The Model 843-251 Process GM Detector, when installed into an existing off-line gas sampler or a Model 841-7 On-Line Sampler, a Model 843-252 Preamplifier and a Model 942A Universal Digital Ratemeter (UDR) or Model 960 Digital Radiation Processor Controller, provide an extended range radiation monitoring system for applications where the direct measurement of ionizing radiation in a release path is required. When used as a main steam monitor, pressurized water reactor (PWR) primary to secondary leakage may also be detected. The Model 843-251 GM detector consists of a GM tube detector and a radioactive bias source mounted in a phenolic tube. The bias source is a small chip of 238U added to provide a 10 to 15 CPM output to verify detector operation. The phenolic tube is provided to protect the thin walled, glass sealed GM tube from physical damage when handling the tube. An integral nine (9) foot coaxial cable with mating connector is provided to connect the GM tube to its associated preamplifier. The detector is available for high, medium, or low sensitivity applications. The sensitivity is specified by an alpha suffix added to the detector model number (i.e. 843-251D, uses a low sensitivity GM tube). 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 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 ft 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, maintaining the output and resultant readout display at full scale. The preamplifier is designed for mounting within six (6) feet of the detector. The preamplifier enclosure functions as a waterproof enclosure for preamplifier, provides a mating connector for the detector cable and includes an internal terminal strip for connecting the detector high voltage (HV), signal and power conductors from the control room readout. Field calibration of the detector is accomplished through the use secondary transfer button sources and a Model 843-5 Series fixed geometry source holder.
