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.
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
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
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.
Metal wall (Fe, Cr alloy) ~90 Mg/cm2 thick, neon filled, halogen
Requirements: +400 to +500 V at ~2 mA, +550 V maximum
~440 V to 500 V, ~0.5%/V
+5 V pulse ~25 ms wide
~9.0 x 10-7 rad/pulse
Response: ±10% from 70 keV to 2.1 MeV The response below
70 keV falls rapidly.
0° to 90° end on, ±12% CS137; 0° to 90° side
Less than 1 % of gamma sensitivity for neutrons @2 MeV
Counter wall will stop ~22 MeV protons
Mates with BNC UG 260B/U or equal
Connector: Mates with UG932A/U or equal (SHV upon request)
Cable: RG59/U or equal
Aluminum, waterproof detector housing on end of 10.5 inch (26.7
cm) long, 0.25 inch (O.64 cm) diameter shaft.
GM-1-0.53 inch(l.35 cm); GM-lS-0.75 inch (1.9 cm)
and Weight: Length including connectors 14.5 inches(36.8 cm); weight
Supplied with wooden storage box
Detector packed with Li6F and storage box
Detector with storage box
shield to fit on GM-2