Features

• CANBERRA patented Time-to-Count technique eliminates dead time and saturation effects of conventional GM tubes
• SMART Probe retains probe information in non-volatile memory
• Excellent linearity and accuracy
• Detectors are easily interchanged
• Detectors do not require operator adjustments
• Calibration cycles of up to two years
• Optional mounting brackets
• Optional connection to CANBERRA's area monitor and display units

Quality

• Commitment to meet or exceed your quality expectations
• Cost-effective for safety and compliance

Description

A pair of rugged halogen-quenched GM tubes serves as the basic detection elements in the GP Series detectors used for wide range gamma radiation measurement. The GM tubes operate under the unique and patented CANBERRA Time-to- Count technique which removes many of the limitations associated with the use of GM tubes operated in conventional mode.

In addition, the GP Series of detectors are SMART probes, which retain probe information in nonvolatile memory. When calibrated, data such as probe calibration constants and identifying information are stored and verified in the EEPROM memory in the probe circuitry. This arrangement allows the CANBERRA GP Series detectors to be interchangeable.

The GP Series of detectors are "maintenance free" in design and require no routine servicing or preventive maintenance.

The GP100M and GP100MS are used in conjunction with a preamplifier, PA100M. This arrangement allows components that are susceptible to damage by extended exposure to radiation to be placed up to 200 feet away from the detectors.

The GP Series can be connected to multifunction control and display unit such as ADM606M.

Principle of Operation

A pair of halogen quenched GM tubes serves as the gamma radiation detector in the GP Series of detectors. This design allows for wide range measurements - the low range tube monitors daily environmental changes while the high range tube is able to cover accident levels of radiation.

Readings of the radiation rate are a function of the number of pulses (counts) produced by the tube per unit time. Conventionally, a GM tube operated with a fixed dc voltage continuously applied is characterized by "dead-time", increasing non-linearity as the field intensity increases and saturation.

In the Time-to-Count technique employed in CANBERRA GM tube detectors GP Series, the dead time and saturation effects are eliminated.

A low dc bias voltage is abruptly raised to 500 V dc carrying the tube into its operating region. The rise time of this voltage is less than 0.2 microseconds. At the same time, this rapid increase in voltage is applied, a crystal controlled, 1 megacycle oscillator (clock) is gated on and time, in the form of 1 microsecond cycles, starts being counted. Time counting continues until a GM tube pulse is obtained. At that point, time counting is stopped and the accumulated time is recorded. At the same time, the anode voltage is reduced to the low bias level. The anode voltage is maintained at the low bias level for 1.5 to 2 milliseconds, a time period which is long compared to the dead time and recovery time of the tube. Voltage at anode is applied again when the GM tube is fully recovered.

Only one GM tube pulse can occur in any one 'on' time. Since the tube is fully recovered between 'on' times, the pulses produced by the tube are full size. The process is repeated many times to obtain a statistically reliable average time-to-count.

Thus, the radiation field intensity is proportional to the reciprocal of the time required to obtain a GM count. This precise microprocessor controlled relationship forms the design basis for the GP Series and enables many decades of linear performance for the GM tubes involved.