Appendix B INSTRUMENTS AND DECAY This appendix contains a detailed description of the detecting instruments of primary importance to the project, together with certain early-time de ay curves used throughout the body of this report. This information is not currently available in the literature. B.1 INSTRUMENTS Since the GITR 103 is basic to the project, it is described in greater detail. This instrument is a dose-increment recorder consisting of: (1) two concentric ionization chambers with recycling electrometers, (2) a magnetic-tape recorder, (3) a mechanical timer, and (4) a control circuit and a battery power supply (Figures 1.4 and B.1). The externally mounted detector unit is connected to the main instrument assembly within the case itself. The detector consists of a low-range ionization chamber constructed around a high-range fonization chamber, with each chamber connected to a recycling electrometer. These electrometer circuits use a CK 5886 tube connected as a cathode-coupled blocking oscillator with the interelectrode capacity of the first grid below the predetermined triggering level of the positive voltage shift on the first grid (Figure B.2). When predete-mined voltage level is reached, the circult is triggered and generates a pulse of fixed amplitude at the cathode. The pulse causes the first grid to conduct and to transfer a constant, predetermined charge to the chamber. Simultaneously, the pulse is recorded on magnetic tape. The pulse terminates at the cathode in approximately 500 usec, and the tube is left nonconducting with a negative voitage on the first grid, thus completing the cycle. The gamma dose increment required to discharge the ionization chamber its directly proportional to the amount of charge transferred to the chamber (Figures B.3 and B.4). The charge transferred during each cycle is constant but dependent upon the triggering level of the electrometer, which is controlled by the adjustable bias voltage of the second grid. Calibration of detectors is achieved by adjustment of the bias voltage until a predetermined dose increment causes the electrometer to cycle (Section C.1). The calibration control for each chamber is located on the moistureproof electrometer housing attached to the base of the chamber assembly. The ionization chambers are constructed of thin-walled spun-aluminum shells mounted concentrically. Cylindrical and hemispherical surfaces are used wherever possible to establish optimum voltage gradients for efficient charge collection. The chambers are filled with pure argon at 7.5 psi and sealed by softsoldering over nickel-plated surfaces. The volumes of the two chambers are 1,475 and 14.0 cm’ for the low-range and high-range chambers, respectively. The sensitivity ratio of 1,000 between the two ranges is 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 provides uniform energy response from about 100 kev to 2 Mev (Figure C.1). The record is made on 900-foot lengths of instrumentation-quality magnetic tape spooled on standard 5-inch reels. The tape is 0.25 inch wide and has a polyester backing 0.001 inch thick. A Brush Electronic Company BK 1303-1 three-channel recording head, driven to tape saturation, records unidirectional pulses on the tape. The maximum usable pulse packing is 400 pulses per inch of tape. Recording intervals of 12 hours and 60 hours_are used with tape transport speeds of 0.25 in/sec and 0,05 in/sec, respectively. These speeds are accurate to + 2 percent for the entire recording interval. Both recorders are of identical construction with the exception of the drive motors. A single 6.7-volt mercury-battery stack having a 359 “A nk cw OF