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Model 945A Wide Range Ion Chamber Area Monitor

Wide Range Ion Chamber Area Monitor

Model 945A

The Model 945A is a wide-range area monitoring system with a detectable range of 10-1 to 107 mR/h!

Instruction Manual

945-100 Datasheet

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Key Features

  • Range: 10-1 to 107 mR/h
  • Energy response: 10% from 55 keV to 3 MeV
  • Life expectancy: up to 107 rads if preamplifier is remote or shielded
  • No external power is needed at detector location 3 twisted pair cable with ground conductor and overall shield between detector and ratemeter for maximum noise immunity
  • Environmentally hardened for operation in moisture laden environments
  • Seismically tested with high reliability
  • Microprocessor controlled


The system is comprised of the Model 977-210 Wide Range Ion Chamber Detector, with local preamplifier, and a Model 946A-200 Universal Digital Ratemeter (UDR). The detector and preamplifier are connected by two 5 foot cables, encased in a flexible conduit. Other cable lengths, up to 100 feet between preamplifier and detector, are available. Both the detector and preamplifier are wall mounted in an area designated by the customer. The Model 946A-200 UDR is mounted in a rack in the control room.


Area monitors are used for the detection of x-ray or gamma radiation in selected areas. The monitor should be used in any location where personnel may be exposed to an adverse amount of radiation. Applications include nuclear reactors, accelerators, hot cells, irradiators and any area where radiation sources are handled. These monitors can be used as single channel monitors or grouped together as a multichannel area monitoring system.


The Model 977-210 Wide Range Ion Chamber Detector is a gamma sensitive radiation detection device, which measures radiation in the range of 10-1 mR/h to 107 mR/h with an energy dependence of less than ± 10% from 55 keV to 3 MeV. The preamp is housed in a gasket sealed enclosure. Interconnection between the detector and preamplifier is accomplished via two five foot cables, encased in a flexible conduit. The wide-range ion chamber detector design includes a dual, concentric ion chamber with a volume of approximately 1000 cc. The chamber walls are made of tissue equivalent plastic. An outer wall, made of aluminum, is provided to protect the ion chamber and to mechanically interface with the Model 848-8 Field Test Source. The ion chamber is biased at a nominal 500 volts and produces an output current proportional to radiation absorbed in the chamber. The current is approximately 8 x 10-11 A/R/h. The ion chamber assembly makes use of a double seal design, where the actual ion chamber is filled with nitrogen at atmospheric pressure. The connector area is sealed against moisture and particulates through the use of a sealed, liquid tight flexible conduit between the detector and preamplifier electronics.


The preamplifier contains a microprocessor controlled, auto-zeroing, integrating electrometer, a programmable gain amplifier, an analog-to-digital converter, an asynchronous serial communication interface, an electronic check source and the detector high voltage power supply. Communications with the UDR is accomplished via the optically isolated serial communication loop driver/receiver circuitry.

Universal Digital Ratemeter

Standard features for the instrument consist of a three digit display of the radiation value and a multicolored bargraph indicator which covers the entire range of the detector. The bargraph will change color in the event of an alarm condition (green for normal, amber for warning and red for high). Front panel alarm indicators and rear panel output relays for alarm annunciation are also included. Front panel pushbuttons are provided to apply power, to display alarm limit setpoints, for alarm acknowledgment and for the activation of a check source function.

Models & Accessories

Model Name



Wide-Range Ion Chamber Area Monitor, includes:
Wide-Range Ion Chamber Detector with local preamplifier Detector (977-210)
Universal Digital Ratemeter (946A-200

Miscellaneous Accessories


Field Test Source

Field Test Source (part# 848-8)
Model number: 848-8

Rack Chassis

Rack Chassis (part# 948-1)
Part number: 948-1
Model number: 948-1
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