Fluke Biomedical 956A-201-M2 - Manual (Page 27)

3-1
Section 3
THEORY OF
OPERATION
3.1 OPERATION
This section describes the operation and set up of a Digital Area Monitoring System. The system consists
of a Model 956A-201-M2 UDR and a Model 897A Series G-M Detector. The detector preamplifier
provides the electronics interface between the detector and the UDR. The preamplifier provides input
pulse discrimination, pre-divide capability, anti-jam circuitry, pulse amplification and output drive
capabilities. The output of the preamplifier is transmitted to the UDR. The 956A-201-M2 UDR uses this
information to generate the dose rate display, error codes, alarms, and analog outputs.
Operation of the 956A-201-M2 UDR consists of operator functions and configuration functions. All
operator functions are performed using the front panel shown in Figure 1-2. Configuration functions are
performed using internal switches and/or jumpers, which are accessible with partial removal of the UDR
from the mounting enclosure. Calibration involving trim pots is described in Section 4.
For applications where EMI/RFI compliance is required, the S157033A2 Line Filter/Surge Suppressor
Assembly, designed to provide EMI/RFI protection for the 956A Series digital ratemeter and 897A Series
G-M detectors, is available. The assembly consists of two (2) separate and independent line conditioning
circuits that provide filtered AC power to each 956A Series ratemeter. The S157033A2 is designed to
mount in the center section of the 948B-1A-5 dual module rack chassis, to provide EMI/RFI protection for
two (2) 956A Series ratemeters.
3.2 DETECTOR OPERATION, MODEL 897A-2XX
The Digital Area Monitor uses a Model 897A-2XX G-M tube as a gamma radiation detector. The tube is
filled with a mixture of gases, one of which acts as a quenching agent, while the others tend to support
ionization. Electrically, it consists of a positive electrode (anode), which is maintained at a steady potential
of 500/650 Vdc and a negative electrode (cathode) that is near ground potential.
When a gamma photon penetrates the shield encasing the G-M tube, an ionizing event occurs. An ion
pair is produced, triggering an avalanche of ion pairs. The current pulses produced (one pulse per ionizing
event) are independent of the energy of the initiating particle. Multiple discharge, caused by the release of
electrons from the cathode due to excess energy, is eliminated after a short time (called dead time,
typically 20-45 microseconds) by a quenching gas within the tube. The resulting pulses are conditioned
and transmitted to the preamplifier.
The integral detector preamplifier provides input pulse discrimination and amplifies pulses received from
the G-M tube to 5 Vdc amplitude. The current drive output of the preamplifier will drive a signal up to 2000
feet through a 50-ohm transmission line, without signal degradation.