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Ordering Info


The FWT GM-l and GM-2 are miniature Geiger counters designed for measurement of gamma-ray exposure rates in a mixed gamma neutron field. The GM-1 is packed with Li6F around the detector. The GM-S is a removable Li6F shield for the GM-2. The GM-2 has no Li6F shield.

This detector has been patterned after the work of Wagner and Hurst (Health Physics, 5, 209 1961). The basic detector is a small, diameter GM tube, with a 0.64cm long active volume. This small detector has been enclosed in an energy flattening shield that eliminates the low energy over-response typical of metal-walled GM tubes.

The detector housing is aluminum and is attached to the end of an aluminum rod. The center of the active volume is 0.90 inches (2.3 cm) from the end of the housing. The GM-1/1S/2 counter wall consists of 90 Mg/cm2 Fe, 807 Mg/cm2 Pb, 200 Mg/cm2 Al. The GM-1/1S have an additional 480 Mg/cm2 Li6F.

The Li6F shielding is provided in the GM-1 by tightly packing powder around the Geiger tube during assembly. The GM-1S configuration provides the same shielding with a removable Li6F shield. When used without the shield, the instrument is identical with the GM-2 counter.

The connector block contains the preamplifier and the high voltage and signal connectors. It is sturdily made and suitable for clamping the detector. The preamplifier employs an integrated circuit with all active components on one chip and draws its power from the high voltage supply; no extra power supply is required. The preamplifier will drive at least 50 feet of cable.
Operational Characteristics

The Model GM-l/S/2 is designed to plug directly into a NIM bin scaler. The +5 V pulse will easily drive such a device. This counter has a fixed dead time of approximately 40 ms per pulse. It is blocked during this time; consequently, there will be increasingly severe counting losses as the exposure rate is increased. A 10% loss occurs at approximately 2 R/hr. Operation at higher rates is possible, but usual counting statistics will not be applicable. Pulsed accelerators will normally cause the device to lock into the accelerator pulse rate (or a sub-multiple) and will therefore result in meaningless data.

The gamma energy response is flat within ±10%from 70 keV to 2.06 MeV. The response falls rapidly below 70 keV, and corrections should be made for gamma-ray exposure below this energy for accurate work.

Measurement by Wagner and Hurst in low-energy neutron fields indicate 0.5% response relative to gamma tissue kerma. Higher energy neutrons have a calculated response of 0.2%.

The Model GM-l/1S/2 is not designed for use as an absolute gamma-ray detector. Every effort has been directed towards stability of the calibration; however, it is recommended that frequent comparisons be made with a standard ion chamber in a known gamma field.

GM Tube: Metal wall (Fe, Cr alloy) ~90 Mg/cm2 thick, neon filled, halogen quenched
High Voltage Requirements: +400 to +500 V at ~2 mA, +550 V maximum
Plateau: ~440 V to 500 V, ~0.5%/V
Dead Time: ~40 ms
Signal Output: +5 V pulse ~25 ms wide
Gamma Sensitivity: ~9.0 x 10-7 rad/pulse
Gamma Energy Response: ±10% from 70 keV to 2.1 MeV The response below 70 keV falls rapidly.
Angular Response: 0° to 90° end on, ±12% CS137; 0° to 90° side on:±5% Co60
Neutron Sensitivity: Less than 1 % of gamma sensitivity for neutrons @2 MeV
Proton Shielding: Counter wall will stop ~22 MeV protons
Signal Connector: Mates with BNC UG 260B/U or equal
Signal Cable: Any
High Voltage Connector: Mates with UG932A/U or equal (SHV upon request)
High Voltage Cable: RG59/U or equal
Construction: Aluminum, waterproof detector housing on end of 10.5 inch (26.7 cm) long, 0.25 inch (O.64 cm) diameter shaft.
Housing Diameter: GM-1-0.53 inch(l.35 cm); GM-lS-0.75 inch (1.9 cm)
Overall Size and Weight: Length including connectors 14.5 inches(36.8 cm); weight 180 grams
Accessories: Supplied with wooden storage box

Ordering Information

GM-1: GM Detector packed with Li6F and storage box
GM-2: GM Detector with storage box
GMs: Li6F shield to fit on GM-2
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Specifications subject to change without notice.
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