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955A Geiger-Mueller Area Monitor

Geiger-Mueller Area Monitor

Model 955A

The Model 955A is a single-channel area radiation monitoring system capable of operating over the ranges of 10-2 to 103 mR/h, 10-1 to 104 mR/h, or 100 to 105 mR/h, depending on the detector selected. The system monitors gamma radiation over a 5-decade range and provides indication when the radiation level decreases below a fail threshold, exceeds a warn set point, exceeds a high set point, or exceeds an overrange set point.

Instruction Manual


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Key Features

  • Range: 5 decades between 10-2 and 105 mR/h
  • Energy response: 15% from 80 keV to 1.5 MeV
  • No external power needed at detector location
  • Preamplifier integral with detector
  • Ratemeter may be remotely located up to 2000 feet
  • Single cable between ratemeter and detector
  • 8 µCi 36Cl check source
  • Life expectancy: up to 105 rads
  • Seismically tested with high reliability
  • Microprocessor based digital readout
  • Bargraph radiation value display


The overrange feature on the 955A provides two significant benefits. First, it prevents the system from displaying an on-scale, but inaccurate, reading should the detector become saturated. Second, it lessens the risk of damaging the detector by disabling it during an overrange condition. Relay outputs are available to activate alarm annunciators. Analog outputs are available for trend display on a strip chart recorder or computer. In addition, the monitoring system has an integral check source to verify operational integrity. The system consists of a Model 897A Series Geiger-Mueller (GM) Detector with integral preamplifier and the Model 956A Universal Digital Ratemeter (UDR).

  • GM Radiation Detector/Monitor
  • Area Monitoring
  • Personnel Protection
  • Detects X-ray or Gamma Radiation


Area monitoring is used for the detection of x-ray or gamma radiation in a selected area. 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.

Universal Digital Ratemeter

The Model 956A-201 UDR, when connected to a Model 897A Series GM Detector, comprises a monitoring system which operates over a five decade range. The UDR provides display, control and annunciation functions for the monitoring system, and will display readings in the range of 10-2 to 105 mR/h. 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 UDR. 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 relay outputs for alarm annunciation are also included. Front panel pushbuttons are provided to apply power, display alarm limit set points, acknowledge alarms and activate the check source. Analog outputs of 0 to 10 VDC (1) and 4 to 20 mA (2) are provided for recording and computer monitoring. The outputs may also be used to drive a remote meter or a local (i.e. near the detector) indicator. All electronics required to interface with a Model 897A Series detector are included within the Model 956A UDR. The electronics consist of a high voltage power supply, low voltage DC power supply and the hardware/software required for UDR operation. The system also includes an over-range indicator to preclude the possibility of on-scale readings when the radiation field is beyond the range of the detector.

Detector and Preamplifier

The Model 897A Series detectors use a thin walled energy compensated GM tube to detect ionizing radiation. Each Model 897A series detector has a GM tube, a check source and a preamplifier. The check source is a low-level radioactive source actuated by a + 15 VDC meter movement. For positive operation, the check source is electrically driven in both the ON and OFF positions. The preamplifier provides the pulse conditioning and cable driving capability necessary to drive a Model 956A UDR. All Model 897A Series detectors are functionally identical. They differ only in housing material, tube type and range. The Model 897A series detectors operate in the voltage range between 500 to 650 VDC. The Model 897A Series detector is designed to operate with the Model 956A UDR or with other readouts. Each detector measures a five decade range. Three measurement ranges are available: low, medium and high. The low range covers 10-2 to 103 mR/h, the medium covers 10-1 to 104 mR/h, and the high covers 100 to 105 mR/h. The Model 897A Series detector may also be used with the Model 960 Digital Radiation Monitoring System (DRMS) equipment or the Model 856 Analog Readout.

Models & Accessories

Model Name


955A Single-channel area radiation monitoring system, includes:
- Geiger-Mueller Detector (897A Series)
- Universal Digital Ratemeter (956A)

Miscellaneous Accessories


Field Test Source

Field Test Source (part# 848-8-105)100 mCi (nominal), with adapterfor use with Universal Digital Ratemeter (956A-201)
Model number: 848-8-105

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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