The Model 943-5 Process GM Detector, when directly mounted in the release path or used with the Model 940-4 In-Line Sampler or 841-334 Off-Line Sampler and a Model 942A Universal Digital Ratemeter (UDR) or Model 960 Digital Radiation Processor Controller, provides a radiation monitoring system for applications where the direct measurement of ionizing radiation in the release path is required. The Model 943-5 GM detector consists of a GM tube detector and a preamplifier. A Model 942A UDR or Model 960 Readout provides the high DC voltage necessary to operate the detector, the low DC voltage required to operate the preamplifier and the pulse counting electronics necessary to quantify the detector pulse output. The detector is a thin-walled electronic tube with a negative electrode (cathode) and a positive electrode (anode). A positive high voltage (HV) is applied across the anode and cathode.
The tube is filled with a mixture of halogen, or noble gasses, and a quenching gas. When a radioactive particle penetrates the wall of the tube, an ionizing event occurs. The event produces an ion pair, which triggers the discharge of the tube, producing an avalanche of ion pairs and a current pulse. The current pulse, one per ionizing event, is independent of the energy of the initiating particle. Multiple discharges are eliminated via the use of a quenching gas in the tube. The resultant pulses are then coupled to the preamplifier.
The preamplifier receives the pulses from the GM tube and amplifies the pulses to a nominal 5 VDC level. The preamplifier also includes signal conditioning and line driver circuitry capable of driving the pulses through a 50 ohm transmission line a distance of 1,500 feet without degradation. In the event the radiation field is greater than the maximum range of the detector, the tube will saturate, resulting in a relatively constant output from the preamplifier. This will result in the display of a radiation value that is much less than the actual radiation field. To prevent understating the radiation value, an anti-jam circuit is provided in the preamplifier. When the preamplifier detects a constant output condition, a square wave with a frequency equivalent to the maximum range of the detector is sent to the readout.
The preamplifier is integrally mounted to the top of the detector. The preamplifier enclosure functions as a waterproof enclosure for preamplifier and provides a method for connecting the detector HV, signal and power cables to the control room readout. Field calibration of the detector is accomplished through the use secondary transfer button sources and a Model 943-5-100 Fixed Geometry Source Holder.