
Electronics Company BK 1303-1 three-channel recording head, driven to tape saturation. recorded unidirectional pulses on the tape.
The maximum usable pulse packing was 400 bits per inch of tape.
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SOS
Re-